EG&G 128A Service manual

MODEL 128A
LOCK-IN
AMPLIFIER
SEE SAFETY NOTICE
PRECEDNG SECTION I
;EFORE OPERATING INSTRUMENT
OPERATING AND SERVICE MANUAL

M128A; 9/83·500·MIC
MODEL 128A
LOCK-IN AMPLIFIER
OPERATING AND SERVICE MANUAL
n ~~ G BROOKDEAL ELECTRONICS
~~ .:;;;;; ~ I:.. PRINCETON APPL/ED RESEARCH
Copyright ©1983 EG&G PRINCETON APPLIED RESEARCH
Printed In U.S.A.

SHOULD YOUR EQUIPMENT REQUIRE SERVICE
A. Contact the factory (6091452-2111) or your local factory
representative to discuss the problem. In many cases it
will be possible to expedite servicing by localizing the
problem to a particular plug-in circuit board.
B. If it is necessary to send any equipment back to the fac-
tory, we need the following information.
(1) Model number and serial number.
(2) Your name (instrument user).
(3) Your address.
(4) Address to which instrument should be returned.
(5) Your telephone number and extension.
(6) Symptoms (in detail, including control settings).
(7) Your purchase order number for repaircharges (does
not apply to repairs in warranty).
(8) Shipping instructions (if you wish to authorize ship-
ment by any method
other than normal surface
transportation).
C. U.S. CUSTOMERS-·-Ship the equipment being returned
to:
EG&G PRINCETON APPLIED RESEARCH
7 Roszel Road
(Off Alexander Road, East of Route 1)
Princeton, New Jersey
D. CUSTOMERS OUTSIDE OF
U.S.A.-To avoid delay in
customs clearance of equipment being returned, please
contact the factory or the nearest factory
distributor for
complete shipping information.
E. Address correspondence to:
EG&G PRINCETON APPLIED RESEARCH
P. O. Box 2565
Princeton, NJ 08540
Phone:
6091452-2111
TELEX: 84 3409
WARRANTY
EG&G PRINCETON APPLIED RESEARCH warrants each in-
strument of
its manufacture to be free from defects in material
and workmanship. Obligations under
this Warranty shall be
limited to replacing, repairing or giving credit for the purchase
price, at our option, of any instrument returned, freight
prepaid, to our factory
within ONE year of delivery to the
original purchaser, provided
prior authorization for such return
has been given by our authorized representative.
This Warranty shall not apply to any instrument which our in-
spection shall
disclose to our satisfaction, has become detec-
tive or unworkable due to abuse, mishandling, misuse, acci-
dent, alteration, negligence, improper installation or
other
causes beyond our control. Instruments manufactured by
others, and included in or supplied
with our equipment, are not
covered by
this Warranty but carry the original manufacturer's
warranty which is extended to our customers and may be more
restrictive. Certain subassemblies, accessories or corn-
ponents may be specifically excluded from this Warranty, in
which case such exclusions are listed in the
Instruction
Manual supplied with each instrument.
We reserve the right to make changes in design at any time
without incurring any obligation to install same on units
previously purchased.
THERE ARE NO WARRANTIES WHICH EXTEND BEYOND THE
DESCRIPTION HEREIN. THIS WARRANTY IS IN LIEU OF, AND
EXCLUDES ANY AND ALL OTHER WARRANTIES OR REPRE·
SENTATIONS, EXPRESSED, IMPLIED OR STATUTORY, IN·
CLUDING MERCHANTABILITY AND FITNESS, AS WELL AS
ANY AND ALL OTHER OBLIGATIONS OR LIABILITIES OF
EG&G PRINCETON APPLIED RESEARCH, INCLUDING, BUT
NOT LIMITED TO, SPECIAL OR CONSEQUENTIAL DAMAGES.
NO PERSON, FIRM OR CORPORATION IS AUTHORIZED TO
ASSUME FOR EG&G PRINCETON APPLIED RESEARCH ANY
ADDITIONAL OBLIGATION OR LIABILITY NOT EXPRESSLY
PROVIDED FOR HEREIN EXCEPT IN WRITING DULY EXE-
CUTED BY AN OFFICER OF EG&G PRINCETON APPLIED
RESEARCH.

Section
II
III
IV
V
TABLE OF CONTENTS
CONDENSED
OPERATING INSTRUCTIONS
CHARACTER ISTICS
2.1
Introduction
2.2 Specifications
INITIAL CHECKS
3.1
Introduction
3.2 Equipment Needed
3.3 Procedure
OPERATING INSTRUCTIONS
4.1 Introduction
4.2 Preliminary Considerations
4.2A Power Requirements
4.2B Fusing
.....
4.2C Warm-Up Period
4.20 Operating Frequency
4.2E
Grounding...
4.2F Noise .....
4.3 Operating the Model 128A
4.3A Introduction..
4.3B Reference Channel
4.3C Signal Channel
4.30 Output Channel Controls
4.4 Mixer
Function and Harmonic Sensitivity
4.5 Interface Connector .
4.6 Battery Operation .
4.7 Operation
with the Internal Reference Oscillator
4.7A
Introduction
4.7B Operation . . . . . . . . . . .
4.7C Installation .
4.8 Operation
with the Internal Tuned Amplifier
4.8A Introduction
4.8B Operation.........
4.8C Installation .
4.9 More Reference Channel Operating Hints
4.9A Reference Channel Slewing Rate
4.9B
Phase Errors with Small Reference Signals
ALIGNMENT .
5.1
Introduction
5.2 Required Equipment
5.3 Preliminary Steps
5.4 Procedure
5.4A +15 V Adjust (R310), -15 V Check, and +5 V Check
5.4B Reference Board Adjustments
5.4C Signal Board
Adjustment
5.40 Mixer Adjustments
5.4E Other Adjustments
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TROUBLESHOOTING
6.1 Introduction
6.2 Equipment Required
6.3 Initial Steps . . . .
6.4 Power Supply Checks
6.5 Reference Checks
6.6 Signal Channel Amplifiers
6.6A Preamplifier
6.6B Intermediate AC Amplifiers
6.6C Final AC Amplifier
6.7 Mixer
6.8 DC Amplifiers .
SCHEMATICS, Table of
FIGURES
Model 128A Lock-In Amplifier .
Ground-Loop Suppression by Ten-Ohm Input Ground
Differential Measurement of "Single-Ended" Signal
Differential Measurement of
"Off-Ground" Signal
Example of Everything
"Done Wrong"
Errors Depicted in Figure IV-4 Corrected
Typical Model
l28A Noise Figure Contours
Pulse Train as Reference Drive
Net Phase
Shift Between Signal and Reference Channels as a Function of Frequency
Amplitude and Phase Characteristics of Hi-Pass Filter
Amplitude and Phase Characteristics of Low-Pass Filter
Examples of
Output Filter Interactions . . . . .
Mixer
Output for In-Phase and Quadrature Signals
Internal Oscillator Board Installed .......
Tuned Amplifier Installed .
Phase/Amplitude Characteristics of Tuned Amplifier
Model 128A Adjustments and
Testpoints .....
TABLES
Interface Connector Pin Assignments
Frequency Range as a Function of Capacitors
ii
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SAFETY CONSIDERATIONS
A. INTRODUCTION
The apparatus to which this instruction manual
applies has been supplied in a safe condition.
This manual contains some information and warn-
ings that have to be followed by the userto ensure
safe operation and to retain the apparatus in a
safe condition. The described apparatus has been
designed for indoor use.
B. INSPECTION
Newly received apparatus should be inspected for
shipping damage. If any is noted, immediately
notify EG&G PARC and file aclaim with the car-
rier. The shipping container should be saved for
possible inspection by the carrier.
WARNING!
THE PROTECTIVE GROUNDING COULD BE
RENDERED INEFFECTIVE IN DAMAGED AP-
PARATUS. DAMAGED APPARATUS
SHOULD NOT BE ·OPERATED UNTIL ITS
SAFETY HAS BEEN VERIFIED BY QUALI-
FIED SERVICE PERSONNEL. DAMAGED AP-
PARATUS WAITING FOR SAFETY VERI-
FICATION SHOULD BE TAGGED TO INDI-
CATED TO A POTENTIAL USER THAT IT
MAY BE UNSAFE AND THAT IT SHOULD
NOT BE OPERATED.
C. SAFETY MECHANISM
As defined in IEC Publication 348 (Safety Require-
ments for Electronic Measuring Apparatus), the
Model 128A is Class Iapparatus, that is, apparatus
that depends on connection to a protective con-
ductor to earth ground for equipment and operator
safety. Before any other connection is made to the
apparatus, the protective earth terminal shall be
connected to a protective conductor. The protec-
tive connection is made via the earth ground
prong of the M128A's power cord plug. This plug
shall only be inserted into asocket outlet provid-
ed with the required earth ground contact. The
protective action must not be negated by the use
of an extension cord without aprotective conduc-
tor, or by use of an "adapter" that doesn't main-
tain earth ground continuity, or by any other
means.
The power cord plug provided is of the type illus-
trated in Figure 1. If this plug is not compatible
with the available power SOCkets, the plug or
power cord should be replaced with an approved
type of compatible design.
WARNING!
IF IT IS NECESSARY TO REPLACE THE
POWER CORD OR THE POWER CORD
PLUG, THE REPLACEM ENT CORD OR PLUG
MUST HAVE THE SAME POLARITY AS THE
ORIGINAL. OTHERWISE A SAFETY HAZARD
FROM ELECTRICAL SHOCK, WHICH
COULD RESULT IN PERSONAL INJURY OR
DEATH, MIGHT RESULT.
L= LINE OR ACTIVE CONDUCTOR (ALSO CALLEU "LlV£" OR "HOT")
N= NEUTRAL OR IDENTIFIED CONDUCTOH
E=EARTH OR SAFETY GROUND
Figure 1. POWER CORD PLUG WITH POl.ARITY INDICATIONS
D. POWER VOLTAGE SELECTION AND
LINE FUSES
Before plugging in the power cord, make sure that
the equipment is set to the voltage of the ac power
supply.
CAUTION!
THE APPARATUS DESCRIBED IN THIS
MANUAL MAY BE DAMAGED IF IT IS SET
FOR OPERATION FROM 110 V AC AND
TURNED ON WITH 220 V AC APPLIED TO
THE POWER INPUT CONNECTOR.
Adetailed discussion of how to check and.tt nec-
essary, change the power-voltage setting follows.
The line voltage is selected by means of a rear-
panel switch. FOR SAFETY, UNPLUG THE
POWER CORD WHEN CHECKING THE LINE
VOLTAGE SETTING OR WHEN CHECKING THE
FUSES. FUSES SHOULD ONLY BE CHANGED BY
QUALIFIED SERVICE PERSONNEL WHO ARE
AWARE OF THE HAZARDS INVOLVED. Depend-
ing on the switch position, either "115" or "230"
(both are printed on the swltchj-wl!l be visible to
the viewer. For operation from a line voltage from
100 V ac to 130 V ac, 50-60 Hz, "115" should show.

SECTION I
CONDENSED OPERATING INSTRUCTIONS
The following condensed operating instructions are pro-
vided as an assistance in placing
the Model 128A Lock-In
Amplifier into operation as quickly as possible. Generally
speaking, these condensed instructions will allow good re-
sults to be obtained in most instances. However, because of
the brevity of these instructions, many considerations hav-
ing
importance in particular applications have been fore-
STEP ONE
PRELIMINARY STEPS; Check that the
rear-panel 115/230 V switch is in the proper
position. Remove the top cover and verify
that the two Reference Lock-On Speed
switches are properly set. For operation
below 5 Hz, they should be set to SLOW.
For operations above 5 Hz, they should be
set to FAST. (Units are shipped with these
switches set to FAST.) Replace the cover
and plug in the line cord. Turn the power
on.
STEP TWO
REFERENCE CHANNEL: Check that the
Reference Mode switch is set to the proper
position. With the switch set to "f", the
detector is driven at the frequency of the
applied reference signal. Set to "2f", the
detector is driven at twice the frequency of
the applied reference to facilitate second
harmonic measurements. Next connect the
reference signal (100 m V pk -pk or greater)
to the Reference Input connector and wait
for the REF. UNLOCK light to go out
before proceeding. Usually this will be but a
few seconds. However, at low frequencies, a
longer time is required.
STEP THREE
SIGNAL CHANNE L: Set the Input Selector
to "A" (single-ended), "A-B" (differential)
or "-B" (single-ended), whichever is
appropraite. Set the Sensitivity switch to
"250 mV". If signal channel filtering is
desired, set the HI-PASS and LO-PASS
Filter switches as required. Connect the
signal source to the Model 128A Input.
1-1
gone. For this reason, it is advisable to read Section IV, the
complete Operating Instructions, to be assured of achieving
optimum performance. NOTE: As written, these condensed
instructions do not apply to units having either the Internal
Oscillator or Tuned Amplifier modifications. If the unit in
question has either or both of these modifications, the
operator is referred to Subsections 4.7 and 4.8.
STEP FOUR
OUTPUT CONTROLS; Set the Zero Offset
toggle switch to the center (OFF) position.
Then set the DC Prefilter to 100 ms and the
Time Constant switch to 0.3 SEC.
STEP FIVE
FINAL ADJUSTMENTS: Rotate the
Sensitivity switch counterclockwise until
the panel meter begins to deflect. Adjust the
Phase Quadrant switch and Phase dial for
maximum meter indication. When the
Sensitivity and Phase adjustments are
optimally set, the meter should indicate the
maximum possible on-scale reading. If the
Overload light comes on, increase the
prefilter and time constant settings. Also try
"bracketing" the signal frequency with the
HI PASS and LO PASS filters. If these
techniques don't help, rotate the Sensitivity
switch as far clockwise as is required to
eliminate the overload. Set the time
constant as required to reduce the output
noise to an acceptable level.
STEP SIX
READING: The input signal level is read
from the panel meter. If the input signal is a
sinewave, the meter directly indicates the
rms amplitude of the input signal. However,
if the signal frequency is near enough to a
selected HI or LO PASS filter frequency to
be influenced by the filter, the filter
attenuation effects must be taken into
account. If the input signal is not a
sinewave, then the harmonic response of the
instrument must be taken into account as
well.

1-2
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SECTION II
CHARACTERISTICS
2.1 INTRODUCTION
The Model 128A Lock-In Amplifier enables the accurate
measurement of signals contaminated by broad-band noise,
power line pickup, frequency drift, or other sources of
interference. It does this by means of an extremely narrow
band detector which has the center of its passband locked
to the frequency of the signal to be measured. Because of
the frequency lock and narrow bandwidth, large
improvements in signal-to-noise ratio can be achieved,
allowing
the signal of interest to be accurately measured,
even in situations where it is completely masked by noise.
Signals applied
to the input (single-ended or differential)
1 routed through aseries of amplifiers which allow
..rll-scale sensitivity ranges down to one microvolt. Switch
selectable low-pass and high-pass filters allow considerable
noise reduction ahead of the phase-sensitive detector. This
pre-detector noise reduction can be further enhanced by
making use of the optional plug-in (internal) selective
amplifier. At
the phase sensitive detector, the signal is
compared with the reference signal derived from the
experiment. Only those signal components which are
synchronous with the reference yield a net dc detector
output. Noise and other non-synchronous signals do not
contribute a net dc output, but only ac fluctuations which
can be reduced to any arbitrary value according to the
amount of filtering selected with the Time Constant switch.
This switch allows time constants as large as 100 seconds to
be selected, with provision for achieving larger externally
determined time constants if necessary. Post-detector dc
amplifiers drive
the panel meter and signal output
connectors. Other features include provision for calibrated
zero suppression of up to 10 x full scale, a two-position dc
prefilter, and the capability of driving the reference input
the detector at double the frequency of the signal
applied to
the Reference Input connector to facilitate
second harmonic measurements. An optional plug-in
oscillator
(internal) is available for use in applications where
the experiment does not produce a reference signal itself,
but is capable of being driven by a signal furnished by the
Lock-In Amplifier.
With its wide range of capabilities
and ease of operation,
the Model 128A Lock-In Amplifier should find extensive
application in situations where the accurate measurement
of signals is complicated by the presence of noise and
interference.
2.2 SPECIFICATIONS
SIGNAL CHANNEL
(1) INPUT TYPE
Single-ended or differential as selected by front-panel
switch.
11-'
(2) INPUT IMPEDANCE
100 Mr2 shunted by no more than 20 pF.
(3) SENSITIVITY
12 full-scale ranges in 1-2.5-10 sequence from 1 J.1V to
250 mV.
(4) FREClUENCY RANGE
0.5 Hz to 100 kHz.
(5) COMMON MODE
REJECTION
At least 100 dB at 1kHz.
(6) MAXIMUM COMMON MODE
VOLTAGE
3 V pk-pk to 20 kHz; then --6 dB/octave above 20
kHz.
(7)
DETECTOR BIAS
Internal
network allows dc bias current of either
polarity to be provided at the "A" Input to facilitate
operation with diode detectors which require biasing.
(See page VII-3
and Parts Location Diagram on page
VII-2.)
(8) NOISE
At 1 kHz
the signal channel noise will not exceed 10
nV/H
zY2.
(9) LOW PASS FILTER
Switch selectable 6 dB/octave low-pass filter which
can be set to 3 dB down frequencies of 100 Hz, 10
kHz, or MAX (greater than 100 kHz).
(10) HIGH PASS FILTER
Switch selectable 6 dB/octave high-pass filter which
can be set to 3 dB down frequencies of 50 Hz, 5 Hz,
or MIN
(below 0.5 Hz).
(11) OVERLOAD DETECT
Front-panel indicator lights if applied signal plus noise
is large
enough to cause overload at any of several
critical
overload monitor points.
(12) GAIN STABILITY

(13) GAIN LINEARITY
0.05%.
(14)
OVERALL GAIN ACCURACY
±2%.
REFERENCE CHANNEL
The Model 128A reference channel automatically locks
onto and tracks an applied reference signal over the entire
operating frequency range of the instrument. As a result,
the instrument is immune to frequency and phase shifts as
long as
the reference and signal to be recovered change
together.
(1) TRACKING RANGE
5 Hz to 100 kHz (FAST) or 0.5 Hz to 100 kHz
(SLOW) as
determined by the setting of two internal
switches.
Faster lock-on time and slewing rate
obtained with switches set to FAST make this range
preferable
except when operating below 5 Hz.
(2) MODES
Either of two modes, f and 2f, can be selected by
means of a front-panel switch. In
the "f" position, the
phase-sensitive detector is driven at the same
frequency as the applied reference signal. In the "2f"
position, the phase-sensitive detector is driven at twice
the
frequency of the applied reference signal to
facilitate second harmonic measurements.
(3) INPUT IMPEDANCE
10
Mrl shunted by no more than 20 pF.
(4) MINIMUM REFERENCE SIGNAL REQUIREMENT
100 mV pk-pk, any waveshape crossing its mean only
twice each cycle. Minimum time required on either
side of the mean is 100 ns. Amplitude excursions must
be at least 50 mV on each side of the mean. Maximum
input signal is 5 V (pk-to-rnean}. Best phase accuracy
is obtained with a 1 V rms sinewave.
(5) LOCK-ON TIME
A
function of internal switch setting as follows.
Selected Range Lock-On Time
SLOW (0.5 Hz to 100 kHz) 20 sec. per octave
FAST (5 Hz to 100 kHz) 2 sec. per octave
(6) PHASE
Calibrated Phase
controls allow the phase of the
reference drive to the Phase-Sensitive Detector to be
set at any angle relative to
the input signal. The
controls consist of a Phase Dial with arange of 100°
and a Phase Quadrant switch which provides
incremental phase shifts of
90°. The phase shift
11-2
accuracy of the dial is better than 0.2
0over the
entire
frequency range. The resolution of the dial is better
than 0.10. The incremental phase shiffs provided by
the
Quadrant switch are accurate to 0.2°. The overall
phase accuracy of
the instrument, including shifts in
both the reference and signal channels, is typically
better than 50.
(7) DETECTOR BIAS
Internal
network allows de bias current of either
polarity to be provided at the REF. IN connector to
facilitate
operation in situations where the reference
signal is
taken from diodes requiring biasing. (See page
VII-6 and Parts Location Diagram
on page VII-5.)
PHASE
SENSITIVE DETECTOR, DC AMPLIFIER
(1) OUTPUT DRIFT
(2) OVERLOAD CAPABILITY
1000 times full scale up to a maximum at the input of
650 mV rms. Overload capability is defined as the
ratio, at the input of the Model 128A, of the
maximum pk-pk non-coherent signal which can be
applied
without overloading the Model 128A to the
pk-pk coherent signal required to yield full scale
Model
128A output. Note that, expressed as the ratio
of the pk-pk non-coherent signal to the rms value of
the coherent signal required for full-scale output, this
number can be as great as 2800. Maximum acceptable
signal is a 650 mV rms sinewave.
(3)
NON-COHERENT REJECTION
50 ppm maximum. Non-coherent rejection is defined
as that offset which results from applying a
non-coherent signal having a pk-pk amplitude 1000
times the pk-pk amplitude of the coherent signal
required to obtain full-scale output.
Example: With a non-coherent signal applied having a
pk-pk
amplitude 1000 times the pk-pk coherent signal
required
to obtain full-scale output, there will occur
an offset at the output caused by the non-coherent
input signal. The amplitude of this offset will be no
greater
than:
50 X 10-
6X
1000
=50 X 10-
3of f.s.
output
=50 mV (f.s. =1 V)
(4) TIME
CONSTANT
Front-panel switch allows selection of 6 dB/octave
filter time constants of 1 ms, 10 ms, and .1 s through
100 s in 1-3-10 sequence. Also MIN (time constant
:::=0.7 ms) and EXT, which allows time constants
longer than 100 s to be achieved by means of external

capacitors. A separate front-panel toggle switch allows
another 6 dB/octave filter to be inserted, if desired.
This filter has a
time constant of either 100 ms or 1 s,
whichever is selected.
(5)
ZERO OFFSET
A calibrated, ten-turn, Zero Offset dial, with up to ten
times full-scale capability is provided.
(6) FULL-SCALE
OUTPUT
±1 V.
(7)
OUTPUTS
(a) Panel meter, ± full scale.
(b)
Front-panel BNC connector. One volt out
corresponds to full-scale panel meter deflection.
Output resistance is 600 ohms.
(c) Rear-panel Recorder Out binding posts, spaced to
accept standard double-banana connector.
Output resistance is 600 ohms.
GENERAL
(1) AMBIENT OPERATING TEMPERATURE RANGE
(2) AUXILIARY POWER OUTPUT
±15.5 Vregulated dc at 20 mA is provided at
rear-panel
connector.
(3) POWER REQUIREMENTS
100-130 Vac or 200-260 Vac, 50-60 Hz. May also be
powered from ±24 Vde source (such as batteries).
Power consumption: 15 watts.
(4) SIZE
17-3/4" W x 3-1/2" Hx 14" D (45 cm W x 9 cm H x
36 cm D).
11-3
(5) WEIGHT
14 Ibs (6.4 kg).
(6)
MODIFICATIONS
(a) Model 128A/97 Monitor Modification
Three rear-panel BNC connectors are installed
which permit monitoring of: (1) Output of Signal
Channel
before demodulation (please note that
the Signal Channel monitor is provided as part of
the Tuned Amplifier modification as well), (2)
Squarewave output of Reference Channel, and
(3) Full-wave demodulated Mixer output before
Time Constant filter.
(b) Model
128A/98 Tuned Amplifier Modification
Internal plug-in
board is available which provides
a tuned bandpass or notch characteristic at a
Q-of-5.
The frequency can be adjusted over a 3: 1
range by
means of arear-panel adjustment, and
can be set to any frequency from 1 Hz to 100
kHz by changing capacitors mounted on
component clips located on the plug-in circuit
board.
(c) Model 128A/99 I nternal Oscillator Modification
An internal low distortion oscillator is available
which provides asinewave output adjustable
from 0-to-l0 V pk-pk at 600 ohms. The
frequency is adjustable over about a 3: 1 range by
means of a rear-panel
adjustment, with the actual
frequency range spanned by the adjustment being
determined by a pair of internal capacitors
mounted on the oscillator circuit board.
Operation from about 1 Hz to 100 kHz is
possible. This
option, in conjunction with the 2f
mode of operation, is particularly useful for
harmonic detection where the modulation
frequency must be at one half the detected
frequency.

SECTION III
INITIAL CHECKS
3.1 INTRODUCTION
The following procedure is provided to facilitate initial
performance checking of the Model 128A. In general, this
procedure should be performed after inspecting the
instrument for shipping damage (any noted to be reported
to the carrier and to Princeton Applied Research
Corporation), but before using the instrument for
experimental
measurements. Should any difficulty be
encountered in carrying out these checks, contact the
factory or one of its representatives. It might be noted that
it is not the purpose of these checks to demonstrate that
the instrument meets all specifications, but rather simply to
show that it is functioning normally. If normal indications
e obtained for the functions checked, one may reasonably
assume that those functions which are not checked are
working properly as well.
3.2 EQUIPMENT NEEDED
(1) Sinewave Oscillator to provide a 100 mV rms sinewave
at 1 kHz.
NOTE: If the instrument to be checked is
equipped with the Tuned Amplifier modification, then
the oscillator will have to provide a 100 mV rms
sinewave at
the tuned frequency. If the instrument in
question is equipped with the Internal Oscillator
modification, no external oscillator will be required.
Th is applies whether or not the unit is equipped with
the Tuned Amplifier modification.
(2) General purpose oscilloscope. This item is required
only in the case of units having the Tuned Amplifier
modification.
'3) Suitable cables for interconnecting the above instru-
ments.
3.3 PROCEDURE
(1) Check the position of the rear-panel 115/230 switch.
Be sure
the number showing in the window corre-
sponds to the line voltage to be used.
(2) With the Power switch
set to OFF, plug in the line
cord.
(3)
Set the Model 128A controls as follows.
Input Selector: A
Sensitivity:
100 mV
Filters
HI-PASS: MIN.
LO-PASS: MAX.
Phase
Quadrant switch: 270°
Phase dial: 90°
Mode: f
Zero
Offset
switch: OFF (center position)
111·1
dial: 1.00 (one turn from fully counterclockwise
position)
Time Constant: .3 SEC.
DC Prefilter:
OUT
Reference Tracking-Rate switches (two internal
switches); FAST (unless unit is equipped with
Tuned Amplifier' or Internal Oscillator set for
frequency below 5 Hz, in which case switches
should be set to SLOW). NOTE: Instruments are
normally shipped with these switches set to
FAST.
Power. ON
(4) Applies
only to units not equipped with the Internal
Oscillator modification. If the Model 128A has this
modification, go to step 5.
Connect the output of the external oscillator (set for
100 mV rms sinewave out at 1 kHz) to both the "A"
and Reference Inputs. If the unit in question has the
Tuned Amplifier modification, set the oscillator to the
tuned frequency specified when the unit was ordered.
(5) Applies only to units having the Internal Oscillator
modification.
Connect a cable from the rear-panel REF. OSC. OUT
connector to the front-panel "A" Input. There is no
need to connect th is signal to the Reference Input
connector (labeled MONITOR in units equipped with
the Oscillator modification). The connection to the
Reference Channel of the Model 128A is made
internally at the factory. The rear-panel REF. AM-
PLITUDE adjustment should be set so that the
amplitude of the signal at the REF. OSC. OUT
connector is 100 mV rms. An accurate ac voltmeter
may be useful for setting this level (units leave the
factory set for a nominal 100 mV rrns out).
(6) Applies only to units having the Tuned Amplifier
modification. In the case of units not having this
modification, go d irectl y to step 7.
(a)
Connect the oscilloscope to the rear-panel SIG.
MON.
connector.
(b) Vary the frequency of the signal applied to the
"A" Input (use the rear-panel REF. OSC. FREQ.
ADJ. control in units equipped with an Internal
Oscillator)
for peak signal amplitude as observed
with the oscilloscope.
(7) Set the Phase Quadrant switch to 0°. Then adjust the
Phase dial for "0" panel meter indication.
(8) Set the Phase Quadrant switch back to 270°. The
panel meter should indicate full scale to the right ±a
few
percent of full scale. The accuracy of th is readinq

will depend on the amplitude accuracy of the signal
applied
to the "A" Input.
(9) Adjust the amplitude of the signal applied to the "A"
Input as required to obtain exactly full-scale panel
meter indication.
(10)
Set the Phase Quadrant switch to 180°. The panel
meter should indicate "0" ±5% of full scale.
(11)
Set the Phase Quadrant switch to 90°. The panel
meter should indicate negative full scale ±5% of full
scale.
(12)
Set the Phase Quadrant switch to 270° to restore the
111-2
positive full-scale panel meter indication.
(13)
Set the Zero Offset toggle switch to "+". The panel
meter indication should go to "0" ±5% of full scale.
(14) Begin
rotating the Offset dial counterclockwise. The
panel meter indication should increase linearly, track-
ing
the dial setting. When the dial is fully counter-
clockwise, the panel meter should indicate full scale
±5%. Reset
the Zero Offset toggle switch to the center
(OFF) position.
This completes the initial checks. If the indicated results
were
obtained, one can be reasonably sure that the Model
128A is functioning normally.

SECTION IV
OPERATING INSTRUCTIONS
4.1 INTRODUCTION
Even though operation of the Model 128A is straight-
forward, there are a number of factors of which the
operator should be aware to be assured of achieving
optimum performance in all situations. This section of the
manual treats these considerations in some detail. Topics
covered include grounding, noise performance, harmonic
sensitivity, operation in conjunction with the plug-in
accessories,
and others. For an overall "quick look" at how
the instrument is operated, the operator is referred to
Section I, the condensed operating instructions.
4.2 PRELIMINARY CONSIDERATIONS
4.2A POWER REQUIREMENTS
The Model 128A requires 100-130 V ac or 200-260 V ac,
50-60 Hz. The power consumption is 15 watts. The unit
may also be powered by batteries by applying ±24 V to the
appropriate pins of the rear-panel octal- connector (see
BATTERY OPERATION, page IV-l0). Arear-panel slide
switch
determines whether the ac power circuits are
connected for operation from 100-130 V or from
200·260 V. For operation from 100-130 V, "115" should
show in the window. For operation from 200-260 V, "230"
should show.
4.28 FUSING
The Model 128A is protected by a single fuse mounted on
its rear panel. A
slow-blow 1/4 A fuse is used for operation
from 115 V. A slow-blow 1/10 A fuse is used for operation
from 230 V. It occasionally happens that a slow-blow fuse
fails in
shipment as a result of shock and vibration. Hence,
if
the fuse is found to be bad when the instrument is
operated for the first time it is advisable to try and change
the fuse. If normal operation follows, chances are there are
no
other problems. However, if the replacement fuse fails,
there is something wrong which will have to be corrected
before proceeding.
4.2C WARM-UP PERIOD
For most applications, five minutes. Where it is desired to
achieve
the best possible gain and output stability, allow an
hour.
4.20 OPERATING FREQUENCY
Although one can, in principle, make equally accurate
measurements at any frequency within the operating range
of
the instrument, operation is simplest and least subject to
error over arange having as its lower limit, perhaps a few
hundred Hz, and as its upper, perhaps 10k Hz. At very low
frequencies,
phase offsets occur which could matter if one
is interested in the absolute phase of the input signal.
Another problem of low frequency operation is that of l/f
noise, including both that which develops in the Model
128A and that which originates in the experiment itself to
degrade the signal-to-noise ratio ahead of the lock-in
amplifier. Increased
response and settling time could be a
IV-1
significant problem if one were operating in conjunction
with the optional plug-in tuned amplifier. At high fre-
quencies, radiation and associated pick-up tend to be
bothersome. Another high frequency problem is that of
signal attenuation as a result of the input cable capacitance.
Th is is especially a problem when working from a high
source resistance. Other frequencies to avoid are 60 Hz and
its lower order harmonics. By avoiding these frequencies,
the operator assures that he will be measuring the signal of
interest only, uninfluenced by power frequency pick-up,
either internal or external.
4.2E GROUNDING
In any system processing low-level signals, proper grounding
to minimize the effects of ground loop currents, usually at
the power frequency, is an important consideration. In the
case of the Model 128A, special design techniques have
been
employed to give a high degree of ground-loop signal
rejection in single-ended applications. Even so, it will often
prove advisable to operate differentiallv. even when ex-
amining asingle-ended signal source, 10 achieve the greatest
possible rejection. Figures IV-l and IV-2 illustrate this
point. Note from Figure IV-l that the signal source is
located inside a grounded enclosure (shield), to which signal
source common is attached at one point. The braid of the
/
'U ,
INDUCED emf
(DISTRIBUTED AROUND
LOOP)
RESISTANCE OF AC GROUND
PATH--J
Figure IV·l. GROUND·LOOP SUPPRESSION BY
TEN-OHM INPUT GROUND
SHIELD
l_ 10"
L
INDUCE~.mf '"
(CJSTRIBUTED AROUND LOOP) J
RESISTANCE OF AC GROUND PATH
Figure IV-2. DIFFERENTIAL MEASUREMENT OF
"SINGLE-ENDED" SIGNAL

Figure IV-4. EXAMPLE OF EVERYTHING "DONE WRONG"
The reduction of power frequency interference is not the
only benefit to be derived from proper grounding and
differential operation. A much more serious source of
interference is coherent interference at the signal frequency
which results when drive signal current is allowed to flow
through the braid of the signal cable. Figures IV-4 and IV-5
are
provided to illustrate this problem and the steps which
can be taken to prevent it. To begin with, Figure IVA
shows the experiment with just about everything possible
"done wrong". The lock-in amplifier is operated single-
ended. The ground connections at the experiment are made
to the enclosure, allowing currents to flow through it, and,
in particular, the drive signal currents have the opportunity
to flow th rough the braid of the signal cable. The drive
signal, in
addition to providing the reference input signal to
the Model l28A, can be presumed to be driving other
components of the system as well. Depending on the nature
of the experiment, these currents could range from very
small to
quite large, perhaps even amperes if the experiment
involves driving a low impedance coil. Note that the various
loads for the drive are represented by a single resistor
returned to ground somewhere on the enclosure. Most-of
this drive signal current can be presumed to flow through
the shield back to the drive signal source. However, asmall
but significant part of it will flow through the parallel path
consisting of the braid of the signal cable, the ten ohm
resistor, and the braid of the reference signal cable. The
voltage drop of this current across the resistance of the
signal cable brai-J, even though attenuated by the ratio of
the ten ohm resistor to the braid resistance, can constitute a
serious source of interference at low signal levels, particular-
ly in that this interference is coherent, in phase, and
directly adds to the signal of interest. It is not hard to
envision situations where this interference signal could well
be larger at the Input of the lock-in amplifier than the signal
of
interest itself.
signal
cable is grounded directly to signal source common as
well,
thereby assuring that no signal currents or ground-
loop currents will flow through the shield, 'a desirable
condition for the best possible shielding. The Model l28A
is operated single-ended, using the "A" input. Note that the
"low" side of the amplifier input is not grounded to the
chassis directly but by way of a ten ohm resistor. Further
note that the braid of the signal cable is returned to this
resistor, and not to the chassis. A ground loop generator is
indicated as being connected between the chassis of the
Model l28A and siqnal source common. This path would
ordinarily consist of the ac ground "third wire", paralleled
by the braids of other cables connecting the system
components. The ground loop generator will cause currents
at the power frequency to flow through the braid of the
signal cable, through the ten ohm resistor, and back
through the ac wound path to complete the loop. Because
of
the ten ohm ground employed in the Model l28A, these
currents are attenuated over what they would be if the
Model 128A input were returned directly to the chassis.
More
importantly, most of the ground loop signal is
dropped across the ten ohm resistor and little across the
braid of the signal cable, the ratio being the ten ohms of the
resistor to the 10 to 20 milliohms (typical) of the braid
resistance, As far as the input of the Model l28A is
concerned, the ground loop signal is reduced by this ratio,
and the ground loop interference is thus perhaps a factor of
five hundred or one thousand less than would be the case
without the ten ohm qr ound.
However, in some appl ications, th is would not be enough.
Figure IV-2 shows how this same signal could be measured
operating the Model 128A differentially. In this instance,
the Model l28A Input Selector is set to "A-B" and two
input cables are used, one connected to the signal source
and the other to siqnal source common. At the source end,
the braid of both cables is returned to signal source
common. At the lock-in amplifier end, the ten ohm ground
serves to attenuate the wound loop currents and maintain a
small
ground loop signa! drop across the braids the same as
in Figure
IV-l. However, in the first instance, the amplifier
"looked" at the potential difference between the center
conductor of the cable and the braid. In the second, it sees
the potential difference between the "A" Input and "8"
Input. The ground loop siqnal current flowing in the signal
cable braid is of no consequence. The very high common
mode rejection of the amplifier assures that common mode
power frequency pickup will not be a problem either.
However, when operating differentially, it is important to
take a little trouble to assure that common mode inter-
ference arising in ground loops is just that, that is, without
a significant differential component. This should not prove
a problem as lonq as both signal cables follow the same
path,
Figure IV-3 shows the Model 128A operated differentially
to measure an "off ~JI ound " siqn al. The most important
consideration in an application of this type is to be sure
that the common mode siqnal component is not so large as
to
exceed the common mode input limit of the Model
128A. (See Specs.)
IV-2
DRIVE
~ ~:£"
0'"'''''' ,.0'" -1
AS FUNCTION OF EXPERIMENTAL
PARAMETEI L..- ...I
Figure IV-3. DIFFERENTIAL MEASUREMENT OF
"0 FF-GROUND" SIGNAL

DRIVE
LOAD
k= Boltzmann's constant = 1.38 x 10-
2 3 joules/kelvin
T =
absolute temperature in kelvins
Rs=Resistance in
ohms of the resistive component of
the impedance across which the voltage iii mea-
sured
B = Bandwidth over which the measurement is made
Mathematically expressed, noise figure can be stated as:
NF(dB) =
2010
9 10 Noise V.olta~_~~g~!':l_~t of Amplifier
That Portion of Numerator Attributable IV-2
to Source Thermal Noise
Figure IV-6.
TYPICAL MODEL 128A NOISE FIGURE CONTOURS
~~
where Etis the total noise referred to the input in volts rrns
and all other terms are as defined previously. Note thar
IV-3
10~
Et =V4kTBR X 10NF/20
o
11>
N
~
Ul
2
:r
o
~
""
o
z
c(
Ii;
~
0:
"" U
~
~ 102::-:~ __ ~~---9:::'-'-_--.-l.- __ .L, __ --.J
0.5 10
CENTER fR
Noise figure is not constant but varies as a function of the
source resistance, frequency, and temperature. When the
loci of all points having the same noise figure are plotted as
a
function of frequency and source resistance (temperature
fixed), the result is a noise figure contour. A full set of
contours completely specifies the noise characteristics cf
the antpti#er ruler its working range. Figure IV-6 contains a
full
set of contours for a typical Model 128A. The utility of
these contours are, first of all, that they clearly indicate the
best noise performance region in terms of operating
frequency and source resistance, and secondly, that they
allow one to directly compute the total noise accompany-
ing the signal (amplifier noise and source thermal noise
considered, other noise sources neglected). .The relating
formula is:
IV-l
4.2F NOISE
Any electronic signal processing system adds noise to that
already accompanying the signal to be measured, and a
Lock-In
Amplifier is no exception. Even though the
method of signal processing used in a Lock-In Amplifier
allows very large improvements in signal-to-noise ratio to be
achieved,
the amount of noise contributed by the Lock-In
Amplifier itself
affects its performance and limits the
achievable improvement.
Figure IV-5 shows the steps which can be taken to
circumvent this problem. All of the drive signal current is
returned directly to the drive signal source except for the
very small component (reference input resistance of Model
128A is 10 MS1) which is applied to the Model 128A by
way of
the Reference Input. Second, no current, whether
drive current, reference current, or signal current, is allowed
to flow through the experiment shield; the shield contacts
ground at one point only. The only coherent signal which
can flow through the parallel path of the signal cable braid
is a small
portion of that allowed by the ten megohm
Reference Input resistance. Furthermore, the use of differ-
ential operati on assures that even th is small amou nt can
have no effect. By using the arrangements indicated, one
could operate with very large drive currents without
concern that they might contaminate the signal of interest.
If electrostatic coupling of the drive signal to the detector is
a
problem, mounting a conducting material around the
signal source detector should prove helpful. The electro-
static shield should be connected to the system at but one
point, signal source common.
Figure IV-5. ERRORS DEPICTED IN FIGURE IV-4 CORRECTED
One convenient way of specifying the noise performance of
an amplifier is to speak of its noise figure, which indicates
the amount of noise the amplifier adds to the source
thermal noise. Source thermal noise is used as the basis for
comparison because it is completely predictable, always
present, and is the least amount of noise which can possibly
accompany any signal. Its value, in volts rms, is given by the
following formula.
where:
En =rms noise voltage within the bandwidth of the
measurement
IV·3

with anoise figure of 3 dB, the amount of noise
contributed by the amplifier is 1.4 times the source thermal
noise. At 1.4 times the thermal noise, the amplifier noise
just begins to be noticeable. At lower noise figures, the
amplifier for all practical purposes may be regarded as
noiseless.
Generally speaking, if one can operate anywhere
inside the 3 dB contour, amplifier noise considerations may
be neglected.
As critical as amplifier noise is in certain applications, it is
nevertheless possible to overemphasize its general impor-
tance. For example, if the signal amplitude is significantly
higher than the amplifier noise, the subject becomes purely
academic. Similarly, if preamplification is provided ahead
of the Model 128A, with the result that the amplified
source noise at the input to the Model 128A is far greater
than the amplifier noise, there is little point in striving to
operate inside the 3 dB contour of the Model 128A.
However, where a preamplifier is used, it is important that
these same considerations be carefully evaluated for the
preamplifier. In other words, when using a preamplifier, try
to operate inside the 3 dB contour of the preamplifier. A
quick check of Fiqure iV-G followed by a computation of
the total noise (Equation IV-3) should give one arealistic
idea of the
importance of amplifier noise considerations to
the measurement at hand.
Where amplifier noise is a consideration, one should try to
operate inside the 3 dB contour by appropriately adjusting
the operating frequency or source resistance. The choice of
operating frequency is usually determined by the type of
sensor used and by the capabilities of the chopper, where
one is used. Often, the ex perimenter has some control over
his choice of frequency and so can adjust things so that he
is
operating at the low noise end of the frequency range
available to
him.
The situation with regard to source resistance may be less
flexible
because the source resistance is usually determined
solely by the type of sensor used. Once the commitment to
a particular sensor has been made, it can be difficult to
adapt the system to another one. Hence, in applications
where noise is a uotcntial problem, the choice of sensor
should be made carefully and with full regard for the noise
characteristics of the amnlifier.
Two situations deserve special attention, the first being
operation from a source resistance very much lower than
optimum, and the second being operation from a source
resistance very much higher than optimum. In either case,
there may be a temptation to "improve" the source
resistance situation by the use of resistors. In the case of
the low source resistance, one may be tempted to connect a
resistor in series
with the source. In the case of the high
source resistance, one may be tempted to connect a resistor
in parallel with the source. Unfortunately, neither approach
to the problem does any good and, in fact, both will result
in further degradation of the signal-to-noise ratio ahead of
the lock-in amplifier. The series resistor adds its thermal
noise to that already accompanying the signal. Although
the amplifier shows a "better" noise figure than before, it is
only
because the ampl ifier noise is now less relative to the
thermal noise of the combined resistances (source plus
IV-4
series resistor). The signal is no larger (in fact, it may well
be
attenuated), the noise is greater, and the improved noise
performance is illusory. Recall that noise figure only relates
amplifier noise to thermal noise, and does not denote the
absolute value of amplifier noise.
Connecting aparallel resistor to lower a high source
resistance has a similar effect. Even though the thermal
noise does go down, the signal amplitude goes down even
more. For example, if a source of resistance R were
paralleled by another resistor of the same value, the signal
ampl
itude would go down by a factor of two. However, the
thermal noise would only be reduced to .707 of its initial
value
(thermal noise varies directly with the square root of
the resistance), with a net degradation in signal-to-noise
ratio.
In operating from low source resistances, however, one can
usually
improve the situation dramatically by using a
transformer to raise the source resistance seen by the
amplifier. The improvement one obtains with a transformer
is real because the amplitude of both the signal and the
noise is increased by the turns ratio. The source resistance is
increased by
the square of the turns ratio. For example, if
one had a ten ohm source, one could use a 1: 100 step-up
transformer, in which case the amplifier would see a source
resistance of 100 kn. At 100 kS1 the amplifier adds little
additional noise. Even though the thermal noise of the
transformer adds to th at of the source, a very considerable
improvement is usually achieved. P.A.R.C. manufactures a
line of
suitable signal transformers, each designed for
optimum operation over agiven frequency range. Perfor-
mance information can be obtained from the factory or one
of its representatives.
In using an external transformer in conjunction with 'the
Model 128A, single-ended operation of the lock-in amplifier
is advised. The extremely high inherent common mode
rejection of the transformer makes differential operation of
the lock-in amplifier unnecessary.
When working from a high source resistance, one could, in
principle, use a transformer in the same manner to improve
noise performance. Unfortunately, practical transformer
design considerations usually prevent one from doing so. As
a
result, the options available to an experimenter working
with a high source resistance device, such as a photo-
multiplier, are limited. Practically speaking, the best one
can do is to make the load resistor as large as possible. The
larger the source resistor, the less the shunting effect it will
have,
and the better the signal-to-noise ratio at the input of
the amplifier will be. That this is so becomes clear when
one recalls that the signal amplitude varies directly with the
load resistance, while the thermal noise varies with the
square root of the resistance.
Note that the entire preceding discussion of noise is based
on
comparing the noise generated by the amplifier with the
source thermal noise. In many situations, other types of
noise of interference may accompany the signal as well and
could even dominate it. Where this is the case, the amplifier
can only perform "better" than the noise figure contours
indicate because the noise figures are based on acornpari-

son of amplifier noise with the minimum possible noise
which can
accompany any signal, namely, the source
thermal noise.
4.3 OPE;RATING THE MODEL 128A
4.3A INliRODUCTION
Operation/of the Model 128A is straightforward. In most
instances, the operator simply connects the reference signal,
waits
for the REF UNLOCK light to go out, and then
connects the signal to be measured. The Sensitivity and
Phase controls are then adjusted for maximum output
without overload. Should overload occur, the dc filtering
(Prefilter and Time Constant) is increased and/or the
sensitivity is reduced as required to eliminate the overload.
The readinq can then be taken .
•n many Situations, achieving asuccessful measurement will
depend not so much on critically adjusting the Model 128A
controls as on taking the proper steps indicated by the
preliminary considerations discussed in Subsection 4.2.
Factors such as proper grounding and operating inside the 3
dB contour are most important in making low level
measurements of noisy signals.
4.38 REFERENCE CHANNEL (see Subsection 4.9
for additional information)
Referende Signal Requirements
An outsltanding feature of the Model 128A is its unique
reference channel circuitry which allows it to lock onto and
track a wide range of possible reference input waveforms.
Once locked on, the reference remains locked on, even if
the
reference input signal changes in frequency. There are
no
Reference Channel controls of any kind which must be
adjusted for proper reference channel operation. Once the
light goes out, all that remains is to adjust the Phase
controls so that the Reference signal applied to the mixer is
-t the proper phase relative to the signal to be measu red. As
stated in the' specifications, the only requirements on the
reference signal are that it swing at least plus and minus 50
mV
with respect to its mean, that it cross its mean twice
each cycle, and that it remain on each side of the mean for
at least
100 ns. Sinewaves, square waves, triangle waves, and
many others are all suitable. However, for best phase
accuracy, a 1 V rms sinewave is recommended. One
waveshape
which at first glance may seem to suffice, but
which does not, is the very narrow low duty factor pulse.
For example, suppose one intended to use as a reference
signal the pulse train depicted in Figure IV-7, that is, pulses
having an
amplitude of 1 V, a duration of 1 JiS, and a period
of 1 ms. The mean of this signal would be about 1 mV, and
the excursions relative to the mean would be +999 mV and
-1 mV. Thus, the individual pulses, even though they
exceed the minimum excursion requirement on one side by
some 950 mV, lack meeting the excursion requirement on
the other side of the mean by a full 49 mV (50 mV
required), with the result that the Reference Channel will
not function properly. Thus, when using pulses as the
reference input, take care that the duration of the pulses,
relative
to the pulse period, is great enough to give a mean
of at least 50 mV. In the example just cited, if the pulse
duration were increased to 100 JiS as shown in Figure
IV-5
AMPLITUDE : 1 V
J
~
ER I OO : 1 ml
DURATION: 1 III
MEAN: ImV
'- -'-_____ EXCURSIONS: .!"f~vmv
A. PULSE TRAIN WHICH WILL NOT TRIGGER BECAUSE
EXCURSION CRITERIA NOT MET
J
~
~
A M PL I TU D E :1V
PEROO: 1ms
~tti~~~~ ~~ liS
EXCURSIONS :+900 mV
- 100 mV
-~
B PULSE TRAIN WHICH MEETS All. CRITERIA, INCLUDING
EXCURSION REQUIREMENTS
Figure IV-7. PULSE TRAIN AS REFERENCE DRIVE
IV-7B, the mean would increase to 100 mV, and the
excursions (900 mV one side, 100 mV the other) would be
more than adequate for proper triggerinq.
Even though the Model 128A can accept and track a wide
range of possible reference siqnals, it is nevertheless
important that the reference signal used be relatively noise
free. Any superimposed noise can cause small zero crossings
to occur in the region of the main waveform zero crossings,
with the result that the Reference Channel momentarily
"sees" a much higher reference frequency than what is
really
there. When this happens, the reference "lock" can
be lost.
Frequently, moderately noisy siqnals can be cleaned
up sufficiently for satisfactory operation by interposing a
single-section low-pass
filter between the reference siqnal
source and the Reference I nput connector of the Model
128A.
On later instruments, space is provided for mounting the
filter
components on the Reference printed circuit board.
Location of the mounting holes is indicated on page VII·5.
To install the filter, transfer the wire which norrnally goes
to
quick-disconnect J419 over to quickd isconnect J429.
Then install the filter components, a resistor for RX4 and a
capacitor for CX 1 (RX4 and CX 1 are de siqnations given on
the schematic and parts-location diagram for these corn-
ponents). In most instances, optimum performance is
obtained by setting the filter corner frequency (f ::::
1/4rrRC) to the intended reference frequency.
Switches
Three switches are associated with operation of the
Reference Channel. One of them, the f/2f SWitch, is located
at the front panel. The other two are internal. The front
panel switch determines whether the mixer will be driven at
the frequency of the applied reference signal or at twice the
frequency of the applied reference signal. The "2f" position
is used for second harmonic studies. For normal operation,
the switch is set to "f". To examine harmonics higher than
the second, the operator would have to supply a reference
signal at the frequency of the harmonic to be measured.
One should check the position of this switch before
connecting the Reference signal, when the switch posi tion
is changed during operation, the re"ference channel will
unlock, and time will be lost III waitill~] for it to lock on

5 101 5 10 2
510 2510 2
25°2
FREOUENC.,. ---.
0
0
""-
'SLOW' REF.
r<.:0OE
'FAST' REF
"....,....",
,MODE
-1\
~
I---
\
0
/'
IREF SIG IN IV rml SINE WAVE
SIGNAL IN' 100 mV
,ml SINEWAVE
IIIII
,
111
,I
,I
'"
-2
0
+2"
°
+1
-1
+3
again. In most situations, the lost time would be but a few
seconds
and of little consequence. However, if one were
operating at a low frequency, it could be lengthy.
The internal switches determ ine the lock-on range, either .5
Hz -
100 kHz or 5 Hz 100 kHz. With these switches set
to either FAST or SLOW, the unit will lock onto reference
signals in the frequency range of 5 Hz to 100 kHz without
difficulty. However, only when the switches are set to
SLOW will it lock onto reference signals which are below 5
Hz.
The switches should be set to SLOW only for operation
below 5 Hz, because the time required to achieve frequency
lock is longer with the switch set to this position than when
it is set to FAST.
Detector Biasing
There is no
provision for detector biasing at the Reference
Input by means of an internal network. The network is not
provided but must be furnished by the operator accord ing
to
the impedance and voltage requirements of his detector.
The parts location diagram on page VII-5 shows where to
install
the resistors.
Fi~re IV-B. NET PHASE SHIFT BETWEEN SIGNAL AND
REFERENCE CHANNELS AS A FUNCTION OF FREQUENCY
Phase Controls
A high resolution potentiometer covering a range of
0-to-l00 degrees works in conjunction with a Quadrant
switch to determine the phase of the synchronous detection
process with respect to the phase of the applied reference
signal.
The Phase Sensitive
Detector provides a dc output propor-
tional to the amplitude of the input signal. This output
varies with the cosine of the angle between the reference
and input signals. When the Phase controls are adjusted for
maximum output, the signal amplitude can be read from
the panel meter and the phase of the signal can be read
from
the Phase dial.
It may
happen that meter fluctuations due to noise will
make it difficult to find the setting which gives maximum
output. Where this is the case, it will usually prove more
accurate and expedient to adjust for the null obtained when
the reference phase is set at 90° relative to the signal phase.
Once
the null is achieved, the Phase Quadrant switch can
then be rotated the one position necessary to achieve
maximum output so that the amplitude can be read from
the meter.
The absolute accuracy and resolution of the Phase controls
are stated in the specifications. Figure IV-8 shows the
typical net phase shift through the unit. Additional phase
shifts are introduced in the Signal Channel at the frequency
extremes. Even at middle frequencies, the HI-PASS and
LO-PASS filters can have an effect on the signal phase. A
phase
calibration can easily be made at any frequency by
connecting the input signal (noiseless) to both the Signal
and Reference
Inputs, followed by adjusting the Phase
controls for maximum output. The Phase controls then
indicate the net phase shitt in both the Signal and
Reference Channels. This
shift can then be subtracted from
any phase readings taken while operating at the calibration
frequency.
Reference Monitor Connector
An optional Reference Monitor output can be provided at a
rear-panel
connector. The Reference Channel output is
available at this
connector. This signal is a square wave
taken from ahead of the Mixer but after the Phase Control
circuitry, and so is at the frequency of the applied reference
signal (twice the frequency in the case of "2f" operation)
and at the phase set with the Phase controls. Standard TTL
logic levels are employed. Logic "0" = 0.2 V ±0.2 V and
Logic"1"=+3.5V±1 V.
4.3C SIGNAL CHANNEL
Introduction
Operation of the Signal Channel controls is straightforward.
The Input Selector is set to "A", "A-B", or "-B" as
appropriate, the Sensitivity switch is set for as near
full-scale output as possible, and the filters are used to
narrow the bandwidth ahead of the Mixer. In some
applications, it may be desirable to incorporate the optional
tuned amplifier into the Signal Channel as well. A further
discussion of these topics follows.
Input Selector Switch
With this switch set to "A", the signal applied to the "A"
Input is processed by the instrument. Signal applied to the
"B" Input is dead-ended. Similarly, when the switch is set
to "-B", the situation is the same except that the roles of
the "A" and "B" inputs are reversed. Another difference is
that the "B" Input is 180
0
out-of-phase with respect to the
"A" Input. In other words, if a signal which yields positive
output meter deflection when applied to the -"A" Input is
applied to the "B" Input, an equal but negative reading will
be
obtained. In the "A-B" position, the instrument
operates differentially, that is, only the difference between
the signals applied to the two inputs is processed and read
out. As discussed in Subsection 4.2E, it is generally
advantageous to operate differentially, even when process-
ing signals
from asingle-ended source.
IV-6

Sensitivity Switch
The Sensitivity switch should be set to provide as near
full-scale
output as possible. In very high noise situations, it
may be necessary to
operate with less sensitivity than
would be employed if processing a noise free signal of the
same amplitude. If overload proves a problem at the
sensitivity which yields maximum on-scale output meter
deflection, there are a cou pie of th ings the operator can try
before resorting to lowered sensitivity operation. First, he
can increase the
output filtering using the Prefilter and
Time
Constant switches. With a very low time constant,
output amplifier overload can occur when processing noisy
signals. This
type of overload problem can generally be
resolved by
operating with a time constant setting of .3
SEC or higher,
and with the Prefilter set to 100 mSEC or 1
SEC, as required. Additionally,
one can narrow the noise
bandwidth ahead
of the Mixer by means of the high-pass
and low-pass filters. If
neither increased time constant nor
the filters brings the overload under control, there still
remains
the additional step of using the internal tuned
amplifier in the case of a unit equipped with this option.
Assuming none of these steps helps the overload problem,
such as would happen if noise of sufficient amplitude to
cause overload is at the frequency of the signal being
measured,
there is no alternative but to reduce the
sensitivity.
Hi-Pass and Low-Pass Filters
The function of these filters is to eliminate as much
interference and noise as possible while having minimal
effect on the signal of interest. Under most conditions, by
setting these filters so
that they bracket the signal fre-
I
v---
~
,._._.
-: -:
/V
l/
,/
-
V'H'
~H'
,---
--7
f.- /
,~---
V
~~~::;;:~~~~::I~~OSN
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.... -. V.- ._-.-
.' ,"'0
quency as closely as possible, the noise tolerance of the
instrument will be increased and the noise fl uctuations at
the output of the Model 128A will be reduced. However, in
using
the filters, it is important to take their effect on the
signal of interest into account. Figures IV-9 and IV-10 show
the amplitude and phase characteristics of these filters as a
function of switch setting and frequency. To find the net
effect in a region where both filters affect the signal,
multiply the amplitude transfer fractions and add the phase
shifts.
Use of
the MIN. and MAX. POSitions simultaneously
provides aflat response curve over
the full operating range
of
the Model 128A (see specifications). Aflat response is
useful for
operation in situations where the signal fre-
quency changes by large factors during the course of the
measurement.
In certain applications, the operator rnight like to have "3
dB down" frequencies other than those provided. This can
be
done by changing the value of some internal capacitors.
Each of
the two filters has a separate capacitor for each of
the three switch positions. The capacitors which determine
the MIN and MAX response characteristics should not be
changed. The other two can be. The Parts Location
Diagram on page VII-2 can be used to identify these
capacitors. Schematically, they are shown on page VII-4. In
the case of the Low Pass filter, the relationship between
capacitance and 3 dB down frequency is given by:
IV-4
!:[---~.~
"" --+----'-\---cA"--
to- ----
.1 .2
'R(OO(NC,.frtHI
Ptl,lS(IH"IHPQS,11OJ\l
ISUHO(fIH[O
Figure rv-s, AMPLITUDE AND PHASE CHARACTERISTICS
OF HI-PASS FIL TER
IV·7
Figure IV·l0. AMPLITUDE AND PHASE CHARACTERISTICS
OF LOW-PASS FILTER