HP 415E Service manual

- 02152-4
n
OPERATING AND SERVICE MANUAL
MODEL 415E
SWR METER
SERIALS PREFIXED: 719-
See Appendix I and II at rear of manual for:
a) 415E Options 01, 02, апа
b) Serials Prefixed 530-, 545-
co HEWLETT-PACKARD COMPANY 1965
150r PAGE MILL ROAD, PALO ALTO, CALIFORNIA, U.S.A.
00415-90007
Printed: JUNE 1967

Model 415E Lists of Illustrations and Tables
LIST OF ILLUSTRATIONS
Number Title Page Number Title Page
1-1. Model 415E SWR Meter . . . . . . . 1-0 4-1. Block Diagram , . ...... ... 4-1
2-1.
Тһе
Combining
Case
.
. . . ,
.
. .
2-0
5-1.
Test
Set
Юр.
.
.
А
9
DES
2-2. Steps to Place Instrument in 5-2. | Examples of Diode Marking
Combining Case . . . . . 1... 2-0 Methods, . ........ ... 5-9
2-3.
|
Adapter
Frame
Instrument
5$-3A.
Switch
Component
Location
, ,
.
,
,
5-10
Combinations . . .. 2-1 5-3B. Switch Component Location , , . , , 5-10
2-4. Two Half Modules in Rack Adapter I 2-2 5-4. Transistor Biasing and Operating
3-1
Front Panel
Operating
Controls
Characteristics
. ,
.
.
. , . . .
5-12
and Connector , . , FEN 3-2 5-5. Power Supply Waveforms (AC
3-2. Rear Panel Operating Controls ` Operation Only, .,.... 5-14
and Connectors , 3-2 5-6. Power Supply Waveforms (Initial `
3-3
Typical
SWR
Measurement
Setup
.
3-3
Battery
Operation
-
Only),
.
,
.
.
5-15
3-4 General Turn-On Procedure , , .. 3-4 5-7. Signal Flow Waveforms (Input to
3-5.
Expanded
Section
of
Figure
3-6
.
, ,
3-5
Amplifier
Output)
.....
2...
5-16
3-6.
Converting
Decibels
to
SWR
,
,
..
38-6
5-8.
Meter
and
Output
Waveforms
,
.
, ,
5-17
3-7
Attenuation
Measurement
Setup
,
,
.
3-6
5-9.
Schematic
Notes
.
,
.
.
.
.
5-18
3-8 Impedance Measurement Rules 5-10. Circuit Board Component
Summary . . . .. . 3-7 Location , ...... .. . 5-19
3-9.
Shift
об
Minimum
with
Load
and
Short
.
3-7
5-11.
Power
Supply
and
Input
Circuit
|
...
5-19
3-10. Example for Use of Smith Chart . 8-8 5-12, Output and Meter Circuit , , , . . , 5-21
3-11.
415E
Noise
Figure
Curves
.
.
.
.
.
3-10
1-1.
Battery
-
Cover
Assembly
,
,
.
Ll
3-12.
Meter
Noise
Correction
Curve
,
,
,
3-10
1-2.
Connector
Assembly,
.
.
У
1-1
1-1. Circuit Board Component Location-
Instruments Prefixed 530. . . . . . П-1
LIST OF TABLES
Number Title Page Number Title Page
121, збресніїсабіовви Leere oec! v itm oie as 1-0 5-4. | Out-of-Circuit Transistor Resistance
3-1. Panel Descriptions. . 3-3 Measurements ............... 5-12
5-1. Recommended Test Equipment ...... 5-1 5-5. Ohmmeter Ranges for Transistor
5-2 Performance Tests ............. 5-3 Resistance Measurements ....... 5-12
5-3 Etched Circuit Soldering 6-1. Reference Designation Index ....... 6-2
Equipment... mr 5-8 6-2. НБеріасеаМе................... 6-8
6-3. Code List of Manufacturers ........ 6-11
02152-3 Hi

Section I
Figure 1-1 and Table 1-1
Model 415E
Figure 1-1.
Model 415E SWR Meter
Table 1-1. Specifications
Sensitivity: 0.15 шу rms at maximum bandwidth
(1 шу rms on high impedance crystal input).
Noise: At least 7.5 db below full scale at rated sen-
Sitivity and maximum bandwidth with input termi-
nated in optimum source impedance (see Input).
Noise figure less than 4 db.
Range: 70 db in 10 and 2 db steps.
Accuracy: +0.05 40/10 db step; maximum cumula-
tive error between any two 10-db steps, +0.10 db;
maximum cumulative error between any two 2-db
Steps, 20.05 db. Linearity: 20.02 db on expand
Scales, determined by inherent meter resolution on
normal scales.
Input: Unbiased low and high impedance crystal (100
and 5000 ohm optimum source impedance respec-
tively); biased crystal (1 v into 1K); low and high
current bolometer (4.5 and 8.7 ma +3% into 200 ohms),
positive bolometer protection. Input connector, BNC
female.
Input Frequency: 1000 eps, adjustable 7%. Other
frequencies between 400 and 2500 cps available on
Special order.
Bandwidth: Variable, 15 to 130 cps. Typically less
than 0.5 db change in gain from minimum to maxi-
mum bandwidth.
Recorder Output: 0 to 1V, 1000 ohms source imped-
ance, BNC female.
Amplifier Output: Oto 0.3V rms (NORM), 0to0.8rms
(EXPAND) intoat least 10K ohms, dual banana jacks,
Meter Scales: Calibrated for square-law detectors.
SWR: 1 to 4, 3.2 1010 (NORM); 1to1.24 (EXPAND).
DB: 0 to 10 (NORM); 0 to 2.0 (EXPAND). Battery:
charge state.
Meter Movement: 0.25% movement, taut-band sus-
pension, mirror-backed scale with expanded db and
swr scales greater than 4-1/4 in. (108 mm) long.
Power: 115 or 230 volts +10%, 50to 400cps, 2 watts.
Power line frequency or multiples thereof must not
be at the tuned amplifier frequency. Optional re-
chargeable battery provides up to 36 hours contin-
uous operation.
Dimensions: 7-25/32in. wide, 6-3/32 in. high, 11in.
deep from front side rail (190 x 155 x 279 mm).
Weight: Net 7-7/8 1b (3, 5 kg), 9-7/8 lb (4, 4 kg) with
battery.
Options:
01. Rechargeable battery installed.
02. Rear-panel input connector in parallelwith the
front-panel connector.
1-0
02152-2

Model 415E
Section I
Paragraphs 1-1 to 1-7
SECTION 1
GENERAL INFORMATION
1-1. INTRODUCTION.
1-2. Тһе Model415E SWR Meter is a high-gainampli-
fier, tuned to an audio frequency, with a square-law
calibrated meter readout. The Model 415E is designed
for use with square-law detectors inthe measurement
of SWR and attenuation. In addition, because of the
high-sensitivity and tuned amplifier, it can be used as
a null detector for audiofrequency bridges. The Model
415E is shown in Figure 1-1. Operating Specifications
for the Model 415E are given in Table 1-1.
1-3. The Model 415E is a tuned audio amplifier de-
signed to operate at a mean center frequency of 1000 cps
(Hz), adjustable 7% with a variable bandwidth of from
15 to 130 cps (Hz). Operating center frequency andband-
width are both variable at instrument front panel.
Tuned amplifier gain is only slightly changed due to
any change inbandwidth andis typically less than0.5 db.
In addition to the front panel meter readout provided
by the SWR Meter, two rear panel outputs are also
available: An AC amplifier outputis provided to allow
using the 415E as a high-gain(126 db) tuned amplifier;
a DC recorder output providing a convenient means of
obtaining a permanent record of measurement data.
Either or both of these rear panel outputs can be used
without affecting instrument meter operation provided
power line ground is not connected to the instrument
through either rear panel connector.
1-4. INSTRUMENTS COVERED ВУ MANUAL.
1-5. This manual applies directly to the Model 415E
SWR Meters having serial numbers prefixed 719 (first
Bie
02152-4
three numbers of serial number). If the serial prefix
on your instrument is other than 719, there are differ-
ences between the manual and your instrument which
are described in a Manual Changes sheet included with
the manual. If the manual changes sheet is missing,
the information canbe supplied by your nearest Hewlett-
Packard Sales and Service Office (see lists at the rear
of this manual). The manual change sheet may also
include an" ERRATA" section which describes manual
correction information which applies tothe manual for
all instruments including instruments prefixed 719,
1-6. INSTRUMENT OPTIONS.
1-7. This manual provides operating and servicing
information for the standard Model 415E. In addition,
operating and servicing information for Model 415E
instruments with Options 01 and/or 02, described be-
low, is also included.
a. Option 01: Factory installed, 24-volt recharge-
able battery capable of supplying up to 36 hours con-
tinuous operation of the Model 415E. 1f not initially
installed as an option, the same battery is available
on order from Hewlett-Packard (see Paragraph 2-17).
b. Option 02: Additional input connector on rear
panel wired in parallel with the front panel INPUT con-
песіог. If not initially installed as an option, the con-
nector-cable assembly is available on order from
Hewlett-Packard (see Paragraph 2-17).
1-1

Section
H
Model
415Е
Figures
2-1 and
2-2
DIVIDER ASSEMBLY
Ж
DIVIDER LATCH Ж... RETAINER
Figure 2-1. The Combining Case
415Е-А-8
Kr op ss o.
INSERT DIVIDER, ENGAGING
TABS IN TOP AND BOTTOM
MOUNTING SLOTS
Ө!
|
PUSH
RETAINER
DOWN
TO
RELEASE
WIST DIVIDER TO VERTICAL
POSITION
©
а | SLIDE INSTRUMENT
Ф ІМТО САЗЕ
PUSH IN TO LIMIT
Ф
TO SET RETAINER BACK IN
PLACE, ENGAGE HOOKS FIRST
ON ONE SIDE OF DIVIDER,
THEN ON OTHER
PUSH RETAINER UP TO LOCK
@
SLIDE LATCH ТО LOCK
DIVIDER
Figure 2-2. Steps to Place Instrument in Combining’ Case
2-0
02152-1

Model 415E
Section П
Paragraphs 2-1 to 2-16
SECTION Il
PREPARATION FOR USE
2-1. INCOMING INSPECTION.
2-2, This instrument was inspected both mechanically
and electrically before shipment. To confirmthis, the
instrument should be inspected for physical damage in
transit. Also check for suppliedaccessories, and test
the electrical performance of the instrument, using
the procedure outlined in Paragraph 5-3. If there is
damage or deficiency, see the warranty on the inside
front cover of this manual.
2-3. INSTALLATION.
2-4. The Model 415E is fully transistorized; therefore
no special cooling is required. However, the instru-
ment should not be operated where the ambient tem-
perature exceeds 55°C (140°F).
2-5. RACK MOUNTING.
2-6. The Model 415E is a submodular unit that when
used alone canbe bench mounted only. However, when
used in combination with other submodular units it can
be benchand/or rack mounted. The hp combining case
and adapter frame are designed specifically for this
purpose.
2-7. COMBINING CASE. The combining case is a
full-module unit which accepts varying combinations
of submodular units. Being a full-module unit, it can
be bench ог rack mounted analogous to any full-module
instrument. An illustration of the combining case is
shown in Figure 2-1. Instructions for installing the
Model 415E in a combining case are given graphically
in Figure 2-2.
2-8. ADAPTER FRAME. The adapter frameis a rack
frame that accepts any combination of sub-modular
units. It can be rack mounted only. Ап illustration of
the adapter frame is given in Figure 2-3. Instructions
are given below:
a. Placethe adapter frame on edge of benchas shown
іп step 1, Figure 2-4.
b. Stackthe sub-modularunits inthe frame as shown
in step 2, Figure 2-4. Place the spacer clamps be-
tween instruments as shown in step 3, Figure 2-4.
c. Place spacer clamps on thetwo end instruments
(see step 4, Figure 2-4) and push the combination into
the frame.
d. Insert screws oneither side of frame, andtighten
until sub-modular instruments are tight in the frame.
е. The complete assembly is ready for rack mounting.
2-9. THREE-CONDUCTOR POWER CABLE.
2-10. To protect operating personnel, the National
Electrical Manufacturers’ Association (NEMA) rec-
ommends that the instrument panel and cabinet be
grounded. А11 Hewlett-Packard instruments are
02152-1
equipped with a three-conductor power cable which,
when plugged into an appropriate receptacle, grounds
the instrument. The offest pin on the power cable
three-prong connector is the ground wire.
2-11. To preserve the protection feature when орег-
ating the instrument from a two-contact outlet, use a
three -prong to two-prong adapter and connect the green
pigtail on the adapter to ground.
2-12. PRIMARY POWER REQUIREMENTS.
2-13. The Model 415E can be operated from anAC or
DC primary power source. The AC source canbe either
115 or 230 volts, 50 to 400 cps. TheDC sourceis a
24-volt rechargeable battery. The rechargeable bat-
tery is supplied with option 01 instruments only.
2-14. For operation from AC primary power, the in-
strument can be easily converted from 115- to 230-
volt operation. The LINE VOLTAGE switch, 51, а
two-position slide switch located at the rear of the in-
strument, selects the mode of AC operation. The line
voltage for which the instrument is set to operate ap-
pears onthe slider of the switch. А 1/16-ampere, 250
volt fuse is used for both 115- and 230-volt operation.
CAUTION
DO NOT CHANGE THE SETTING OF THE
LINE VOLTAGE SWITCH WHEN THE IN-
STRUMENT IS OPERATING,
2-15. INITIAL BATTERY CHECK.
2-16. The following applies to option 01 instruments
or instruments that have field-installed batteries.
When the battery is used as the power source for the
first time, perform the following steps:
NEC FILLER PANEL
| \
Y 1]
S. о M
©
——À91
О.
|
о | 9
u
| |
4SE-A-IT
Figure 2-3. Adapter Frame Instrument Combinations
2-1

Section П
Paragraphs 2-17 to 2-20
©
ADAPTER
FRAME
эу @
@ INSTRUMENT
SPACER
CLAMP
SPACER
CLAMP
аше
RETAINING.
SCREWS
Figure 2-4. Two Half Modules in Rack Adapter
а. Connect Model 415E to AC source. Set POWER
switch to CHARGE and charge battery for а minimum
of 16 hours or overnight. Note: the battery can be
maintained in the charging state indefinitely without
damaging the battery. It will assume its fullcapacity,
1.25 ampere hour, and no more.
b. Set POWER switch to TEST position, the meter
needle indication should be within the BAT. CHARGED"
area (see Figure 3-1).
2-2
Model 415E
2-17. INSTALLING BATTERY AND INPUT
CONNECTOR.
2-18.
Available
from
Hewlett-Packard
are
parts
re-
quired
for
modifying
any
Model
415E
to
correspond
to
those
instruments
with
Option
01
and/or
Option
02.
A
rechargeable
Battery
Installation
Kit,
hp
Part
Number
00415-606,
contains
the
battery
and
necessary
hard-
ware
for
installation
(corresponds
to
Option
01).
In-
stallation
instructions
are
detailed
in
Appendix
at
rear
of
this
manual.
To
obtain
the
parts
required
for
an
input
connector
on
the
rear
panel
(corresponding
to
Option
02),
order
by
hp
Part
Number
as
found
in
Table
6-1
(listed
under
Option
02).
Instructions
for
instal-
lation
of
this
additional
connector
are
detailed
in
Ap-
pendix
at
rear
of
this
manual.
2-19. REPACKAGING FOR SHIPMENT.
2-20. When returning an instrument to the Hewlett-
Packard Company, use the original packing material
(if foam type) if available or contact an authorized hp
Sales Office for assistance. If this is not possible, first
protect the instrument surfaces by wrapping in heavy
Kraft paper or with sheets of cardboard flat against
the instrument. Then protect the instrument on all
sides (use approximately 4 inches of packing material
designed specifically for package cushioning), pack in
а durable container, mark container clearly for proper
handling, and insure adequately before shipping.
Note
When an instrument is being returned to the
Hewlett-Packard Company for service or ге-
pair, attach atag tothe instrument specifying
the owner and desired action. Allcorrespon-
dence should identify the instrument by model
number and the full (eight-digit) serial number.
02152-1

Model 451Е
Section ПІ
Paragraphs 3-1 to 3-14
SECTION ПІ
OPERATING INSTRUCTIONS
3-1. INTRODUCTION.
3-2. This section contains information and procedures
foroperation of the Model 415E (from either AC or bat-
tery power source) in making swr and attenuation
measurements. Also included isinformation оп slotted
line techniques, instruction in the use of aSmith Chart
for plotting load impedance, and discussion of Model
415E noise performance with various source imped-
ances and noise effect on meter indication.
3-3. FRONT AND REAR PANEL FIXTURES.
3-4. Figures 3-1 and 3-2 identify by number the front
and rear panel fixtures of the Model 415E. The des-
criptions in Table 3-1 are keyed by number (1-12 for
front, 13-18 for rear) to the figures. Further infor-
mation regarding the various settings and uses of the
controls, indicators, connections, and adjustments is
included inthe procedures of this section. Information
on battery is found in Paragraph 3-6.
3-5. GENERAL OPERATING AND
MEASUREMENT CONSIDERATIONS,
3-6. BATTERY OPERATION.
3-7. The Model 415E may be operated froma battery
instead of the 115 or 230 volt AC supply (see Paragraph
2-13). Battery operation requires some slightly dif-
ferent procedures to prolong battery life and to ensure
proper results. The rechargeable nickel - cadmium
battery is factory installed iforderedas Option 01 (see
Paragraph 1-7). The same battery may be ordered
and installed later. To obtain this, order hp Stock
Number 00415-606, Rechargeable Battery Installation
Kit.
3-8. INITIAL BATTERY USE. When the Model 415E
is to be battery operated for the first time, perform
the following steps:
a. Switch the Model 415E POWER switch to BAT-
TERY/TEST position and note meter pointer indication:
A meter pointer indication in the "BAT. CHARGED”
area indicates the internally battery properly charged
and ready for use; A meter pointer indication to the
left of the "BAT. CHARGED" area means that the
battery must be charged as described below.
b. Connect the Model 415E to AC power source. Set
POWER switch to BATTERY/CHARGE and charge the
battery for a minimum of 16 hours or overnight.
с. After at least 16 hours of recharge time, switch
POWER switch to BATTERY/TEST position and check
battery charge. If the battery charge indication is still
unsatisfactory, see Paragraph 5-35.
02152-4
3-9. OPTIMUM BATTERY USAGE. Itis recommended
that the Model 415E be operated by the battery for up
to 8 hours, followed by 16 hours of recharge. If con-
tinuous battery operation is required for more than 8
hours, the recharge time should be double the oper-
ating time. Continuous battery operation is possible
for up to 36 hours but this must be followed by a pro-
longed recharge period.
3-10. BATTERY STORAGE. Storage of the battery at
or below roomtemperature is best. Extended storage
at high temperatures will reduce the cell charge but
not damage the battery if the storage temperature is
less than 140°F. It is suggested that the battery be
charged after removal from storage and before using
the Model 415E for battery operation.
3-11. GROUNDLOOP CURRENTS.
3-12. The 415E SWR Meter audio amplifier has high
sensitivity tolow level signals. To reduce groundloop
currents, the 415E grounds are isolated by а 46.4 ohm
resistor. Ground loops occur when instruments are
-connected to 415E outputs and grounded through power
cords or rack mountings. Ground loops can be min-
imized in the following ways:
а. Connect the 415E to instruments with floating
inputs;
b. Connect the 415E to instruments with high input
impedance; Connect only the signal wire between in-
strument and the 415E;
с. Operation at higher signal levels;
d. An Adapter on the power cord іо float the instru-
ment ground where not prohibited by safety regulations.
3-13. BANDWIDTH AND FREQUENCY SELECTION.
3-14. Twofront paneladjustments are provided to op-
timize operation of the Model 415E tuned amplifier.
The FREQ (frequency) control allows a total variation
of 7% ої the center tuned frequency. When more than
one Model 415E is included in the same measurement
setup, the variable tuned frequency is used to set all
the instruments tothe exact frequency modulating the
source. Тһе high sensitivity and narrow bandwidth
of the amplifier make the Model 415E valuable as а
meter-indicating null detector for audio frequency
bridges. The BANDWIDTH adjustment varies the tuned
filter bandwidth from 15 to 130 cps. A narrow band-
width is best for iow level signals as this improves
the signal to noise ratio. А wide bandwidth would find
more use in fast sweep rate measurements.
3-1

Section HI Model 415E
Figures 3-1 and 3-2
/ І2А 9
` GAIN
VERNIER
15 E SWR METER
WEWLETT + PAOKARSL o
RANGE-DB
EXPAND
E XTAL IMPED —— pi BOLOMETER
1 i " BIASED ЗНА
LOW. 8.7 МА.
HIGH:
12
1
3 4 5 8
415Е-А-9
Figure 3-1. Front Panel Operating Controls and Connector
13
18
RECORDER үш
и П
4i5E-A-IO
15 16
Figure 3-2. Rear Panel Operating Controls and Connectors
3-2 02152-1

Model 415Е
Section ІП
Paragraphs 3-15 to 3-21
Table 3-1. Panel Descriptions
1. Selects desired 415E power source: BATTERY/
CHARGE position allows internal battery re-
charge when power cordis connected to AC line,
2. Indicator lights when power switch is in LINE/
ON or BATTERY/CHARGE position.
Female BNC INPUT connector.
4. Set input of Model 415E for use with a bolom-
eter or crystal detector mount. See Paragraph
3-53.
5. Adjustment allows center frequency variation
by 70 eps.
6. Attenuator adjusts gain in 10 db steps.
7. Allows full scale expansion of any 2.0 db portion
of the 10-db scale.
Changes bandwidth from 15 to 130 cps.
9. Allows initial meter reference setting with a
control range of at least 10-db.
10. Provides fine adjustment of GAIN control meter
settings.
11. Mechanicalzero adjustment allows exact sett-
ing of meter needle to 2.0 db calibration mark.
12. With POWER switch set to BATTERY/TEST, a
meter needle indication within the "BATTERY
CHARGE" area on the meter face (indicated ру
12A) shows that internal battery is charged suf-
ficiently for proper 415E operation; if needle
indicator is to left (area 12B) of "BATTERY
CHARGED" area, then battery is not charged
sufficiently for proper instrument operation
(option 01 - ONLY).
13. Additional input connector (wired in parallel
with front panel connector); supplied as Option
02 for 415E only upon request.
14. DC output for recorder use (0to1voltinto open
circuit or 1000 ohmg.
15. AC output for use as tuned amplifier output.
16. Three-conductor AC power cord receptacle
(NEMA-type).
17. Contains power line fuse.
18. Slide switch to allow 115- or 230-volt AC
operation.
3-15. SWR MEASUREMENT EQUIPMENT
AND TECHNIQUES.
3-16, EQUIPMENT.
3-17. A typical setup of equipment used in SWR meas-
urements is shownin Figure 3-3. The signal source is
usually square-wave modulated at 1000 cps since other
modulating waveforms often cause undesirable fre-
quency modulation of the source. Harmonics from the
Source sometimes cause trouble andcanbe eliminated
with а low-pass filter.
3-18, The detector shouldbe а square-law device (out-
put voltage proportionalto RF power input) such as a
barretter or a crystal diode operated at low signal
levels. The meter of the 415E is calibrated for square-
law detectors. Crystal diodes are normally more sen-
sitive than barretters but barretters are square-law
over a wider dynamic range. Both types of detector
normally maintain accurate square-law responseupto
at least fullscale deflection with the RANGE-DB switch
set to 30 position and coarse GAIN at maximum. (1 хау
RMS sine wave or 2.2 mv peak-to-peak square wave
causes full scale deflection on HIGH. XTAL IMPED
position. On other positions of INPUT switch, 0.15 mv
RMS sine wave or 0.33 mv peak-to-peak square wave
causes full scale deflection.) Above this level these
detectors should be individually checked for departure
from square-law behaviour or manufacturer's data
should be consulted,
02152-3
3-19. A short circuit termination is useful in establish-
ing reference positions along the transmission line and
is measuring transmission line wavelengths.
3-20, SLOTTED LINE PROBE PENETRATION.
3-21. Ageneralrule in slotted line measurement is to
use minimum probe penetration that still picks up ad~-
equate signalto measure. The probe couples to the
@ MODEL 415E
SIGNAL
GENERATOR
ATTENUATOR
—
UNDER
TEST
СС
4iSE-B-4
| й DETECTOR LOAD
SLOTTED LINE
Figure 3-3. Typical SWR Measurement Setup
3-3

Section HI
Figure 3-4
Model 415E
. Set POWER switch to OFF. Meter pointer should
rest at 2 onthe 0-2 DB scale (if not referto Par-
agraph 5-10).
. Set POWER switch to LINE/ON (or BATTERY/
ON).
Note
Xt set to BATTERY/ON refer to Paragraph
3-5 and check battery potential.
. Set INPUT to desired input impedance. (Note see
Paragraph 3-55.)
. Connect audio source to INPUT (i.e., crystal de-
tector, bolometer, audio oscillator, etc.).
. Adjust modulation frequency (audio input signal)
to approximately 1000 cps.
INPUT |
XTAL IMPED --51 ро BOLOMETER
GAIN
VERNIER
V RANGE — DB
AND
4i5E-A-I58
. Adjust RANGE -DB, GAIN, and VERNIER controls
and the amplitude of the input signal for a con-
venient meter reference near mid-scale.
. Adjust FREQ control for maximum meter pointer
deflection.
. Adjust BANDWIDTH control: fully counterclock-
wise rotation is minimum bandwidth and fully
clockwise rotation is maximum bandwidth.
Note
А narrow bandwidth is usually best for low
levelsignals; 30 cps is convenient for most
applications; and a wide bandwidth is usually
best for fast sweep rate measurements.
Figure 3-4. General Turn-On Procedure
02152-2

Model 415Е
transmission line as a shunt admittance which increases
(disturbing the transmission line more) as the probe
penetrates farther. To find out whether a given probe
penetration is too great or not, measure SWR, then
change probe penetration and remeasure SWR. If the
second reading is different, the probe is penetrating
toofar and loading the transmission line significantly.
3-22. PROCEDURE.
3-23. MODERATE SWR. The scales of the 415E are
calibrated for reading standing wave ratio directly from
the meter. Set the slotted line probe ata voltage max-
imum and adjust the gain of the 415E with the RANGE-
DB, GAIN, and VERNIER controls (EXPAND switch to
NORM) for full scale deflection (1.0 оп the 1.0t04 SWR
Section III
Paragraphs 3-22 to 3-24
scale). Now move the probe toward a minimum. If the
meter drops below 3.2, rotate the RANGE- DB switch one
position clockwise and read on the 3.2to0 10 SWR scale.
Ifthe pointer drops below this scale, rotate RANGE-DB
Switch one more position clockwise and readonthe 1.0
to 4 scale and multiply by 10. This pattern continues
for still higher SWR readings.
3-24. The DB scales can be used for a standing wave
ratio measurement by setting the 415E to full scale at
avoltage maximum, then turning the RANGE- DB switch
clockwise for anon scale reading ata voltage minimum
and noting the difference in DB readingsat the maximum
and minimum. A DB reading is obtained by adding
RANGE-DB switch setting and meter indication.
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DECIBELS (SWR) 4ІБЕ-В-Ї
Figure 3-5. Expanded Section of Figure 3-6
02152-2
3-5

Section HI
Paragraphs 3-25 to 3-32
3-25.
LOW
SWR,
Standing
wave
ratio
between
1.0
and
1.24
canbe
read
quiteaccurately
onthe
EXPAND
scales
of
the
meter
when
the
EXPAND
switch
is
set
to
any
position
other
than
NORM.
3-26.
MODERATE
SWR,
HIGH
RESOLUTION.
The
EXPAND
and
-DB
scale
can
be
used
together
with
the
EXPAND
switch
to
read
any
SWR
with
high
resolution
in
DB.
Figure
3-5
and
3-6
are
used
to
convert
DB
to
SWR.
The
reference
level
(full
scale
meter
deflection
ata
voltage
maximum)
can
be
used
with
the
EXPAND
switch
at
NORM
(since
0
db
NORM
and
0
db
EXPAND
correspond)butgreater
accuracy
is
obtained
by
setting
the
reference
level
with
the
EXPAND
switch
to
0.
30
20
STANDING WAVE RATIO
o
о 4 8 12 6 20 24 28
DECIBELS (SWR)
415E-A-2
Figure
3-6.
Converting
Decibels
to
SWR
3-27.
HIGH
SWR.
High
standing
wave
ratios(greater
than
30,
or
sometimes
10)
present
problems
because
of
excessive
probe
penetration
(to
lift
the
minimum
above
the
noise
level)
and
departure
of
detector
behaviour
from
square-law.
Both
problems
are
lessened
or
elim-
inated
by
measuring
only
the
standing
wave
pattern
near
the
voltage
minimum,
where
probe
loading
effects
are
least
disturbing.
3-28.
TWICE-
MINIMUM
POWER
METHOD.
The
basis
for
this
method
(andthe
TEN-TIMES-MINIMUM
POWER
METHOD)
is
the
fact that
for
a
high
SWR,
the
standing
wave
pattern
approximates
a
parabola
in
the
vicinity
of
avoltage
minimum.
The
slotted
line
carriage
must
have
a
good
scale
or
dial
indicator.
Measure
the
distance
(AX)
between
positions
on
the
standing
wave
pattern
where
the
voltage
is
3
db
above
the
voltage
at
the
min-
imum.
Alsomeasure
the
transmission
line
wavelength
^g
(standing
wave
pattern
minima
are
one-half
wave-
length
apart
and
the
sharp
minima
resulting
from
3-6
Model 415E
short-
circuiting
the
transmission
line
are
easy
to
lo-
cate
accurately).
Compute
the
SWR
from
the
following
formula:
swa = Ул Sx).
3-29.
TEN
-ТІМЕЅ-
MINIMUM
POWER
METHOD.
Ап-
other
convenient
"level
above
minimum
method"
to
use
for
computing
SWR
is
alevel
10
db
above
minimum,
The
separation
(A X)
between
these
positions
Should
be
put
in
the
following
formula:
3,
(А
SWR=
Ут
(
gax)
For
standing
wave
ratios
as
low
as
15
(о
1,
the
accuracy
of
this
method
is
within
1%.
3-30.
SWR
MEASUREMENT-SOURCES
OF
ERROR.
Several
possibilities
have
already
been
mentioned:
excessive
frequency
modulation
of
source
(smears
out
sharp,
deep
nulls
of
high
SWR
pattern),
harmonics
of
signal
frequency
from
source,
departure
of
detector
from
square-law
behaviour,
and
excessive
probe
pen-
etration.
Also,
reflections
in
the
transmission
line
between
the
slotted
line
and
device
being
measured
must
be
minimized.
3-31.
ATTENUATION
MEASUREMENT.
3-32.
The
415E
may
be
used
for
high
resolution
in-
sertion
loss
measurements
simply
by
inserting
the
device
to
be
measured
between
signal
source
and
de-
tector
and
noting
the
change
in
DB
indication
on
the
415E.
A
typical
measurement
is
shown
in
Figure
3-7.
The
continuous
coverage
of
the
EXPANDscales
allows
any
attenuation
measurement
to
be
made
on
the
EX-
PAND
scales.
For
accurate
results,
both
the
signal
source
and
the
detector
should
be
well
matched.
Im-
pedance
match
of
source
and
detector
can
beimproved,
if
necessary,
with
padding
attenuators,
isolators,
ОГ
tuners,
dj MODEL 415Е
SIGNAL
SOURCE
о
IMPEDANCE MATCH
HERE
ATTENUATOR
Figure
3-7.
Attenuation
Measurement
Setup
02152-2

Modei 415Е Section Ш
Paragraphs 3-33 to 3-36
IF MINIMUM SHIFTS 3/4,
LOAD EQUALS Z,: SWR
LOAD
MINIMUM
MINIMUM WILL SHIFT TO MINIMUM WILL SHIFT TO
THIS REGION IF LOAD THIS REGION IF LOAD
GENERATOR ІЗ INDUCTIVE LOAD
À À
ШЕ +52
IF MINIMUM DOES МОТ
SHIFT, LOAD EQUALS 2,// SWR RE
Figure 3-8. Impedance Measurement Rules Summary
3-32. LOAD IMPEDANCE MEASUREMENT.
3-33. GENERAL.
3-34. Slotted line techniques provide information to
allow calculation of a load impedance. The following
rules apply tothe indications givenbythe voltage min-
imum when the load is replaced by a short. Figure
3-8 summarizes and graphically presents these im-
pedance measurement rules. Whentheloadis replaced
by a short, then:
a. The shift in the minimum is never more than
31/4 wavelength.
b. If the minimum moves toward the load, the load
has a capacitive component,
c. É the minimum moves toward the generator, the
load has an inductive component.
d. H the minimum does not move, the load is com-
pletely resistive and has a normalized value of 1/swr.
e. If the minimum shifts exactly one-quarter wave-
ength, the loadis completely resistive and has a nor-
malized value equal to the swr.
f. The minimum will always be at a multiple of а
half wavelength from the load.
3-35. IMPEDANCE MEASUREMENT PROCEDURE.
3-36. The procedure for performing the actual im-
edance measurement witha slotted line is as follows:
a. Connect the load under test to the slotted line
section and measure the swr (see Paragraph 3-26 or
3-28). Also note the position of the probe carriage at
һе minimum.
b. Replace the load under test with a short.
с. Locate the minimum with the line shorted.
02152-2
d. Referring to Figure 3-9 and the following for-
mulas, compute the normalized load impedance:
Normalized 71, з EIE ATH талх
180° (+Ad)
Ag/
where X з
415Е-А-?
Figure 3-9. Shift of Minimum with Load and Short
8-7

Section ПІ
Рагасгарһв 3-97 іо 3-38
and: «Ad = Shift in centimeters of the minimum
point when the short is used. Ad takes a positive
sign (+) if the minimum shifts toward the load.
A d takes a negative sign(-)if the minimum shifts
toward the generator.
8/2 = one-half guide wavelength, i.e., the
distance in centimeters between two adjacent volt-
age minima,
Note
The above calculations are based on the as-
sumption that no losses occur in the trans-
mission line. It is assumed that the char-
acteristic line impedance, Zp, is resistive.
3-37. SMITH CHART EXPLANATION.
3-38. When data is obtained from a slotted line sys-
tem, one of the best aids for determining impedance
AB 9 еы
аСТ В
of SOMME о
TOETER
Model 415E
is the Smith Chart. * A Smith Chart with ап example
(see Paragraph 3-39)is shown in Figure 3-10. The
values ofresistance andreactance are basedon a nor-
malized value obtained by dividing the actual valueby
the characteristic impedance, Zp, of the line. Thus
if Z з 5 + j 25 ohms and if 20 =o ohms, then Zw =
0.1 + j 0.5. On the Smith Chart, the circles which
are tangent to the bottom of the chart are for а con-
stant, normalized resistance;lines curvingtothe right
from center are the normalized positive reactance
components; lines curving to the left from center are
the normalized negative reactance components; the
straight line forming the vertical diameter is aline of
zero reactance; the lower half of the zero reactance
line (marked 1 through 50) also represents the stand-
ing wave ratio line.
ж Smith, P. H., "Transmission - Line Calculator, "
Electronics, Jan. 1939, McGraw-Hill.
xu
Ec
m
M
v
T
> їз
Сас
оя
EN
АА
410 TOWARD Z ZŠ
4156 -B-2
Figure 3-10. Example for Use ої Smith Chart
3-8
02152-2

Model 415E
3-39. SMITH CHART CALCULATIONS.
3-40. Use of the Smith Chart for calculating imped-
ance is outlined below. Followingthe generalized рго-
cedure is a numerical example. Other methods are
possible for first enteringthe Smith Chart, but the one
suggested here is practical and easy to use.
a. Determinethe guide wavelength, A gas explained
in Paragraph 3-28.
b. Measure the swr by the method in either Para-
graph 3-26 or 3-28.
c. Locate a convenient minimum with the load still
in place. Record the probe carriage reading.
d. Replace load by a short, relocate the minimum
and record the probe carriage reading. Determine
A d, the difference between this reading and the one
from step c. Note whether the minimum was moved
toward the load or toward the generator.
e. Calculate the shift of the minimum, in terms of
wavelength:
ах Ad
^g
f. Start at center of Smith Chart and draw a circle
with a radius equal to the swr.
g. Enterthe Smith Chart at the top, move inthe di-
rection of probe movement noted in step d and a dis-
tance AA, computed in step e. Use wavelength scale
at the periphery of the Smith Chart.
h. Draw а line from the AA point to the center of
the chart.
i. Locate the normalized impedance as the inter-
section of the swr circle and the line drawn in step h.
j. The actual impedance is the product of the nor-
malized impedance from step i and 70, the line char-
acteristic impedance.
Note
The convention of enteringthe chartas stated
in step g applies only if the minimum is lo-
cated first with the load onthe line and relo-
cated whenthe line is shorted. If it is neces-
sary to first establish the shorted minimum
point, the direction of ДА would be opposite
to the direction of probe movement required to
relocate the minimum withthe load concerned.
3-41. The following example will clarify the above pro-
cedure, Figure 3-10 showsthe important steps involv-
ing the Smith Chart. The assumed characteristic im-
pedance is 50 ohms. Тһе distance between adjacent
minima is 15 cm, therefore A, = 30cm. Тһе swris
measuredas 3.3. А minimum i$1ocatedat22 ст. The
loadis shortedandthe minimum shifts to 19 cm, toward
the generator.
02152-2
Section ІШ
Paragraphs 3-39 to 3-49
Аа = 22 cm - 19 cm = 3 em
АА AW
rg = 3 ст/30 cm = 0.1 wavelength
3-42. The following numbered steps refer directly to
Figure 3-10.
(1) A circle for swr = 3.3 is drawn.
(2) A line is drawn from the 0.12 point (toward the
generator) to the center of the chart.
(3) The normalized impedance at the intersection
of the circle and the line is 0.44 + j 0.63.
The impedance of the load (for 70 = 509) is then:
50 (0.44 + 10.63) = 22 + j 31.5 ohms
3-43. SPECIAL APPLICATIONS.
3-44, The Model 415E is equipped with outputs which
allow applications other than as a meter indicating de-
vice for swr or attenuation.
3-45, RECORDER,
3-46, The rear panel recorder output furnishes an out-
put from 0to 1 volt DC with internal resistance of 1000
ohms and provides a convenient means of obtaining а
permanent record of measured data. For proper op-
eration, the recorder output ground (BNC shell) must
be connected to a floating ground. Adapters are com-
monly available to float the ground of grounded input
instruments at the power cord (see Paragraph 3-11).
3-47. AMPLIFIER OUTPUT.
3-48. The rear panel amplifier output furnishes an
output from 0 to 0.8 volt RMS into 10K ohms or more.
The Model 415E will supply up to 126 db of voltage gain.
For proper operation, the ground terminal (black) must
be connected to a floating ground (see Paragraph 3-11).
With the 415E EXPAND switch set to NORM, a full
scale meter reading willresultin a 0.3 volt RMSout-
put signal, anda minimum scale reading (10 db) will ге-
sult in approximately 0.03 volt RMS. With the 415E EX-
PAND switch set to any position except NORM, a full
Scale meter reading results in a 0.8 volt RMS output
and a minimum scale reading (2 db) results ina 0.5
volt RMS output signal. А zero input signal results in
a zero volt output signal.
3-49. The Model 415E is especially useful as a tuned
amplifier ina measurement setup using an Oscilloscope
andaSweepOscillator. Sweep speeds may be increased
(overthe speeds using a ratio meter in a reflectometer
system) and the Model 415E, used as a high gain amp-
lifier, provides the required sensitivity.* Тһе AM-
PLIFIER OUTPUT (AC) is often more useful for this
purpose than the RECORDER OUTPUT (DC) since the
DC output is filtered to reduce ripple and its response
is too slow to make full use of maximum bandwidth.
* See hp Application Notes 54, 61, 65, and 66.
3-9

Section Ш
Paragraphs 3-50 to 3-51
3-50.
MODEL
415Е
NOISE
FIGURE,
3-51, Figure3-11illustrates a typical value of Noise
Figure that would be encountered in a Model 415Е.
The following example of Model 415E noise figure
measurement is presented to illustrate the particular
considerations that must be made to calculate instru-
ment noise figure.
а. Calculate the meter indication when a 5000 ohm
resistoris connected as a source, assuming the Model
415E is noiseless (0 db noise figure).
b. Any excess indication of 415E meter is then one-
half of its noise figure.
T T T 4] |
Ap 415Е
9 INPUT AT HIGH S3
в IMPEDANCE POSITION
7
ul
5
86
u
° 5
°
24
3
2
І Les
1250 2500 5000 IOK 50K
(OHMS —=)
SOURCE
IMPEDANCE
(Rg)
setas
19
зс
„ттл
зо
9 hp 4\5Е
INPUT AT LOW 7]
З IMPEDANCE POSITION
Rs
т
шб
a
©
5-5
ш
Фа
о
2
ЗЕ.
2
Th 4 |
25 50 100 200 409
(OHMS — )
SOURCE
IMPEDANCE
(Rs)
mnm
Figure 3-11. 415E Noise Figure Curves
3-10
Model 415E
с. Calculation example:
(1) Assume 415E with controls setfor the following
conditions: EXTAL IMPED HIGH (input imped-
ance 200K), 1 uv RMS sine wave at input causes
full scale deflection (0 оп Oto 10 DB scale), 130
cps bandwidth at 3 db points (1.5 db points on
Model 415Е meter which is calibrated for
square-law. Noise equivalent bandwidth is 7/2
times 3 db bandwidth).
(2) The open-circuit noise voltage across а 5000
ohm resistor at 295^ K (22°C) in a bandwidth of
(130 times 7/2) cps is as follows:
Y з Z2IKTBR
v= Жі,38к10:29) (295130 x 7/2) (5000)
V,
=
0.129
x
1076
volts
=
0,129
шу
(3) The 0.129 цу open circuit voltage is reduced to
0.126 uv by the 200K ohm input resistance of
the 415E which is assumed to be noiseless.
(4) 0.126 цу is 18.0 db below 1 uv but square-law
calibrated meter of the 415E, set as above,
would indicate 1/2 of 18 db or 9.0 db below full
Scale. Also, since the 415E meter is average-
reading and calibrated to read RMS value of a
sine wave, it reads 1.05 db below the RMS value
of Gaussian noise. Therefore, the 415E reads
1/2 of 1.05 db or 0.525 db less than 9.0 and the
415E would read 9.525 db with а 5000 ohm
resistor connected to the input as described.
Hence, a 7.525 db (9.525 - 2) meter reading
indicates a 4 db noise figure.
MEASUREMENT ERROR (DB)
2 4 6 8 (9) 12
SIGNAL PLUS NOISE MINUS NOISE ALONE (DB)
везла
Figure 3-12. Meter Noise Correction Curve
02152-3

Model 415Е
3-52. А system error occurs when making meas-
urements on lower 415E ranges due tonoise, For con-
venient reference, a graph (Figure 3-12) is shown to
allow correctionto meter reading forany given meas-
urement. To use this graph, make a signal measure-
ment, then turn RF power source off or disconnect
detector from RF source andnoteaverage meter read-
ing due to noise. Тһе difference between these two
reading is used to obtain the proper correction factor
from Figure 3-12. For example: if the average noise
level is 9.5 db and the measured signal level is 3.5 db
then the difference is 6 db. Refer to Figure 3-12, 6 db
corresponds to an error of 0.1 db. This 0.1 db
correction factor is always added to measured signal.
Hence, 3.5 + 0.1 = 3.6 db (corrected meter reading).
3-53. USE OF CRYSTAL DETECTORS.
3-54. The input impedance of Model415E must always
be higher than the output or source impedance of a
02152-1
Section ПІ
Paragraphs 3-52 to 3-56
bolometer or crystal detector connected to the INPUT
connector (see Paragraph 4-21 for discussion). For low
output impedance devices, such as 100 to 200 ohm
detectors, use 415E XTAL IMPED/LOW switch posi-
tion. For high output impedance devices, such as hp
Models 420B, 423B, 424B, or 786D, use 415E XTAL
IMPED/HIGH position. If improper input impedance is
selected, the crystaldetector may depart from square-
law for which 415E is calibrated. Paragraph 3-55
gives method of checking and calibrating а detector for
square-law response.
3-55. CHECKING SQUARE-LAW RESPONSE.
3-56. Increase the power leveltothe crystal detector
by known increments and note detector response on
415E. Note: a deviation in square-law response may
be due to excessive RF power to the crystal detector
(see Operating literature for specified response
characteristics of crystal detector in use).
3-11/3-12

Model 415Е
Section IV
Paragraphs 4-1 to 4-4
SECTION ІМ
PRINCIPLES OF OPERATION
4-1. GENERAL.
4-2 The 415E is a high-gain tuned amplifier which
takesan input froma bolometer, crystal, or any audio
Source, amplifies itandapplies ittoa meter calibrated
for use with square-law detectors. With bolometer
or biased crystal operation, the Model 415E supplies
the appropriate bias current. Figure 4-1 is a block
diagram which illustrates instrumentoperation. Refer
also tothe schematic diagrams, Figures 5-11and 5-12
which fold out of the manual for easy reference.
St ATTEN INPUT
RANGE AMPLIFIER
А251 A3QI-Q4
INPUT
TRANS
TI
COARSE
GAIN
CONTROL
FINE 27 ATTEN and EXPAND
GAIN RANGE AMPLIFIER ATTEN
CONTROL A2SI АЗ05-06 А252
4-3. INPUT CIRCUITS.
4-4. Тһе input voltage is first routed through INPUT
Switch, A1S1. In the HIGH position it is applieddirec-
tly
to
the
first
section
of
the
range
attenuator,
A2S1.
Whenthe INPUT switch is set to any other position but
HIGH, the input signal passes through transformer T1
whose turns ratio provides a 50 to 1 impedance trans-
formation, converting a 50 to 200 ohm source to 2500
to 10,000 ohms (which is the range of best noise figure
for the INPUT AMPLIFIER).
EXPAND
NOTES
DARK LINES REPRESENT AC SIGNALS
FLOW PATH WHILE LIGHT GRAY LINES
REPRESENT DC SUPPLY AND SIGNAL
FLOW PATH
Figure 4-1.
02152-1
FINAL
AMPLIFIER
A3QIO-13
AMPLIFIER
A3Q8/Q9
TUNED
AMPLIFIER
AMPLIFIER Азат.
OUTPUT я
© [FRED]
«вано
Block Diagram

Section IV
Paragraphs 4-5 to 4-21
4-5. RANGE ATTENUATOR.
4-6. Тһе signal from А151 is fed to the first section
of the RANGE-DB switch, А251, and then to the input
amplifier. The second section of A281 is located be-
tween the input amplifier and the second amplifier.
Тһе RANGE-DB Switch positions are marked in 10
db steps.
4-7. INPUT AMPLIFIER.
4-8. After passing through the first section of the
range attenuator, А251, the signal goes to the input
amplifier (A3Q1/Q2/Q3/Q4) which consists of four
transistors in cascade. The input signal is applied to
the base of A3Q1 and the final amplifier signal is taken
from the collector of A3Q4. The GAIN and VERNIER
controls are associated with this amplifier and vary its
gain over a range of more than 10 to 1. GAIN control
Ri, the coarse control, is a 250K ohm variable resistor
which adjusts the amount of negative feedback from
the collector of A3Q4 to the emitter of A3Q1. VER-
NIER control, R2, is a fine gain control and changes
gain by inserting 0 to 5000 ohms in series with the
output signal.
4-9, SECOND AMPLIFIER.
4-10. Transistors A3Q5 and A3Q6 amplify the signal
from the second section of the range attenuator. AC
feedback provides gain stability and high input im-
pedance. The output of the amplifier is applied through
the EXPAND attenuator, A282, to the third amplifier
А398 and A3Q9.
4-11. EXPAND CIRCUIT.
4-12. The function of the EXPAND switch A282, is to
allow any signal level to be measured on an expanded
scale with continuous coverage while maintaining the
original reference level. Expansion is accomplished
by applying a precise amount ої DC -offset current from
A3Q17 to the meter and simultaneously increasing the
signalto the 3rd amplifier. This increased gain allows
a 2 db change in signal level to deflect the meter across
its full scale. The offset current places the zero signal
indication off scale to the left.
4-13. FREQUENCY SELECTIVE CIRCUITS.
4-14, The frequency response of the third amplifier,
A8Q8 and A3Q9, is shaped by negative feedback. The
feedback path includes a Wien-bridge and amplifier
АЗОТ. At the null frequency of the Wien-bridge, the
negative feedback path is open and the gain of the am-
plifier ismaximum. Off center frequency the negative
feedback through Ње Wien-bridge reduces gain. The
amount of the "off resonance" gain reduction depends
on the setting of the BANDWIDTH control, R3.
4-15. The Wien-bridge is adjusted for a sharp null at
center frequency with BRIDGE STABILITY ADJUST
АЗВ29. Actually, this control is set for a very slight
bridge unbalance to produce just enough positive feed-
back so that signal current to the base of A3Q8 is
supplied mainly by A3Q7. Thus, at resonance, negligible
signal current flows through BANDWIDTH control, R3,
4-2
Model 415E
andgainis independent ofits setting. Center frequency
is set by varying resistors R4 and R5 (these resistors
are gangedand comprise the front panel FREQ control).
4.16. FINAL AMPLIFIER.
4-17. The output amplifier consists of four transis-
tors. The two output transistors, А3012 and А3013,
operate as a push-pull class B amplifier with both
collectors AC grounded. The emitters of these tran-
sistors are tied together and the AC amplifier output
is taken from this point through a coupling capacitor,
A3C28. Large negative feedback makes the gain of
the output amplifier very nearly unity. The AC output
voltage is developed acrossresistor A3R51: The cur-
rent through A3R51 is supplied by A3Q12 and A3Q13
conducting one at atime onalternate half cycles (Class
B operation) and the output signal sine wave is
а composite of this half-cycle operation. In addition,
the collector current of A3Q13 can drive the meter
directly. No rectifier diodes are needed, This meter
driving current is filtered by capacitor A3C26 and
passes through the meter anda 1000 ohm resistor, R6,
to develop а DC voltage for the recorder output.
4-18. GROUND LOOPS.
4-19, Тһе grounding technique used in the 415E con-
sists ої an input connector ground, a circuit board
ground, and output connector grounds. These are
"floating" grounds that are tied together and isolated
from chassis ground except for a 46.4 ohm resistor,
ЕТ, anda 0,05 uf capacitor, СІ, connecting ground
and chassis. A solid connection to chassis-or-earth
ground permits troublesome ground loop currents to
flow causing erroneous instrument operation, For
this reason, connecting grounded instruments to the
415E output connectors сап cause erroneous readings.
Most recorders and oscilloscopes that might be used
with the 415E outputs have differential inputs available
with neither side grounded (see Paragraph 3-11).
4-20. INPUT IMPEDANCE.
4-21. The Model 415E is designed to have an input
impedance much higher than that of any crystal detec-
tor or bolometer normaily used with it. This results
in lower noise figure and the highest possible input
signal tothe 415Е. For example with the 415Е INPUT
switched to LOW, the input impedance is approximately
2000 ohms while the output or source impedance of a
bolometer is approximately 200 ohms.
02152-3