Amalgamated Wireless F242A User manual

DISTORTION AND NOISE METER
F242A
Applies to Serial No. 586 Onwards
AMALGAMATED WIRELESS (AUSTRALASIA) LIMITED
North Ryde Division
422 Lane Cove Road, NORTH RYDE. N.S.W. 2113
AUSTRALIA
Handbook 1-70542R
Issue 3 31 Jan 80
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l-70542R
REVISION
NUMBER
l
2
3
4
REVISION RECORD
REVISION
DATE
ENTERED BY
AUTHORITY ENTERED
NAME( PRINT) SIGNATURE
C/O 75958 l JUL 80
WITHERS
l u 1-L
C/O 75964 l
JUL 80 WITHERS
J L) n:_
C/0 78553 31 JUL 81 WITHERS
J,W~
C/O 80051
27 JUL 82 REID
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l-70542R
(i)
TABLE OF CONTENTS
Page
PART 1
GENERAL INFORMATION
1.1
Introduction 1/1
1.2
Brief Description
1/1
1.3
Performance
1/2
1. 3.1
Environmental 1/2
1.3.2 Distortion Factor
Measurement 1/2
1.3.3 Level or Noise Measurement 1/2
1. 3.4
Input Impedance 1/3
1.3.5
Frequency Response 1/3
1.3.6
Standard Filters 1/3
1.4
General Information 1/3
1.5
Mechanical Details l/4
1.6 Accessories Provided
l/4
1.7
Optional
Accessories
l/4
i
1.8
Ordering Information 1/5
PART
2
INSTALLATION
2/1
2.1
General
2/l
2.2
Mounting
2/l
2.2.1
Rack Mounting
2/l
2.3 Co~nections
2/2
2.3.1 Power
2/2
2.3.2 Input
2/2
2.3.3 Alternative Input Connectors
2/2
2.3.4 Output 2/2
2.4 Optional
Program
Weighting
Network
2/3
2.5 Optional
Telephone
Weighting
Network
2/3
2.6
Optional ''C'1 Hes_,._a,ge Weighting Network
2/3
2.7 Repacking for Shipment
2/3
Revision 2

l-70542R
PART 3
PART 4
TABLE OF CONTENTS
OPERATION
3.1 Operating Controls
3.2
3.1.1
3.1. 2
3.1. 3
3.1.4
3.1. 5
3.1. 6
3.1. 7
3.1.8
Power Switch
Bal-Unbal. Switch
6000 - Bridging Switch
Filters Switch
Function Switch
Distortion Calibrate Controls
Meter Range Switch
Freq. Out Of Range Light
Level or Noise Measurements
3.2.1
3.2.2
Absolute Level or Noise Measurements
Relative Level or Noise Measurements
3.3 ·Distortion Factor Measurement
TECHNICAL DESCRIPTION
4.1
4.2
PrLnciples of Operation
Circuit Description, excluding Auto Notch Filter
4.2.1
4.2.2
4.2.3
4.2.4
4.2.5
4.2.6
4.2.7
4.2.8
4.2.9
Input Circuit
Attenuator & Preamplifier
Program Filter
20 kHz Hi-Cut Filter
400 Hz Lo-Cut Filter
Meter Amplifier & 120 kHz Filter
RMS - DC Converter
SET REF Attenuators and Amplifier
Power Supply
( ii)
Page
3/1
3/1
3/1
3/1
3/1
3/1
3/1
3/2
3/2
3/2
3/2
3/2
3/2
3/3
4/1
4/1
4/1
4/1
4/1
4/2
4/2
4/2
4/2
4/2
4/2
4/2
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l-70542R
TABLE OF CONTENTS
PART 4 Continued
4.3 Circuit Description of Auto Notch Filter
4.3.1
Bridge Circuit
4.3.1.1 Bridge Preamplifier
4.3.1.2 Modified Wien Bridge Circuit
4.3.1.3 Bridge Postamplifier
4.3.2 Nulling Circuits
4.3.2.1 Frequency Locked Loop
In Phase and Quadrature Generator
4.3.2.2 Chopper Preamplifier
4.3.2.3 Chopper & Loop Integrators
4.3.3 Automated Range Control
4.3.3.1 Principle of Operation
4.3.3.2 Coarse Frequency Measurement
PART 5 MAINTENANCE
PART 6
5.1 General
5. 2 Service
5.3 Fault Finding, Repair and Adjustment After Repair
CALIBRATION AND TEST PROCEDURE
6.1 Te.st Equipment Required
6.2 Adjustment and Testing
6.2.1 Level Meter Testing
6.2.1.1 Provisional Level Alignment
6.2.1.2 Frequency Response and A.C. Monitor
6.2.1.3 Attenuator Test at 1 kHz
6.2.1.4 High Level Attenuator Test for
REL. LEVEL, UNBALANCED
6.2.1.5 High Level Attenuator Test for
REL. LEVEL, BALANCED
(iii)
Page
4/3
4/3
4/3
4/3
4/3
4/3
4/3
4/5
4/5
4/5
4/5
4/6
i
5/1
5/1
5/1
5/2
6/1
6/1
6/1
6/1
6/1
6/2
6/2
6/3
6/3

1-70542R
PART 6
PART 7
Revision 2
(iv)
TABLE OF CONTENTS
Continued
6.2.2
6.2.1.6 COARSE SET REF Attenuator Test 6/4
6.2.1.7 FINE SET REF Attenuator Test 6/4
6.2.1.8 Attenuator Test at 100kHz 6/4
6.2.1.9 Noise 6/4
6.2.1.10 Longitudinal Suppression Test 6/5
6.2.1.11 Return Loss 6/5
6.2.1.12 Lo-Cut Filter 6/6
6.2.1.13 Hi-Cut Filter 6/7
6.2.1.14 Test of Program Weighting Network 6/7
6.2.1.15 Test of Telephone Weighting Network 6/7
6.2.1.16 Level Calibration 6/8
6.2 .1.17 Test of "C Message" Weighting Network 6/8
Nulling Loop Testing
6.2.2.1
Bridge Balance Adjustment
6.2.2.2 Auto Ranging Circuit Adjustment
6.2.2.3 Bridge Range Test
6.2.2.4 Frequency Out of Range Test
6,2.2.5 Residual Distortion Test
6/9
6/9
6/10
6/10
6/11
6/11
COMPONENT SCHEDULE 7/1
7/1
7/1
7/1
7.1
7.2
Explantory Notes
Distortion & Noise Meter F242A, 2A70542
7.2.1
7.2.2
Components Mounted on Chassis
Components Mounted on Printed Circuit
Board 2R70543
7.2.2.1 Capacitors
7.2.2.2 Resistors
7,2,2.3 Variable Resistors
7/1
7/1
7/4
7/10
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TABLE OF CONTENTS
Page
PART 7
Continued
7.2.2.4 Resistor Networks
7/10
7.2.2.5 Inductors 7/10
7.2.2.6 Photo-cells 7/10
7.2.2.7
Relays
7/10
7.2.2.8 Transistors 7/10
7.2.2.9 Integrated Circuits 7/11
7.2.2.10 Diodes
7/12
7.2.2.11 Switches
7/14
7.2.2.12 Connectors
7/14
7.2.3
Components on Set Reference Amplifier
Assembly 2R70545
7/14
7.2.4 Meter Range Switch Assembly 2A70546
7/15
f
PART 8
DIAGRAMS
Simplified Block Schematic & Levels Drg 70542-3-56
Circuit Diagram, Main Drg 70542-0-71
Circuit Diagram, Nulling Loop Drg
70542-0-70
Filter Response Curves Drg
70550-3-26
Nulling Loop Block Diagram Drg 70542-3-26
Nulling Loop Range Control
Drg 70542-3-57

l-70542R 1/1
PART 1
GENERAL INFORMATION
This apparatus has been designed according to Class 1 of IEC Publication 348
Safety Requirements for Electronic Measuring Apparatus, and has been supplied
in a safe condition. The present instruction manual contains information
and warnings which shall be followed by the user to ensure safe operation and
to retain the apparatus in safe condition.
1.1 INTRODUCTION
The AWA Distortion & Noise Meter F242A is suitable for the measurement of
total harmonic distortion on balanced or unbalanced audio circuits including
tape recorders. It features fully automatic fundamental rejection frequency
tuning over the range 20 Hz to 20 kHz. The F242A will also measure both
absolute and relative level, noise and hum-plus-noise in audio circuits in
the frequency range 10 Hz to 100 kHz and sinad receiver sensitivity.
1.2 BRIEF DESCRIPTION
The F242A is a sensitive distortion and level meter'which can be used to
determine the performance of all types of audio amplifiers, oscillators, tape
recorders, line equalizers and filters. Parameters which can be readily
measured are: gain, frequency response, signal-to-noise ratio (weighted and
unweighted) sinad receiver sensitivity and the distortion factor over a wide
range of input levels.
Distortion measurements are referred to the incoming signal level and the
meter is calibrated to indicate distortion factor either in dB or as a
percentage. The fundamental rejection frequency is automatically tuned over
the entire 20 Hz to 20 kHz operating frequency range, no manual selection or
tuning being necessary. Residual distortion is typically less than -90 dB.
An out of frequency range warning lamp indicates a frequency outside the
operating range.
i
A simplified distortion calibrate control, independent of attenuator setting,
enables easier calibration and rapid checking of calibration during measurements.
For level measurements, including noise, the meter is calibrated in dBm in
600 ohms (0 dBm = 1 mW) and V r.m.s. The meter circuit is true r.m.s. responding,
necessary to produce accurate readings of the complex distortion waveforms and
noise. The residual noise of the F242A is very low, being less than -110 dBm
on full bandwidth.
Low-cut and high-cut filters are provided, and by the appropriate selection of
these filters, noise above 20 kHz and mains hum-plus-noise below 400 Hz may be
excluded. An optional programme weighting network complying with C.C.I.T.T.
Recommendation Jl6 Annex, Geneva 1972, is available for weighted no·ise
measurements.

1/2
1.3 PERFOR~CE
_1. 3.1 Environmental
Operating Temperature Range
Storage Temperature Range
1.3.2 Distortion Factor Measurement
Fundamental Frequency Range:
Balanced
Unbalanced
Harmonic Measurement Range
Distortion Factor Range
Accuracy of 2nd Harmonic Indication
Input Level Range
1.3.3 Level or Noise Measurement
Meter Ranges (For 0 dB Meter
Indication)
Level Accuracy at 1 kHz
and 0 dB on meter
Absolute Level Accuracy 0 dBm
at 25 °C and 1 kHz
Meter Indication
Meter Calibration
Meter Response
Meter Accuracy
RESIDUAL NOISE
Unbalanced
0 °C to 50 °C
-10 °C to 60 °C
20 Hz to 20 kHz
20 Hz to 20 kHz
100 kHz
Maximum -10 dB
l-70542R
Minimum determined by residual
distortion of -84 dB balanced or
unbalanced, but increasing to -60 dB
at 30 Hz on balanced input.
Typically better than -90 dB
unbalanced.
For inputs below -10 dB re~ 0.775 V,
minimum determined by residual noise of
less than -74 dB. (100kHz bandwidth),
-84 dB (20kHz bandwidth).
+0, -0.5 dB
-30 dB to +30 dB reference 0.775 V r.m.s.
2.5 mV to 30 V.
-90 dBm to +30 dBm in 10 dB steps
(0.1 mV to 30 V FSD)
±0.2 dB (for any attenuator setting)
±0.05 dB
True r.m.s. responding
-30 dB to +2 dB, 0 to 3.2 and 0 to 10
for Volts or Percent
Response time to 99% of FSD in
250 ±50 mS
±1'7o of f.s.d.
Less than -110 dBm (less than 2.5 ~V)
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l-70542R 1/3
1.3.3.1 Relative Level Facility
Variable Gain
1.3 .4
Input Impedance
Balanced and unbalanced
Balanced Bricging
Unbalanced Bridging
RETURN LOSS OF TERMINATED INPUT
Balanced
Unbalanced
LONGITUDINAL SUPPRESSION OF
BALANCED INPUT
1.3.5
Freguency Response
Balanced Input
Unbalanced Input
A.C. OUTPUT
L3.6 Standard Filters
LOW CUT FILTER
HIGH CUT FILTER
1.4 GENERAL INFO&~TION
CONNECTORS
Standard Input Connector
POWER SUPPLY
Mains operation
Revision l
+22
dB to -38 dB
600 n
50 kn (0.1 dB bridging loss)
10 Hz to 30 kHz
100 kn (10 Hz to 100 kHz)
30 dB (10 Hz to 30 kHz)
40 dB (10 Hz to 30 kHz)
34 dB
at 100 kHz
At least 40 dB with respect to transverse
voltage (10 Hz to 30 kHz)
10 Hz to 30 kHz ±0.2 dB (typically less
than ±0.3 dB to 100 kHz)
10 Hz to 100 kHz ±0.2 dB
2 Volts r.m.s. for FSD on meter
1 kn output impedance
±0.2 dB at 400 Hz
-45 dB at 50 Hz
Meets high frequency roll off
requirements of IEC Noise Filter
l dB at 19 kHz
-18 dB at 32 kHz
-40 dB at 63 kHz
Siemens Type 9, Relkli 6a 3-way
balanced or similar
f
110 V -120 V and 220 V - 240 V nominal, ±10%
50 - 60 Hz'

1/4
1.5
1.6
MECHANICAL DETAILS
Height
Width
Depth
Weight
ACCESSORIES PROVIDED
1 x Mains Cable
1 x Instruction Manual
1 x
Mating Input Connector
l-70542R
146 mm (including feet)
133 mm (with feet removed for
rack mounting)
430 mm
397 mm (including handles & rear feet)
345 (behind mounting face, when rack
mounted)
7.1 kg.
(Siemens type Rel stp 6ac or similar)
1 x Spare Fuse
1.7 OPTIONAL ACCESSORIES
Option 001
Option 002
Option 003
Option 004
Option 005
Option 006
Option 007
Option 008
Option 009
CCITT PROGRAM WEIGHTING NETWORK complying with
CCITT Recommendation Jl6 Annex, Geneva, 1972.
CCITT TELEPHONE WEIGHTING NETWORK complying with
CCITT Green Book (1972) Vol V, Recommendation P53
Alternative input connectors consisting of twin
jacks for carrier or tip ring and sleeve plus
an alternative rear panel connector. A mating
connector for the latter is supplied.
Alternative input connector, 3 binding posts-.
Rack mounting kit enabling the unit to be
mounted in a standard 483mm rack, occupying 3
vertical units - 133mm total height.
Adaptor 3 pin male to BNC Female.
Adaptor 3 pin male to 3 binding posts.
A.M. Detector.
Distortion, less than 0.3% 500kHz to 1.7 MHz
less than l% 1.7 MHz to 36 MHz
for 2-10 Volt carriers at 30%
modulation.
R.F. input connector, BNC female.
Plugs into standard 3 pin balanced input
socket of F242A.
A.M. Detector
Specification as for Option 008
Mates with alternative input connector
Option 004, 3 Binding Posts.
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1-70542R
1.7 OPTIONAL ACCESSORIES (contd)
Option 010
1.8 ORDERING INFORMATION
"C" MESSAGE WEIGHTING NETWORK complying with
Bell Systems Technical Reference Publications
41009, May, 1975.
The
instrument is described as A.W.A. Distortion & Noise Meter F242A,
Type 2A70542.
Revision 2
1/5

l-70542R 2/l
PART 2
I N S T A L L A T I 0 N
2.1 GENERAL
The F242A Distortion & Noise Meter is designed for
mounting and for operation from a.c. mains supply.
connections, see below.
bench operation or rack
For power and signal
The equipment should be inspected for any physical damage incurred during
transportation. The accessories should be checked against the equipment
schedule.
Whenever it is likely that the protection has been impaired, the apparatus
shall be made inoperative and be secured against any unintended operation.
The protection is likely to be impaired if, for example:
the apparatus shows visible damage;
the apparatus fails to perform the intended measurements;
after prolonged storage under unfavourable conditions;
after severe transport stresses
Make sure that only fuses with the required rated current and of the specified
type are used for replacement. The use of mended fuses and the short-
circuiting of fuse-holders shall be avoided. The fuse should be of the
current rating as marked on the rear panel for the mains voltage selected.
2.2 MOUNTING
The F242A is fitted with bottom feet and a tilt bar for bench operation,
The tilt bar enables the unit to be tilted upwards for easier operation.
2.2.1 Rack Mounting
If it is required to rack mount the F242A, Option 005, Rack Mounting Adaptor,
must be ordered. This adaptor kit is designed to mount the F242A in a
standard 483 mm (19 inch) rack.
To prepare the F242A for rack mounting, remove the top and bottom covers,
held on by two screws at each end of each cover. Remove the front feet,
tilt bar and rear feet from the bottom cover. Refit the bottom and top
covers.
Next, peal off the adhesive trim strip on the side of the two front handles
and remove the outer two screws from the handles. Using the screws provided
in the Rack Mounting Adaptor Kit, screw the angle brackets onto the ends of
the case, flange forward. The instrument is now ready for mounting·into
the rack.
The F242A does not require any cooling in ambient temperatures up to +50 •c
however the usual care of not mounting above high dissipation equipment
without adequate ventilation should be followed.

2/2
l.-70542R
2.3 CONNECTIONS
2.3.1 Power
The F242A will operate from 110 V to 120 V and 220 V to 240 V nominal a.c.
mains supply, 50 Hz -60 Hz. The voltage range is selected by a slide switch
on the rear panel of the instrument. This voltage range should be checked
before connecting to the supply. Ensure that the correct fuse rating is
fitted for the supply voltage range selected. These are shown on the rear of
the instrument. For 110 V to 120 V range, a 200 mA Anti-Surge fuse should be
used.
For 220 V to 240 V range, a 100 mA Anti-Surge fuse should be used.
An international mains socket is fitted on the rear panel of the F242A. The
power cord supplied should be plugged into this and connected then to the
mains power receptacle.
The mains plug shall only be inserted in a socket-outlet provided with a
protective earth contact. The protective action shall not be negated by the
use of an extension cord without protective conductor.
Warning!
Any interruption of the protective conductor inside or outside the apparatus
or disconnection of the protective earth terminal is likely to make the
apparatus dangerous. Intentional interruption is prohibited.
2.3.2 Input
The standard Siemens Halske Type 9 Rel kli 6a or equivalent input connector is
a 3-contact in-line socket. The top contact is connected to chassis earth.
Balanced input from a 600 ohm line to the other two contacts is normally
bridged but may be terminated by selecting 600 ohm TERM. The bridging
resistance is 50 kilohms. With unbalanced inputs the centre contact is "active"
and the bottom contact is earthed internally. This condition is achieved when
the push-button switch marked "BAL"/"UNBAL" is in the 'out' position. The
unbalanced bridging input impedance is 100 kilohms.
2.3.3 Alternative Input Connectors
Optional twin jacks for carrier or ring tip and sleeve are available. This
kit also provides an alternative rear-panel connector.
If this option is ordered with the instrument it will normally be factory
fitted.
If the option is ordered separately, it is supplied with a set of fitting
instructions, Modification Bulletin 70542/l, for Distortion & Noise Meter
F242A, to enable the easy fitting of these alternative input connectors.
2.3.4 Output
An a.c. output is provided via two binding posts on the right-hand end of the
front panel, so that the distortion products may be displayed on an oscilloscope.
Two volts r.m.s. from this output correspond to full scale deflection of the
meter. The source impedance of this output is 1 kilohm.
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1-70542R 2/3
2.4 OPTIONAL PROGRAM WEIGHTING NETWORK
To fit the optional C.C.I.T.T. Program Weighting Network, remove the covers
of the F242A, plug the Network into the socket on the main printed circuit
board (centre Left), with components side facing the front of the instrument.
Fit clamp supplied over the Network and then replace the.covers of the F242A.
2.5 OPTIONAL TELEPHONE WEIGHTING NETWORK
To fit the optional C.C.I.T.T. Telephone Weighting Network, carry out
instructions as for 2.4.
2.6 OPTIONAL "C" MESSAGE WEIGHTING NETWORK
To fit the optional C.C.I.T.T. Message Weighting· Network, carry out the
instructions as for 2.4.
2.7 RE-PACKING FOR SHIPMENT
General guidance is furnished below in the event that re-packing of the
instrument for shipment may be necessary at·· any time.
1. Should the original container be available, re-pack in the same manner
as received. It is advisable to retain the original container and
packing case for this purpose.
2. If the original container is not available, the unit should be wrapped
in heavy paper or plastic sheet prior to placing in an inner container.
Place liberal quantities of packing materials, which should be reasonably
dust-free, on all sides of the container but DO NOT pack tightly. The
instrument front panel side should receive extra attention when the
container is being packed into the packing case.
3. Mark the case in which the instrument. is shipped "DELICATE INSTRDMENT"
or "FRAGILE".
Revision 2
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l-70542R 3/1
PART 3
OPERATION
Warning!
Any interruption of the protective conductor inside or outside the apparatus
or disconnection of the protective earth terminal is likely to make the
apparatus dangerous. Intentional interruption is prohibited.
The following controls are mounted on the front panel of the F242A.
3 .1. 1 Power
Switch
This switch applies mains power to the instrument. For safety, both line and
neutral are switched.
An associated red l.e.d. lamp indicates the presence of mains power.
Before switching on the apparatus, make sure that it is set to the voltage of
the power supply.
3 .1. 2
Bal-Unbal Switch
This push button switch selects whether the input circuit to the instrument is
balanced (button depressed) or unbalanced. In the balanced condition the two
lower sockets of the input connector are active. In the unbalanced condition,
only the centre socket is active and the lower socket is earthed.
3 .1. 3 600 Q - Bridging Switch
With this switch in its non-depressed position, the instrument input will be in
the bridging condition. The bridging impedance is 50 kilohms on balanced and
100 kilohms on unbalanced.
With the switch depressed the input will be terminated in 600 ohms.
3. 1. 4 Filters Switch
f
This is a bank of three push button switches. When all push buttons are 'out',
then the response of the F242A will be flat. By depressing the 'wtg' button the
CCITT Programme Weighting Network or CCITT Telephone Weighting Network will be
selected, if either of these options are fitted. If neither of these options are
fitted, no reading will be obtained on the meter. The 'Lo Cut' and 'Hi Cut'
filters may be selected separately or together, again by depressing the
appropriate button or buttons. When using the 'Lo Cut' and/or the 'Hi Cut'
filters ensure that the 'Wtg' button is not depressed as this button over-rides
the other two.
3 .1. 5
Function Switch
This is a bank of four interlocked push button switches. With the 'LEVEL dBm'
button depressed the instrument may be used for absolute level and noise
measurements. With the 'LEVEL rel' button depressed the instrument calibration
level may be changed by use of the coarse and fine SET REF controls to allow
measurements to be carried out relative to some reference input. The remaining
two buttons are for distortion measurement. The 'DISTORTION cal' button is again

3/2
l-70542R
used in conjunction with the coarse and fine 'SET REF' controls, to set the
reference level for distortion measurements. The calibration is carried out
independent of the Meter Range Switch setting. Once the reference level has
beg~ set, the 'DISTORTION read' button should be depressed to obtain a
distortion reading.
3.1.6
Distortion Calibrate Controls
These consist of a coarse and a fine control, the coarse control being switched.
These are used when the Distortion Cal switch is depressed to set the reference
level for distortion readings.
3 .l. 7
Meter Range Switch
This switch is used to adjust the meter range for both level and distortion
readings.
3.1.8
Freq. Out Of Range Light
This indicator lamp will light on Distortion measurement only, if the fundamental
frequency of the incoming signal is outside the range of the instrument.
3.2 LEVEL OR NOISE MEASUREMENTS
3.2.1 Absolute Level or Noise Measurements
Select the appropriate input configuration on the 'bal'-'unbal' and '600 n'-
'bridging' switches. Next, if it is required to limit the frequency bandwidth,
i.e., if noise above 20kHz or hum and low frequency noise are to be excluded
from the measurement, select 'Hi-Cut' and/or 'Lo-Cut' as required. If a
weighted measurement is required and the optional weighting network is fitted,
select 'Wtg'.
Select 'LEVEL dBm' on the function switch, set the meter range switch to the
appropriate range and read the meter.
3.2.2 Relative Level or Noise Measurements
Select the appropriate input conditions and filters as for 3.2.1 Absolute Level
and Noise Measurements.
Select 'LEVEL, rel' on the function switch, set the meter range switch to the
OdB position. The coarse and fine SET REF controls may now be used to adjust
the meter reading to set the reference for other relative level or noise
readings. The meter range switch operates normally enabling relative readings
from 30 dB to -110 dB. The meter range switch may be set at other than the
OdB range for adjustment of the SET REF controls, however under these
circumstances the operator must remember the range setting and subtract this from
the relative measurements obtained from attenuator and meter. Example: If ~he
SET REF controls are set with the meter range switch on -40 dB and another signal
is fed in and reads -26 dB the true relative reading would be (-26 dB) - (-40 dB)
i.e. -26 dB +40 dB giving a relative reading of +14 dB.
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l-70542R
3/3
3.3
DISTORTION FACTOR MEASUREMENT
NOTE:
The F241A measures
distortion factor either as a perc~ntage or_in dB
as follows:
v 2 + v 2 + v 2 + N2
Percentage distortion factor =100 2 3 n
v 2 + v 2 + v 2 + N2
1 2 n
v 2 + v 2 + v 2 + N2
10 log
2 3 n
Distortion factor
in dB
= v 2 + v 2 + v 2 +
N2
1 2 n
where vl' v2, etc. represent the amplitudes of fundamental and harmonics, vn
represents the amplitude of the last harmonic in the admitted bandwidth, and
N represents noise.
Select the appropriate input configuration on the 'bal'-'unbal' and '600 n' -
'bridging' switches, If noise above 20 kHz or hum and low frequency noise are
to be excluded from the measurement, select 'Hi-Cut' and/or 'Lo-Cut' as required.
Note however, the use of the filters restricts the range of signal frequencies
that may be used for accurate measurement. Next, select 'Distortion-Cal' and
adjust the associated coarse and fine calibrate controls for a meter scale
reading the OdB or 10. The setting of the meter range switch is immaterial.
Select 'Distortion-Read' and adjust the meter range switch for optimum meter
reading. The automatic frequency notch filter will start to null as soon as
the Distortion-Cal button is depressed and in most cases, by-the time the
calibration controls have been set and the Distortion-Read button selected,
the nulling will be complete. The higher the distortion, the more rapid the
nulling time.
As the calibration procedure is independent of the meter range switch, the
meter range may be left on the appropriate distortion range where a series of
distortion measurements are being taken, thus avoiding the tedious winding back
to OdB/100 percent range for calibration and then back again to the required
distortion range as on earlier generations of distortion meters.
Revision l

l-70542R 4/1
PART 4
TECHNICAL DESCRIPTION
4.1 PRINCIPLES OF OPERATION
Refer to Simplified Block Schematic drawing 70542-3-56.
The incoming signal feeds through the input circuit and then takes one of
three routes to the main meter range attenuator/preamplifier. In the
'Level-dBm' mode, the signal feeds directly to the attenuator, in the
'Level-rel', 'Distortion-Cal' and 'Distortion-Read' modes the signal feeds
through the coarse SET REF attenuator, SET REF amplifier and fine SET REF
attenuator, thence directly to the main attenuator on 'Level-Rel' and
'Distortion-Cal' and via the automatic frequency notch filter on 'Distortion-
Read'.
In all four modes signal path is common from the main meter range attenuator/
preamplifier to the meter. The signal level is adjusted in the main attenuator
and preamplifier and thence to the meter amplifier directly or via the selected
filter/s. The output from the meter amplifier then feeds to the output on the
front of the instrument and to the rms to d.c. converter, the output of which
drives the meter.
4.2 CIRCUIT DESCRIPTION excluding auto notch filter
Refer to Circuit Drawing 70542-0-52.
4.2.1 Input Circuit
Capacitors Cl, C2 block d.c. from the input circuit BUT DO NOT BLOCK d.c. FROM
TERMINATING RESISTOR, hence d.c. should not be applied across input when using
terminated input.
The maximum safe d.c. level unterminated is limited to ±60 Volts.
Input mode is selected by switch SWA. Balanced signals are passed via a
1:1 transformer (TRI). For meter range settings of +20 dB and +30 dB a 30 dB
pad is inserted in the balanced mode. This reduces the distortion generated
by the transformer. The centre tap of the pad is a.c. grounded in order to
maintain longitudinal suppression. If longitudinal suppression for high
levels is unimportant this capacitor may be removed.
4.2.2 Attenuator & Preamplifier
Refer also to Level Diagram & Attenuator Control Circuit 70542-3-56. The level
diagram shows for each input level the corresponding attenuation in the
attenuator and the gain setting of the preamplifier. The attenuator has a
constant 100 kn input impedance. Transmission gates ICl select either 30 dB,
f
50 dB, 60 dB or 80 dB attenuation. As the transmission gates are in ·series with
a high load impedance (100 kn) their on-resistance variation has negligible
effect on accuracy. Transmission gates IC2 are used to prevent attenuator
feedthrough. For meter ranges of -40 dBm and above transmission gate "6" is on
and
"12" is off and thus preventing relay (RLD) feedthrough. The preamplifier
is a three-stage low noise amplifier. Q3 serves as a dynamic load for Q2 to
attain high gain. Overall negative feedback, selected by RLA, RLB and RLC sets
a gain from 6 dB to 36 dB in steps of 10 dB.

4/2 1.·70542R
4.2.3
Program Filter
The LC-network shapes the frequency response to conform to the C.C.I.T.T.
~ecommendation Jl6 Annex Geneva, 1972. Feedback pair QS, Q6 is used to
achieve an overall gain of +12.2 dB at 6.3 kHz. The exact gain is set by
RV40l. For program filter characteristic refer. to DWG 70550-3-26.
4.2.4
20 kHz Hi-Cut Filter
This filter is a five-pole active low-pass Chebyshev design. The first
section contributes a complex pole and uses a complementa~y emitter follower
with near unity gain. The second section provides a real pole as well as a
complex
pole pair. The roll-off meets I.E.C. noise filter specification with
fm = 16000 Hz. For Hi-Cut characteristic refer to DWG 70550-3-26.
4.2.5 400 Hz Lo-Cut Filter
This filter is a three-pole active high-pass Chebyshev design. It uses an
operational amplifier to provide a unity gain stage. For high-pass
characteristics refer to DWG 70550-3-26.
4.2.6 Meter Amplifier & 120 kHz Filter
This filter is a three-pole elliptical low-pass design. It attenuates
out-of-band high frequency noise. The meter amplifier is a multistage,
wideband negative feedback amplifier. An input differential stage is
followed by two voltage gain stages and an output emitter follower. Ql7
serves as a current source for Ql6. L2 and C62 are used in the balanced
mode to compensate for the high frequency response of transformer Tl.
4.2.7
RMS-DC Converter
IC3 and associated components function as a full-wave rectifier. The four
transistors of res square the absolute value input signal and divide this by
the output d.c.-signal. Zener diode D41 is used to protect the meter against
heavy overloads.
4.2.8 SET REF Attenuators and Amplifier
The coarse SET REF control is used to provide from 20 dB of gain to 30 dB of
attenuation in 10 dB steps to set the reference level in conjunction with the
fine SET REF control RV301. The SET REF amplifier is a low distortion, high
input impedance amplifier circuit with switchable gains of 23, 13 and 3 dB.
Transistor Q302 is operated at constant collector-base voltage due to Q301
thereby minimising any source-impedance induced distortion effects. Q302
provides additional loop gain and Q304 and Q305 form a compound emitter
follower. Feedback from Q305 emitter to Q302 emitter sets the amplifier gain.
4.2.9 Power Supply
The power supply circuit consists of a +12 V series regulated supply IC8 and
a
-12 V series regulated supply IC9. Both supplies are internally protected
against short circuits and thermal overloads.
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