
Model 8640B
TM 9-4935-601-14-7&P
SERVICE SHEET 17
PRINCIPLES OF OPERATION
General
Front panel meter M1 indicates one of three quantities selected by Meter Function
switch A2S1. For each function, the metering circuitry performs the following:
1) AM percent modulation (X10%): The ac component of the modulation signal
from MODULATION potentiometer R2 is peak detected and amplified. One
range of autoranging is provided at 30% maximum. Logic circuitry selects the
appropriate amplifier gain and meter scale lamp.
2) FM peak deviation (kHz or MHz): The ac component of the modulation signal
from the Meter Attenuator section of PEAK DEVIATION switch is peak detected
and amplified. No autoranging is provided. The scale lamp is selected by the
PEAK DEVIATION switch.
3) LEVEL, the RF output level (VOLTS or dBm): A positive dc voltage proportional
to the AGC voltage of the A26A4 AGC Amplifier Assembly is amplified. One
range of autoranging is provided at 30% maximum. Logic circuitry selects the
appropriate amplifier gain and meter scale lamp.
The meter has three linear scales (O - 5, 0 - 10, and 0 - 3 or actually 0 - 3.16) with three
lamps to indicate the appropriate scale. The lamps are located on the A6 Annunciator
Assembly. The meter also has a log scale calibrated in dBm 50SI for use in the LEVEL
meter mode.
Positive Peak Detector (A2)
The Positive Peak Detector samples the ac peak of the incoming signal and stores the
voltage on capacitor C5. The AM or FM input signals are ac coupled into Buffer
Amplifier U1 by capacitor C1 and resistor R4. U1 is internally connected as a voltage
follower. Resistor R3 provides input bias current for U1 and presents a constant load
impedance to the inputs.
Amplifier U2 is a voltage comparator. When the input voltage at pin 4 exceeds the
voltage at pin 3, the output rapidly switches to a level equal to the voltage across C5
(which is also connected to pin 3) plus the voltage drop across the forward-biased diode
CR1. The amplifier charges C5 until the voltage at pin 4 equals the voltage at pin 3,
and maintains this condition until the voltage at pin 4 drops. The amplifier output then
swings to a maximum negative value (about -8V), CR1 reverse biases, and C5 slowly
discharges until the voltage at pin 4 again exceeds that at pin 3. Thus the positive peak
value of the input voltage is stored on C5. Resistors R7 and R10 discharge C5
whenever the input signal level is reduced slightly between input peaks. Resistor R11
adds a small amount of gain to the detector.
Meter Drive Amplifier (A4)
Meter Drive Amplifier U1 scales the dc input voltage and drives meter M1. Transistor
Q1 is a switch which operates in the inverted mode (i.e., the emitter functions as a
collector and the collector as an emitter). When Q1 is OFF, the amplifier gain is 10,
when Q1 is ON, the gain is 31.6. With an amplifier gain of 10 and an input of977
mVdc, the meter (which has a nominal 1 mA full-scale movement) is adjusted to read
full scale (on the 0 - 10 scale) by means of R19. Breakdown diode VR2 protects the
meter from being overdriven or driven negative.
A 1 Vpk signal at the peak detector gives 1 Vdc at the input of Meter Drive Amplifier
A4U1. In the AM meter mode, a 1 Vpk input signal corresponds to a full-scale meter
deflection. Similarly, in the LEVEL mode a 1 Vdc input voltage corresponds to a full-
scale deflection. However, in the FM mode a 1 Vpk signal from the PEAK DEVIATION
switch on some ranges requires slightly more than a full scale deflection (e.g., 320 kHz
on the 3.16 scale). A 0.977 Vdc input to the Meter Drive Amplifier gives a full-scale
meter deflection.
Autorange Comparator (A4)
Amplifier U2 functions as a Schmitt Trigger. A reference voltage is established at pin 3
by voltage divider R3 and R4. Resistor R5 adds a small amount of hysteresis to the
comparator. Ripple at the input pin 2 is reduced by R7 and C2.
SERVICE SHEET 17 (Cont'd)
Logic Circuitry (A4)
The logic gates control the meter amplifier gain and scale lamps (except for FM) as outlined in Table 8-5. Whenever OUTPUT LEVEL switch A1S1AF is in an
odd range, the ODD RANGE CODE line is grounded. The ranges on the switch are numbered consecutively from 1 to 16. The highest output range is 16
which corresponds to a maximum output level of 3.16V. For range 15 the maximum output level is 1V; for range 14, 0.316V; etc. When the LEVEL mode is
selected, the 0 - 10 scale lamp is turned on by Q2 when the range is odd and the input above the autorange reference; or the 0 - 3 scale lamp is turned on by
Q3 when the input is below the autorange reference. When the range is even, the lamp sequence is reversed; the reversal is accomplished by Exclusive-OR
Invert function formed by gates U3A, U3B, U3C, and U4B.
In the AM meter mode the logic conditions are the same as for the LEVEL mode on an odd range. In both the AM and LEVEL modes the Autorange
Comparator and AND gate U4A turn on Q1 when the input to the Meter Drive Amplifier is below the autorange reference and turn off Q1 when above it.
Table 8-5. Meter Drive Amplifier Data
Input Below Autorange Reference
Input Above Autorange Reference
Meter
Odd/
Test
Test
Test
Norm.
A4U1
Test
Nom.
A4U1
Mode
Even
Point
Point
Point
Volts
Gain
Scale
Q1
Q2
Q3
Point
Volts
Gain
Scale
Q1
Q2
Q3
Range
6
2
3
In
3
In
AM
Either
L
H
H
0 - 0.3
31.6
0 - 3
ON
OFF
ON
L
0.3-1
10
0 - 10
OFF
ON
OFF
FM
Either
L
L
H
*
*
**
OFF
OFF
OFF
L
0-1
10
**-
OFF
OFF
OFF
Odd
L
H
H
0 - 0.3
31.6
0 - 3
ON
OFF
ON
L
0.3-1
10
0 -10
OFF
ON
OFF
LEVEL
Even
H
H
H
0 -0.3
31.6
0 -10
ON
ON
OFF
L
0.3-1
10
0 - 3
OFF
OFF
ON
NOTES:
* No autoranging in FM mode
** Scale lamps determined by PEAK DEVIATION switch
L = 0 to 0.8 Vdc
H = 3 to 5.2 Vdc
Meter Circuits Troubleshooting (1 of 2)
Test Conditions and
Component or Circuit
Conditions
Normal Indication
If Indication is Abnormal
Control Settings
BUFFER AMPL (A2U1)
Initial conditions and settings.
2 Vp-p (1 Vpk) at U1 pin 6
Check U1 and associated circuitry
Adjust MODULATION for a
2 Vp-p (1 Vpk) signal at TP3
(AC IN).
POSITIVE PEAK
Initial conditions and settings.
1 Vdc at TP2 (DC OUT)
Check U2, CR1, C5 and associated
DETECTOR (A2U2)
Adjust MODULATION for a
circuitry
2 Vp-p (1 Vpk) signal at TP3
(AC IN).
AUTORANGE
Initial conditions and settings.
4.5 Vdc at TP3 (A)
Check U2 and associated circuitry
COMPARATOR
Adjust MODULATION for
(A4U2)
250 mVdc at TP4 (DC IN).
Adjust MODULATION for
-100 mVdc at TP3 (A)
350 mVdc at TP4 (DC IN)
AMPL GAIN LOGIC
Initial conditions and settings.
4.0 Vdc at U4A pin 3
Check U4 and associated circuitry
(A4U4A)
Adjust MODULATION for
250 mVdc at TP4 (DC IN).
Adjust MODULATION for
50 mVdc at U4A pin 3
350 mVdc at TP4 (DC IN)
Set Meter Function to FM
50 mVdc at U4A pin 3
RF Vernier (A1A1, A26)
SERVICE SHEET 16
SERVICE SHEET 17 (Cont'd)
TROUBLESHOOTING
It is assumed that a problem has been isolated to the
meter circuits as a result of using the troubleshooting
block diagrams. Troubleshoot by using the test equipment
listed below, performing the initial test conditions and
control settings, and following the procedures outlined in
the table.
Test Equipment
Digital Voltmeter..................................... HP 3480B/3484A
Oscilloscope.................................. HP 180A/1801A/1820C
Initial Test Conditions
Top cover, trim strip, and front panel window removed
(see Service Sheet F for removal procedure). Use
extender board to extend desired circuit board assembly
(set instrument LINE power switch to OFF while removing
or inserting circuit boards).
Initial Control Settings
Meter Function ............................................................. AM
AM ..............................................................................INT
MODULATION FREQUENCY ................................... 1kHz
FM ..............................................................................INT
PEAK DEVIATION ................................................... 5 kHz
OUTPUT LEVEL ..................................................... 0 dBm
.................................................. (2 steps ccw from full cw)
RF ON/OFF ................................................................. ON
Figure 8-51. A2 Meter Switch/Detector Assembly Component
Locations
8-52
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