
Model 8640B
TM 9-4935-601-14-7&P
SERVICE SHEET 3
PRINCIPLES OF OPERATION
Divider/Filters (Service Sheets 10 and 1 1)
Except for the 256 512 MHz (and the doubler) frequency range,
the RF signal from the Divider/Filter Buffer Amplifier (Service
Sheet 2) is routed through a series of binary frequency dividers
(i.e., 2) by slide switches on the filter section of the A10
Divider/Filter Assembly. The RF signal is divided to the
selected range.
The divided signal passes through the Modulator Preamplifier,
the Modulator, and then to the RF Filters. The filters remove
unwanted harmonics from the signal (which is approximately a
square wave after being divided). The upper frequency ranges
have two filters per range one for the lower half (Low Band
Filters) and one for the upper half (High Band Filters) of the
band. This is necessary to effectively remove the second
harmonic on the lower half of the band. The midpoint of the
band is sensed by a Schmitt Trigger which compares a
reference voltage to a voltage proportional to the frequency
tuning. On the four lowest frequency ranges the RF signal has
little second harmonic content because of good waveform
symmetry; therefore, each range has only one filter.
AM/AGC Circuits (Service Sheets 12 and 13)
The output of the RF Filters is amplified by the Output Amplifier.
The amplifier's output is peak-detected and buffered by the
Detector Buffer Amplifier. The detected voltage, which is
negative, is summed (in the Summing Amplifier) with a positive
AGC reference voltage from the OUTPUT LEVEL vernier. The
AGC reference may also have the amplitude modulation voltage
superimposed on it. The sum of the detector and reference
voltages is amplified by the Summing and Modulator Driver
Amplifiers. The Modulator Driver Amplifier supplies control
current to the Modulator which adjusts the RF output level.
In the pulse modulation mode, the Modulator Driver Amplifier is
switched on and off by input pulses from the Schmitt Trigger.
To maintain a constant detector voltage into the summing
amplifier, the peak detector's output voltage is sampled during
the RF-on period and then stored in the Sample And Hold
section of the Detector Buffer Amplifier when the RF is off. The
Pulse Overload Detector senses any large errors in the leveling
circuit which can occur when the OUTPUT LEVEL vernier is
reduced. In such a case, the hold function is defeated until
equilibrium occurs. The
FM Circuits and RF Oscillator Troubleshooting
SERVICE SHEET 2
Rate Detector senses pulses of low repetition rate and turns off the meter circuit when the rate is so low that the meter is
no longer accurate.
The Modulation Overload Detector senses when the AGC reference, the AM signal, or a combination of the two is
beyond the Modulator's capability to deliver power. The REDUCE PEAK POWER lamp is then turned on. The Meter
Amplifier produces an output voltage proportional to the detected output voltage (and hence the output level) to drive the
meter circuits. The AGC reference voltage originates in the AM Offset Amplifier where it is summed with any AM input
signal. The voltage out of the amplifier then passes through the OUTPUT LEVEL vernier to the modulation Summing
Amplifier. The Modulator can be disabled (i.e., maximum modulator attenuation) by the RF ON/OFF switch.
Meter Circuits (Service Sheet 17)
The meter can be set to measure either percent AM, peak frequency deviation'(FM), or output level. In measuring AM
and FM, the modulation signal is peak-detected by the Positive Peak Detector and amplified. For output level, the output
of the Meter Amplifier, which is proportional to the detector output, is amplified by the Meter Drive Amplifier. On both AM
and LEVEL, one range of autoranging is provided. The Autorange Comparator senses the autorange condition and
switches the gain of the Meter Drive Amplifier. Logic circuits control gain switching of the Meter Drive Amplifier and turn
on the proper scale lamps.
TROUBLESHOOTING
It is assumed that a problem has been isolated to the AM/AGC circuits and output amplifier as a result of using the
overall block diagram. Troubleshoot by using the test equipment and procedures specified below.
Test Equipment
Digital Voltmeter ............................... HP 3480B/3484A
Oscilloscope ............................HP 180A/1801A/1820C
Initial Test Conditions
Top and bottom covers removed (see Service Sheet F).
Procedure
Set the generator's controls as specified in the box at the right-hand side of the diagram. To check a voltage at a test
point, change the control setting as specified in the box associated with that test point, check the voltage, then reset the
controls to the settings specified in the box at the right-hand side.
The blocks are keyed, by the numbers located in their lower right-hand corners, to the Service Sheets that have the
circuit schematics.
NOTE
The last two foldouts in this manual have top and bottom internal views of the instrument that
show the locations of the test points, assemblies, and cables (all RF cables are accessible from
the bottom of the instrument).
NOTE
After repairs are complete, see Table 5-2 for appropriate post-repair tests and adjustments.
8-22
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