
Model 8640B
TM 9-4935-601-14-7&P
Overall Block Diagram
SERVICE SHEET 1
SERVICE SHEET 2
PRINCIPLES OF OPERATION
R F Oscillator (Service Sheet 5)
The full frequency range of the RF Oscillator is 230 to 550 MHz (nominally 256 512 MHz). The oscillator uses a single high-frequency transistor in a foreshortened
cavity. Frequency is controlled by varying the capacitive loading of the cavity. The oscillator drives two output amplifiers. The Frequency Counter Buffer Amplifier
drives the frequency counter; the Divider/Filter Buffer Amplifier drives the dividers which drive the amplitude modulating and leveling circuits. The oscillator can be
disabled by the RF ON/OFF switch which interrupts its +20V supply. The oscillator's cavity has two varactor diodes that allow the capacitive loading to be varied by
voltages (at the anode and cathode) to provide FM and phase lock.
FM Circuits (Service Sheets 6, 7, and 8)
The RF oscillator's varactor cathode is driven by the FM Amplifier which provides accurate amplification or attenuation of the modulation signal and shapes the signal
to compensate for the non-linear characteristics of the varactor diodes.
Separate shaping circuits are used for positive and negative voltage excursions. The PEAK DEVIATION switch, which controls basic FM amplifier gain, is
mechanically linked to the RANGE switch since, for a given amount of peak deviation, the percent deviation (i.e., the amount of deviation relative to the carrier
frequency) changes as the frequency range is changed. Also, as the frequency is tuned, the FM deviation changes. An FM Gain Compensation circuit with a
potentiometer, which is geared to the FREQUENCY TUNE control, adjusts for the change in FM sensitivity with tuning.
Inputs to the FM circuits are routed through the FM switch. In the CAL position, an accurate 1 Vdc is applied to the FM input. External inputs are applied in AC and
DC, and an internal modulation signal in INT. The PEAK DEVIATION vernier adjusts the input level into a unity gain Buffer Amplifier. In addition to driving the FM
amplifier, the Buffer Amplifier drives the Over-Deviation Detector and the Meter Attenuator. In the event that the input signal exceeds ±1.1V, the Over-Deviation
Detector turns on the REDUCE FM VERNIER lamp.The Meter Attenuator scales the input signal to the meter circuits in such a way that a 1 Vpk input corresponds to
the deviation selected when read on the meter.
Modulation Oscillator (Service Sheets 9 and 9A)
Internal AM and FM is provided by the Modulation Oscillator. The oscillator drives either the AM modulation circuits and AM OUTPUT port or the FM modulation
circuits and FM OUTPUT port or all four. The oscillator is enabled whenever either the AM or FM switch is in INT.
The standard modulation oscillator (shown on Service Sheet 9) has two fixed frequencies 400 Hz and 1 kHz. The oscillator supplied with Option 001 (shown on
Service Sheet 9A) has in addition five variable frequency ranges covering from 20 Hz to 600 kHz.
Power Supplies and Fan (Service Sheets 22 and 23)
The instrument has five regulated supply voltages, +44.6V, +20V, -20V, +5.2V, -5.2V. All supplies are protected against overloading, over voltage, and reverse
voltage. An LED annunciator on each supply indicates proper operation when on. The cooling fan is driven by a dc brushless motor controlled by the Fan Driver
circuits.
TROUBLESHOOTING
It is assumed that a problem has been isolated to the FM circuits and RF oscillator 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 listed in the box at the right-hand side of the diagram. To check a voltage at a test point, change the control settings as specified in
the box associated with that test point, check the voltage, then reset the controls to
8-20
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