Model 8640B TM 9-4935-601-14-7&P
SERVICE SHEET 1
PRINCIPLES OF OPERATION
General
The Hewlett-Packard Model 8640B Signal Generator is
a mechanically-tuned, solid-state RF generator. The
frequency range is 0.5 to 512 MHz covered in ten
octave bands. The output is leveled and may be
continuously varied over an 18 dB range or attenuated
in 10 dB steps from +19 to -145 dBm. In addition,
calibrated AM and FM (either internal or external) and
external pulse modulation are provided. The RF output
frequency is read on an internal counter which may also
be used to count external signals up to 550 MHz or to
synchronize the generator to a stable reference
oscillator.
FM Circuits and R F Oscillator (Service Sheet 2)
The RF source is a 256 to 512 MHz cavity-tuned
oscillator that is mechanically tuned by the
FREQUENCY TUNE and FINE TUNE controls. The
oscillator can also be electrically tuned over a smaller
range by the FM and the counter/lock circuits. The FM
circuits amplify and shape the modulation input to
provide linear, calibrated frequency modulation. The
phase lock circuits tune the oscillator to phase lock it to
a reference. FM inputs can be either external (ac or dc
coupled), internal from the modulation oscillator, or an
accurate 1 Vdc useful for FM calibration.
AM/AGC Circuit and Output Amplifier (Service Sheet
3)
The RF oscillator drives the RF dividers (a chain of
binary dividers) which divide the RF for the lower nine
frequency ranges. The RF filters remove the harmonics
from the RF signal.
The AM/AGC circuits form a feedback system to control
the amplitude of the output and to provide AM or pulse
modulation. The detector senses the level of the RF
signal from the RF output amplifier. A summing
amplifier compares the detector output against an input
reference and drives the modulator. The modulator acts
as a current controlled attenuator to control the RF level.
The reference to the summing amplifier consists of the
level reference, which comes from the output level
vernier, and the modulation signal, if present. The
modulation signal can be either external (ac or dc
coupled) or internal (from the modulation oscillator). In
the pulse modulation mode, external modulation pulses
switch the modulator off and on. Amplitude leveling is
maintained in this mode by storing the detector output
between pulses.
The 10 dB RF step attenuator further controls the output
level. The meter circuits monitor either the detector
output (and hence the output level), the positive peak of
the AM modulating signal (calibrated to given % AM), or
the positive peak of the FM modulating signal
(calibrated to give peak deviation).
Counter/Lock Circuits (Service Sheet 4)
In the internal count mode, the counter always counts
the 256 - 512 MHz signal from the RF oscillator. The
time base period is adjusted to give the correct
frequency reading for the difference frequency ranges.
In the external count modes, external input signals are
counted directly. In the phase lock mode, the counter
compares the count of the RF signal against the count
just before acquisition of phase lock and adjusts the
frequency of the RF oscillator to make the counts
coincide. The counter time base reference may be
either the internal or an external 5 MHz.
TROUBLESHOOTING
Use this overall block diagram to isolate the trouble to a
specific section of the instrument. Then turn to the
troubleshooting block diagram that covers that section
of the instrument and use the information on the
diagram to isolate the trouble to the defective assembly.
Next, turn to the Service Sheet that covers that
assembly and isolate the trouble to the defective
component or replace the assembly.
For example, suppose the AM functions are out of
specification. The block diagram on Service Sheet I is
keyed to the troubleshooting block diagrams that follow
it - in this case, Service Sheet 3. Service Sheet 3 gives
a list of generator control settings (the list is located in
the box on the right-hand side of the sheet) and the
voltages and waveforms that should be found at the test
points and along the signal paths. 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 the settings specified
in the box on the right-hand side.
8-18
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