Model 3585A
Circuit Functional Descriptions
6-27. TRACKING GENERATOR DESCRIPTION (Service Group El
The Tracking Generator simply produces a sine wave at the frequency at which the -hp-3585
is tuned. In other words, the Tracking Generator produces frequencies from 0 to 40 MHz.
The obvious way to do this is to simply take the LO signal of 100.35 to 1 40.35 MHz and sub
tract (mix) a 100.35 MHz from it. This is in fact what is done; however, the circuit is slightly
more complex than what we have implied. Because the IF section of the -hp-3585 drifts very
slightly with time, the calibration system takes this into account by adjusting the LO slightly
so that it matches up with the IF section. If the LO was then used by the Tracking
Generator, the results would be erroneous. What happens is that during the calibration cy
cle, the Tracking Generator is offset slightly to match up with the IF section and the LO.
The A5 1 board begins with two latches and two DAC's through which the 10.35 MHz
VCXO reference can be controlled during the calibration cycle. This is done so that the
Tracking Generator will truly follow the -hp- 3585 's frequency. The DAC is separated into
two parts, a coarse tune and a fine tune. During the calibration cycle, the bits of the two
DACs are set one at a time until the Tracking Generator is matched up with the IF peak . A
-:- 100 circuit causes the actual fine tune signal path.
The DAC output voltages are summed to control the frequency of oscillation of the 10.35
MHz VCXO. A 90 MHz signal from A21 and a 100.35 MHz signal from A52 are mixed to
produce a 10.35 MHz signal. The two 10.35 MHz signals are then phase detected, their phase
difference causing a pulse which is integrated to form the VCO Control Voltage Out signal.
The VCO control voltage enters the A52 board and then the A53 board where it becomes the
tuning voltage for the 100.35 MHz VTO. An amplifier stage provides amplitude control for
the Tracking Generator output. The LO signal enters the A52 board and is mixed with the
100.35 MHz signal from the A53 to yield a signal from 0 to 40 MHz. This signal is low pass
filtered and then given 20 dB of gain before being output to the front panel.
During the calibration cycle, both the Tracking Generator signal and a 10 MHz signal are
needed at different times . The A52 board, therefore, contains an active switch network
which allows one or the other through when needed. An output amplifier with a gain of
about three is included as the final stage. Note that when the (L)CAL goes high, the output
amplifier is turned off allowing neither of the signals throught to the input section (AI).
6-35/6-36
Scans by ARTEK MEDIA =>
