HP 3585A Service Manual page 201

Model 3585A
6·20. Reference Section (A2 l and AB 1 )
The A8 1 boad is simply an oven oscillator that produces a stable 1 0 MHz reference for use
internal to the instrument. The main part of this circuit is the oven oscillator. The oscillator
output is controlled by a "switch" that, when properly adjusted, allows time for the oven to
heat up before allowing the 1 0 MHz signal off the board. When the instrument is first turned
on, the heater in the oven oscillator draws a very high current from the 15Vl source, pulling
its voltage to something below 1 5 volts. A comparator circuit then switches the 10 MHz out­
put off. When the heater reaches proper operating temperature, it draws less current thus
allowing the comparator to close the switch.
It was stated eariler that the local oscillator section contains three PLL's. In fact, a fourth
PLL exist in the Reference Section on the A21 board. The basis of this board is a simple
PLL with a fixed
and then fed into a phase detector where it is compared with an external 10 MHz signal,
which usually comes from the A8 1 via a connector on the rear panel. The remaining circuitry
on this board is gating and level adjustment circuitry used to achieve the signals required
elsewhere in the instrument. There is also a
6·2 1 . Sum Loop (A22, A24, A25, A27 and A2B)
The Sum Loop provides the local oscillator output frequency of 100.35 to 1 40.35 MHz by
briJ;1ging together all three loops involved in mulitple loop operation. Thus the name, Sum
Loop. You should note that the Sum Loop is not operational during single loop operation,
with the exception of the A22 board which is the first LO VCO during single loop operation.
A22 begins with an oscillator whose frequency is determined by the First LO VTO Tune In.
This tuning voltage determines the value of the vari-caps, which in turn determine the fre­
quency of the oscillation. The oscillator output then goes through a gain amplifier (approx­
imately 7 dB) and through two buffers. One output then goes to the Input Section (AI ) and
the other goes to the next board in the Sum Loop, A24, First LO Buffer.
A24 is the First LO Buffer and its purpose is to buffer the LO signal and split it into three
outputs. One buffered output (top buffer on the schematic) goes to the
the Fractional N Loop. Notice that this buffered output can be switched on for single loop
and off for multiple loop. The other two buffered outputs are always on. One of them pro­
vides a signal for the Tracking Generator (A52) and the other for the next board in the Sum
Loop, A25 , Sum Loop Mixer.
The A25, Sum Loop Mixer, board receives the 100.35 to 140.35 MHz LO signal from the
A24 board and the 98 to 1 38 MHz Step Loop signal as inputs. It buffers each input signal
and them mixes them to get a 1 .75 to 3.00 MHz result. It then runs the resultant signal
through a low pass filter and gain amplifier before it sends it on to the next board in the Sum
Loop, A28, Sum Loop Phase Detector.
The A28, Sum Loop Phase Detector, board receives the 1 .75 to 3 .00 MHz signal from the
A25 board and the 1 .75 to 3 .00 MHz signal from the Fractional N Loop as inputs. The input
from the A25 board is squared up in the shaper. The two inputs are then compared in a
phase detector whose output is a pulse with a width proportional to the difference in phase
between the two inputs. This pulse train then goes through a low pass filter and a buf­
fer/limiter to the next and last board in the Sum Loop, A27, First LO VTO Control.
-;-
9. The VCO is a 90 MHz crystal oscillator. Its output is divided by nine
Scans by ART EK MEDIA =>
Circuit Functional Descriptions
-;-
1 00 to get a 1 00 kHz output.
-;-
4 circuit on A3 1 of
6-1 9