HP 8340A Service Manual page 202

SWEEP GENERATOR/YO LOOP - OVERALL DESCRIPTION
The Sweep Generator and YO Loop circuitry generate the signals
used to tune the YO according to the mode of operation selected.
The assemblies included in this section are the A44 YO,
Preleveler, A46
Assembly,
A48 YO Loop Sampler,
Loop Interconnect Board,
A55 YO Driver Assembly,
diagram of the sweep Generator/YO Loop is shown in Figure 8D-l.
The inputs to this section come from the instrument processor,
the 20-30 Loop,
Output. Other important outputs are PRETUNE,
and the SYTM; BVSWP
Level Control board;
sweep widths <5MHz;
sweep events.-
The operation of each block can best be described by considering
the two main instrument modes of operation:
Frequency. The swept mode can be in one of three conditions:
single band sweep,
CW/MANUAL SWEEP OPERATION
When the CW mode is selected,
Circuitry phase-locks the YO at the appropriate frequency. The
processor addresses and sends a number to the A54 YO Pretune
Delay Compensation board which is converted to a voltage,
and summed with the constant DC voltage,
PRETUNE voltage,
frequency (ie, to within the capture range of the YO Phase Locked
Loop). VSWP and LKICK have no contribution to the Pretune voltage
in CW operation.
The processor also sets the frequencies of the M/N and 20-30
Loops according to the CW frequency selected. The outputs of
these two loops are used by the YO Loop to exactly tune and
phase-lock the YO. The A45 Preleveler couples a portion of the YO
RF Output back through a low-pass filter to the A48 Sampler,
where it is mixed with the Nth harmonic of the M/N Output. This
generates a difference signal,
MHz which is input to the A49 YO Loop Phase Detector. There the
phase is compared to the 20-30 MHz Output signal. The resultant
error signal, YO TUNE,
frequency portion of YO TUNE
Model 8340A - Service
SWEEP GENERATOR/YO LOOP
THEORY OF OPERATION
7 GHz Low Pass Filter,
A49 YO Loop Phase Detector,
A54 YO Pretune Delay Compensation board,
and A58 Sweep Generator board. A block
and the M/N Loop. The main output is YO RF
(Buffered Voltage SWeeP),
20-30 SWP,
and MKR RAMP, used as the reference for all
multiband sweep,
the sweep Generator/YO Loop
which tunes the YO to approximately the desired
Sampler IF,
is used to phase-lock the YO. The high
(>100 Hz) is applied to the YO FM
Scans by HB9HCA and HB9FSX
A47 Sense Resistor
used to tune the YO
used on the A27
used to sweep the 20-30 Loop for
CW/Manual,
or narrow band sweep.
VREF. This produces the
in the range of 20-30
A45
A50 YO
and Swept
VDAC,
•
8-279