HP 8340A Service Manual page 208

YO LOOP DESCRIPTION
Introduction
The assemblies
(documented in this functional group)
comprise the YO Loop are A44 YO,
Pass Filter, A48 YO Loop Sampler,
YO Loop Interconnect,
YO Driver.
In the CW and Manual sweep instrument modes,
phase-locks the YO at the appropriate frequency. The information
to tune the YO comes from two sources,
combine to form the YO FM COIL DRIVE and the YO MAIN COIL DRIVE.
Pre tune
The PRETUNE voltage is generated on the A54 YO Pretune/Delay
Compensation board. It is the scaled sum
a tune voltage generated from inputs from the instrument
processor, and VREF,
sensitivity of -2.5V/GHz YO.
LKICK and VCOMP have no contribution to PRETUNE in the CW/Manual
sweep instrument modes. LVSX
to shunt out any unwanted noise contribution from the VSWP line.
YO Tune
The YO TUNE voltage is generated by the A49 YO Loop Phase
Detector from the 20-30 MHz input
Sampler IF, which is the down-converted product of the YO RF
OUTput and the Nth harmonic of the M/N OUTput. The A45 Preleveler
couples a portion of the YO Output back through the A46
Pass Filter to the A48 YO Loop sampler. There it is mixed with
the Nth harmonic of the M/N Output to produce the Sampler IF,
which is in the frequency range of 20-30 MHz. This signal and the
one from the 20-30 Loop are compared in the Phase/Frequency
Detector (A49, Block
resulting error signal is amplified and integrated to produce YO
TUNE, with a sensitivity of -3 MHz/Volt. When HLEY
Enable Yig Oscillator)
is LOW, the Sample And Hold
phase-locked value of YO TUNE,
voltage ramp (VSWP)
Model 8340A - Service
A45 Pre-Leveler,
A49 YO Loop Phase Detector,
A54 YO Pretune/Delay Compensation,
a constant de offset voltage. PRETUNE has a
(Low Voltage sweep disable)
(from the 20-30 Loop)
D) of the A49 YO Loop Phase Detector. The
is HIGH, the YO Loop is closed. When HLEY
(A49, Block
thus breaking the loop to allow a
to be summed into PRETUNE to sweep the YO.
Scans by HB9HCA and HB9FSX
that
A46 7 GHz Low
the YO Loop
PRETUNE and YO TUNE,
(A54, Block C)
(High Lock
B) holds constant the
A50
and A55
which
of VDAC,
is LOW
and the
7 GHz Low
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