HP 8340A, Service Manual

Page: 232 / 326

Model 8340A - Service
A48 YO LOOP SAMPLER
INTRODUCTION
The A48 YO Loop Sampler mixes the output of the A44 YO (via the
A45 Pre-leveler and A46 7 GHZ Low Pass Filter) with the Nth
harmonic of the output of the M/N-Reference Loop (M/N IN). The 20
to 30 MHz difference signal, SAMPLER IF, is output to the A49 YO
Loop Phase Detector and is compared with the 20-30 MHz output
from the A36 PLLl
vco
assembly for the purpose of phase-locking
the YO.
A48 YO LOOP SAMPLER CIRCUIT DESCRIPTION
Sampler Drive Amplifier A
The output of the M/N-Reference Loop (M/N IN) is applied to
common-base amplifier Q3. The output of Q3 is AC coupled to
common-emitter amplifier Q8. The output of Q8 is passed through
an impedance matching network that provides maximum drive power
to A48Ul Sampler. Adjustments Cl and C2 optimize this impedance
match.
A48Ul Sampler B
A48Ul Sampler contains a step recovery diode (SRD) circuit to
create harmonics of the M/N signal which are mixed with the
low-level (-15 dBm) signal from the A44 Yig Oscillator via the
A46 7 GHz LOW Pass Filter (LPF).
When the YO Loop is phase-locked, the m1x1ng product of the Nth
harmonic of the M/N signal and the A44 YO signal is precisely
equal to the 20-30 signal from the A36 PLLl
vco assembly. This
allows the YO Loop to become phase-locked to that harmonic.
IF Preamplifier C
The IF Preamplifier consists of common-source amplifier Q4,
common-emitter amplifier Q2, and feedback divider R20/Rl6.
Overall gain provided is approximately 14 dB.
•
Buffer Amplifier D and
70 MHz
LPF E
The A48Ul Sampler output, after being amplified, is buffered by
emitter-follower Q7 and applied to the 70 MHz Low-Pass Filter
(Block
E).
This filtering is done to remove any unwanted signals
produced by the mixing action of the sampler.
I
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Scans by HB9HCA and HB9FSX