HP 8340A, Service Manual

Page: 66 / 326

Model 8340A - Service
M/N LOOP DESCRIPTION
The M/N Loop generates the 177-197 MHz signal that drives the A48
Sampler in the YO Loop. Harmonics of the M/N signal are generated
in the Sampler and are used to down convert the 2.3-7.0 GHz YO
output to the 20-30 MHz range so that the YO can be phase locked
to the output of the 20-30 Loop.
The M/N Loop consists of three assemblies:
1. A31 M/N Phase Detector board
2. A32 M/N
vco
Assembly
3. A33 M/N Output board
The M/N Phase Detector Board contains a phase detector, a mixer,
and two programmable frequency dividers. (Refer to Figure 8B-l,
Reference and M/N Simplified Block Diagram) The mixer is used to
down convert the
vco output signal (from A33) from 355-395 MHz to
5-45 MHz by using the 400 MHz reference signal from A30 100 MHz
VCXO. This 5-45 MHz IF signal is then divided in frequency by the
M divider, one of the two identical programmable dividers on this
board. The M divider is programmed to divide by an integer ranging
from 8 to 27. The phase of the output of this divider is compared
to the output of the other programmable divider, the N divider, in
the phase detector whose differential output is used to tune the
vco. This tune voltage keeps the M/N Loop phase locked to the
output of the N divider. The N divider is programmed to divide the
20 MHz reference signal by an integer from 13 to 36.
The M/N vco is a foreshortened cavity resonator that is varactor
tuned from 355 to 395 MHz. The M/N vco assembly consists of the
cavity oscillator and a small �C board. This assembly is mounted
on A33 M/N Output Board. On A33, the output of the vco is split
two ways. One output is used for closing the phase locked loop,
and the other is used to generate the YO Loop sampler drive. The
phase lock path contains an amplifier that drives the down
converting mixer on the A31 M/N Phase Detector board. The YO Loop
Sampler path contains an amplifier and then a divide-by-2 circuit
that produces a Sampler Drive signal in the range of 177-197 MHz.
The YO Loop Sampler drive output of the M/N Loop (after being
divided by 2) can be related to the M and N numbers by the
following equation:
fM/N
=
200 - 10
*
(M/N) MHz
The Nth harmonic of this signal is used in the YO Loop Sampler
(A48) to down-convert the 2.3-7.0 GHz YO output to 20-30 MHz. For
every increment in M number, the YO output will decrease by 10
8-87
Scans by HB9HCA and HB9FSX