HP 35601A Operating And Service Manual page 95

Model 35601A
Service
SERVICE GROUP ONE
A3 BOARD: LOW FREQUENCY INTERFACING
6-33. INTRODUCTION TO THE A3 BOARD
The -hp- 35601 A3 board provides the mixdown for the 3585A IF Output. This is ac­
complished by mixing the 350kHz IF signal from the -hp- 3585A with either a 350kHz or a
370kHz signal synthesized by the A3 board. The output is either DC or 20kHz and is ana­
lyzed by the -hp- 3582A.
6-34. The A3 Board in the Narrow Band Analysis Mode
In the Narrow Band Analysis mode, the A3 board is configured as follows: The 350kHz IF
from the -hp- 3585A is bandpass filtered before being mixed with 370kHz to generate
20kHz. The 20kHz is then amplified and analyzed on channel B of the -hp-3582A. The
370kHz is synthesized from the -hp- 3585A 10MHz oven reference.
6-35. The A3 Board in the AM/PM Analysis Mode
In the Side Band Analysis Mode, the -hp- 3585A 350kHz IF output is used as a reference for
a phase locked loop on the A3 board. The IF is amplified and mixed with 350kHz. The mix­
ers output is amplified and filtered to control a 10MHz VCXO (voltage controlled crystal
oscillator). The loop is completed by dividing the 10MHz VCXO to 350kHz. An integrator
can be switched into the loop, forcing the mixer output to a nominal zero volts DC. The
signal from the mixer represents the phase fluctuations of the IF signal. The signal can then
be spectrum analyzed with the -hp- 3582A. The 350kHz driving the mixer is also 90 degrees
phase shifted and used to drive a second mixer. The output of the second mixer represents
the amplitude fluctuations on the IF signal; these fluctuations are analyzed on channel A of
the -hp- 3582A.
6 36. DETAILED DESCRIPTION OF THE A3 BOARD
6-37. Input Section
There are two channels in the input section of the A3 Board. One is an amplified channel
with a gain of 2.5. The other channel is a bandpass filter section which consists of four
350kHz bandpass filter stages. The bandpass filter provides image rejection and amplitude
compensation for the roll-off of the -hp- 3585A IF signal. The amplitude compensation is
accomplished by peaking each 350kHz section at about ±5kHz from their centers. At the
front of each channel is discrete diode switching.
6-38. Mixer Section
There are two ring diode mixers, one for AM, the other PM. The switching signal for the
AM mixer is 90 degrees out of phase with the switching signal for the PM mixer. The outputs
of the mixers are fed into separate 50kHz lowpass filters.
. 6-49