A5 If Section - HP 8560A Service Manual

A5 IF Section

The input switch connects the IF to either the A4 Cal Oscillator or the 10.7 MHz IF output
from the Al5 RF assembly. The Automatic IF Adjustment uses the A4 Cal Oscillator at
instrument turn-on and between sweeps to align the IF filters and step-gain amplifiers. During
sweeps the input switch selects the 10.7, MHz IF output from A15.
The LC filters are variable-bandwidth filters that provide resolution bandwidths from 30 kHz
to 2 MHz. The Automatic IF Adjustment sets the bandwidths and center frequencies of each
filter stage.
The crystal filters are variable-bandwidth filters that provide resolution bandwidths from
300 Hz to 10 kHz. The Automatic IF Adjustment sets the filter bandwidths and symmetry.
The step-gain amplifiers consist of the first step-gain stage, second step-gain stage, and third
step-gain stage. These amplifiers provide gain when the analyzer reference level is changed.
The amplifiers also provide gain range to compensate for variations in the IF filter gains,
which change with bandwidth and environmental conditions, and band conversion loss in the
front end. Fixed-gain amplifiers shift the signal levels to lower the noise of the IF chain.
The assembly has two variable attenuators. The fine attenuator provides the 0.1 dB reference
level steps. The reference 15 dB attenuator provides a reference for automatic adjustment of
the step-gain amplifiers and the A4 Log Amplifier assembly. The reference 15 dB attenuator
also provides gain for changes in analyzer reference level.
Various buffer amplifiers provide a high-input impedance to prevent loading of the previous
filter pole and a low-output impedance to drive the next filter pole.
Digital control signals from the W2 Control Cable, the "analog bus," drive the control
circuitry. At the beginning of each sweep the analog bus sets each control line for instrument
operation. At the end of each sweep the analog bus sets each control line for the next portion
of the Automatic IF Adjustment routine. IF adjustments continuously remove the effects of
component drift as the analyzer temperature changes.
The assembly contains a reference limiting amplifier. This amplifier provides a known amount
of limiting for the Automatic IF Adjustment routines. (Limiting occurs only during the
Automatic IF Adjustment routines.) The LC34-Short switches are open during sweeps. The
current in the reference limiter is increased during sweeps to prevent limiting.
Caution
For troubleshooting, it is recommended that you use an active probe, such as
an HP 85024A, and another spectrum analyzer. If an HP 1120A Active Probe
is being used with a spectrum analyzer having dc-coupled inputs, such as the
HP 8566A/B, HP 8569A/B and the HP 8562A/B, either set the active probe
for an ac-coupled output or use a dc-blocking capacitor between the active
probe and the spectrum analyzer input.
Caution
Do not short control voltages to ground. These voltages are not short-circuit
protected. DACs damaged by shorting these voltages might not fail until
several weeks after the shorting takes place.
8-22 IF Section