432 Chapter8
IF Section
A5 IF Assembly
A5 IF Assembly
The input switch connects the IF to either the cal oscillator on the A4
assembly or the 10.7 MHz IF output from the A15 RF assembly. The
automatic IF adjustment uses the cal oscillator on A4 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 spectrum 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 log amplifier. The reference 15 dB
attenuator also provides gain for changes in spectrum 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 spectrum 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.
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