Chapter 6 133
Troubleshooting the IF Section
A8 Analog IF Assembly Description
A8 Analog IF Assembly Description
Purpose. The A8 analog IF assembly receives a 21.4 MHZ signal from
the A10 3rd Converter. The 21.4 MHz signal is amplified, pre-filtered,
mixed with the 28.9 MHz fourth LO to provide a 7.5 MHz final IF, post
filtered, and amplified again.
The analog IF assembly also provides anti-alias filter protection, and
routes the 21.4 MHz signal, that is used in ranging, to the A7 digital IF
assembly.
The analog IF assembly performs the following functions:
• prefilters the 21.4 MHz third IF frequency
• uses a variable gain amplifier to provide the
optimum level to the mixer
• provides a 21.4 MHz anti-alias filter
• provides the final down conversion
• performs post down-conversion filtering
• provides calibrator oscillator and sample rate oscillator
• provides a clock generator
• performs frequency down conversion to 7.5 MHz
•triggering
• provides the power supply switching frequency generator
Pre-filters
LC and XTAL prefilters are used to attenuate the out of band signals
that are not in the measurement bandwidth, in order to improve TOI
distortion and spurious responses. The pre-filters are adjusted to 2.5
times the resolution bandwidth, except in RBWs less than 1 kHz and
greater than 1.2 MHz. If the resolution bandwidth is less than 1 kHz,
the XTAL prefilter bandwidth stays at 2.5 kHz. If the resolution
bandwidth is 1.2 MHz or wider, the prefilters are bypassed.
Main Gain Amplifier
Provides the following functions:
• Sets the gain between the 21.4 MHz input on the Analog IF, to the
A7 Digital IF Assembly output. This is accomplished during the
Auto Align routine by reading the power detector at the input of the
Analog IF assembly. Then, adjusting the main gain until the proper
signal level is achieved.
• When the prefilter BW is changed, compensates for amplitude
changes.
• When the frequency band is changed, compensates for conversion
loss.
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