Keysight NFA Series Noise Figure Analyzers Service Guide 251
Analog/Digital IF Troubleshooting
25 MHz BW IF Section
Post Down Conversion Amplification and Prefiltering
The 22.5 MHz I.F. goes through a fixed gain amplifier. Then prefilters are
switched in and out. The signal then goes through a variable gain amplifier.
The prefilters provide four single-pole filters to limit the bandwidth of the
signal reaching the ADC on the A3 Digital I.F. assembly. There are five
different signal paths for the 22.5 MHz I.F. when using Signal Analyzer
swept mode:
1. LC wide
2. LC narrow
3. Crystal wide
4. Crystal narrow
5. Through path
The LC wide signal path is used when the analyzer Res BW setting is
180 kHz to 390 kHz.
The LC narrow signal path is used when the analyzer Res BW setting is
30 kHz to 160 kHz.
The Crystal wide signal path is used when the analyzer Res BW setting is
4.3 kHz to 27 kHz.
The Crystal narrow signal path is used when the analyzer Res BW setting is
<3.9 kHz.
The through path is used when the analyzer Res BW setting is 430 kHz and
greater in analyzer swept mode.
In IQ Analyzer mode or in Spectrum Analyzer mode with Sweep Type of
FFT, the analyzer span setting determines the pre-filter settings.
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