138 Keysight NFA Series Noise Figure Analyzers Service Guide
RF Section Troubleshooting (N8973B, 74B, 75B Analyzers)
RF Section Description
RF Section Theory of Operation
A9 Input Attenuator A
This assembly has two 2 dB attenuator sections, a DC block and a cal signal
input port. With the DC block switched in (AC coupled mode), the low end
minimum frequency range increases from 9 kHz to 10 MHz due to capacitive
effects.
A10 Input Attenuator B
This assembly has a total of 66 dB of attenuation to control the RF level going
to the mixer assemblies in the A13 RF Front End Assembly. There is a 6 dB, 10
dB, 20 dB, and 30 dB section in the A10 assembly. Total input attenuation with
the A9 and the A10 combined is 60 dB in 10 dB steps (2 dB steps if Option FSA,
Fine Step Attenuator, is installed).
A11 Low Band Switch
This assembly operates across the entire frequency range of the analyzer. The
switch has 3 main functions:
— Switch RF input frequencies from 9 kHz to 3.59 GHz RF Output to A13 RF
Front End Assembly.
— Switch RF input frequencies from 3.6 GHz to 26.5 GHz Microwave RF
Output to A12, YTF Preselector.
— Optional high band preamplifier, which has a typical gain of 30 dB and a
noise figure from 9-12 dB up to 18 GHz.
A12 YTF Preselector
The YTF Preselector is used for input frequencies greater than 3.6 GHz. Signals
less than 3.6 GHz bypass the YTF assembly and are routed directly to the A13
RF Front End Assembly. The YTF Preselector is a YIG tunable bandpass filter
whose main purpose is to filter out unwanted spurious or image response
signals. When the YTF is calibrated, the tunable bandpass will track the RF
input signal. The YTF Preselector has a 3 dB bandwidth greater than 40 MHz
and an insertion loss of ~ 6.5 dB.
The preselector requires centering for optimum amplitude accuracy. The Auto
Align routine will perform a rough centering during the preselector two-point
tuning algorithm. However, when troubleshooting, press Amplitude, Signal
Path, Presel Adjust to manually center the preselector.
The YTF Preselector is not present in the N8973B.
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