136 Keysight NFA Series Noise Figure Analyzers Service Guide
RF Section Troubleshooting (N8973B, 74B, 75B Analyzers)
RF Section Description
1. RF input frequencies < 3600 MHz route through the low band path. Refer
to Chapter 12, “Block Diagrams” for details.
The RF input signal level can be optimized by either Input Attenuator A or
Input Attenuator B. The low band switch is biased in such a way that the
signal is routed to the input of the A13 RF Front End assembly. There are
different signal paths on the input of the RF Front End assembly that the
signal can be routed through depending on installed options and front
panel settings.
— Low Band Preamplifier path, 20 dB nominal gain (Option Pxx)
— Direct to Mixer #1
The RF input signal is then mixed with the 1st L.O. from the A14 L.O.
Synthesizer assembly in Mixer #1 to a 1st intermediate frequency of
5122.5 MHz. Since the RF input signal is <3600 MHz for low band, this RF
input signal is upconverted in Mixer #1 to the 1st I.F. This 1st I.F. signal is
then downconverted in Mixer #2 using the 4800 MHz 2nd L.O. generated
on the A16 Reference assembly to a 2nd intermediate frequency of
322.5 MHz (5122.5 MHz (1st I.F.) − 4800 MHz (2nd L.O.)). The 2nd I.F.
output is at A13J7 on the A13 RF Front End Assembly. This signal path is
used for all frequency range options when the RF input frequencies are
< 3600 MHz. The control voltages and biasing for these assemblies come
from the A15 Front End Control assembly.
2. RF input frequencies from 3.6 GHz to 13.6 GHz go through high band path
#1. Refer to Chapter 12, “Block Diagrams” for details.
This signal level can be optimized by either Input Attenuator A or Input
Attenuator B. The signal is then routed to the low band switch, which will
route the signal to the A12 YTF Preselector.
The signal then routes to the A12 YTF Preselector before going to the A13
RF Front End Assembly. The input switch in the RF Front End Assembly
routes the RF input signal to Mixer #2. The RF input signal is then mixed
with the 1st L.O. from the A14 L.O. Synthesizer Assembly in Mixer #2 to a
1st intermediate frequency of 322.5 MHz. The 322.5 MHz I.F. output is at
A13J7 of the RF Front End Assembly. This is the signal path for N8974B,
7.0 GHz Frequency Range and N8975B, 26.5 GHz Frequency Range (up to
13.6 GHz). The control voltages and biasing for these assemblies come
from the A15 Front End Control Assembly.
All RF input frequencies < 3600 MHz go to Mixer #1.
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