A2 Analog I.f. Assembly Theory Of Operation - Keysight N8973B Service Manual

Analog/Digital IF Troubleshooting
25 MHz BW IF Section

A2 Analog I.F. Assembly Theory of Operation

Refer to
Input Switch and Filter
Step Attenuator and Amplifier
Image Filters
Mixer
3rd LO
I.F. Comb Cal
250
Chapter 12, "Block
A 322.5 MHz input signal is received from the A13 RF Front End Assembly.
The signal then goes through a band pass filter centered at 322.5 MHz with
a 25 MHz bandwidth. A cal comb signal can be automatically switched in at
the input of the assembly to calibrate the prefilters.
A 1 dB step attenuator follows to compensate for band gain differences.
The signal then goes through a high-dynamic range amplifier.
One of three different filter paths will be selected automatically, depending
on the analyzer mode.
• The through path is selected in IQ Analyzer Mode and wide-band
demod.
• The 12 MHz ceramic bandpass filter is used for normal Signal
Analyzer swept and FFT operation.
• The 300 kHz Surface Acoustic Wave (SAW) is used for ACP modes.
There are two mixers that reside on this assembly. The first mixer is used to
down convert the 322.5 MHz input to the final 22.5 MHz I.F. The other mixer
is described in the I.F. Comb Cal section.
The 300 MHz 3rd L.O. signal comes from the A16 Reference Assembly and
conditions it for use as an L.O. in the two mixers that reside on this
assembly. The third L.O. power is ~ +10 dBm to optimize the conversion
efficiency of the first mixer.
The 2nd mixer is used to mix the 300 MHz L.O. from the Reference
Assembly with the 22.5 MHz cal comb signal from the A3 Digital I.F. to
allow calibration of the prefilters and overall passband phase and
amplitude. The signal is attenuated by 20 dB if necessary. A limiter is used
to reduce amplitude variation effects.
Diagrams".
Keysight NFA Series Noise Figure Analyzers Service Guide