Agilent Technologies E4440A, Service Manual

Page: 127 / 436

Chapter 6 127
Troubleshooting the IF Section
A10 Third Converter Assembly Description
A10 Third Converter Assembly Description
Purpose : The 3rd converter assembly levels the 321.4 MHz IF to
compensate for system loss, before mixing the 321.4 MHz IF to a
21.4 MHz IF. The 3rd converter also provides the 50 MHz and the
321.4 MHz calibrator signals.
Path : Three inputs supply the 321.4 MHz IF signal to the 3rd
converter: (1) an input from the low band assembly, (2) an input from
the RYTHM, and (3), an optional IF input from the front-panel on
instruments with Option AYZ (external mixing). After input filtering,
the variable gain amplification circuitry compensates for system loss.
The 321 IF is then mixed with a 300 MHz IF from the reference
assembly before being routed to the A8 analog IF assembly. In addition,
both the 50 MHz and the 321.4 MHz cal signals are processed through
the 3rd converter and routed to the A14 input attenuator.
Step Attenuator
Provide 30 dB of attenuation available in 2 dB steps.
System Variable Gain Compensation Circuitry
The variable gain circuitry provides flatness compensation over the
frequency range of the analyzer. The gain setting depends on stored
flatness compensation values. The frequency response adjustment
gathers and stores these values.
Linearization Circuit
The circuitry that provides flatness compensation for the 321.4 MHz
signal is non-linear. The linearization circuit, which is driven by the
variable ramp generator on the A13 front end driver board, adjusts the
amplification of the variable gain circuitry so that the resulting flatness
compensation throughout low and high bands is linear.
50 MHz Calibrator ALC
The 50 MHz levelling loop ensures that the
−
25 dBm, 50 MHz calibrator
signal is stable and accurate. This 50 MHz signal is routed to the A14
attenuator for system level calibration. This signal is routed through
pin 24 on J8.
321.4 MHz Filter
The 321.4 MHz bandpass filter has a 10 MHz bandwidth and image
rejection of
−
73 dBc.