Chapter 6 139
Troubleshooting the IF Section
A7 Digital IF Assembly Descriptions
A7 Digital IF Assembly Descriptions
Two versions of the A7 Digital IF assembly are available. Instruments
with Option 124 (Video Out) or Option 122 or 140 (Wideband IF)
contain the optional Digital IF assembly E4440-60206. This assembly
requires slightly different troubleshooting techniques than the
standard assembly. Therefore when following the procedures below be
careful to use the correct information for the digital IF installed in your
instrument.
Purpose: The A7 digital IF assembly digitizes the 7.5 MHz final IF, by
processing the time domain continuous data into I/Q (in-phase and
quadrature) signals, and delivers the data to the CPU for further
processing and display.
The digital IF assembly performs the following main functions:
• digitizes the 7.5 MHz IF
• pre-adjusts the variable amplifier for optimum ADC input levels
using the gain range/select rules circuitry
• performs analog to digital conversion
• performs dither generation
• provides video out (Option 124)
• Final IF overload detection
7.5 MHz IF
The 7.5 MHz IF comes from the A8 analog IF assembly. The maximum
input level to the A7 digital IF assembly is +4 dBm. Exceeding this level
will cause distortion and will clip the ADC causing erroneous
measurement results.
Gain Range Select/Rules
This input path is a tapped 21.4 MHz 3rd IF from the A8 analog IF
assembly, which bypasses the 10 MHz anti-alias filters. The gain range
of the variable amplifier before the ADC is 0 to 18 db (in 6 dB steps).
The gain range select/rules “pre-adjusts” the variable amplifier for
optimum ADC input levels before the 7.5 MHz third IF arrives at the
ADC. Overdriving the ADC will cause distortion and will clip the ADC,
resulting in erroneous measurements.
Offset Adjust and Dither
These signals are requirements for the complex ADC chip to function
properly. The dither generator adds noise to the ADC, which converts
quantization errors into noise, and the noise on the signal can be
removed.
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