Agilent Technologies PSA Series Service Manual page 50

Overall Troubleshooting
Troubleshooting Using Auto-Align Tests
Table 2-2 Sequence for Auto-Align Procedures
Procedure Name
IF Gain Curve
321 MHz Ampl Curve
Comb Teeth
LC Prefilter Wide
LC Prefilter Narrow
XTAL Prefilter Wide
XTAL Prefilter Narrow
ADC Autorange
AIF Main Gain
Atten Steps
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Procedure Description
The A8 Analog IF assembly has a gain control DAC that is called the "Main Gain". It is used
to compensate for analog IF Prefilter BW gain variations, so that the gain of the entire IF
path (before ADC) remains approximately constant. It is the only gain stage that gets
varied during normal instrument operation. This alignment generates the curve coefficients
which characterize the gain vs. DAC number "shape" associated with this stage.
The 321 MHz cal osc signal is used for many internal alignments. This alignment merely
calibrates the amplitude control DAC for this signal, which is used by the other alignments
when necessary to set accurate amplitude levels.
The relative amplitude of each of the 20 individual comb teeth is determined by comparison
to the 50 MHz amplitude reference. The LO is moved to position the 50 MHz signal to the
same frequency in the IF path as the comb tooth.
This alignment is run for wide LC bandwidths (BW > 1.2 MHz). The 321 MHz Comb
calibrator oscillator is used to align BW and CF for the analog IF LC path. Coefficients are
generated for a curve that maps BW-DAC to requested BW. Another curve is generated
which represents Gain vs. BW. Both of these curves are then used during Prefilter and IF
gain setup during a measurement. The prefilter is located on the A8 Analog IF assembly.
This alignment is run for narrow LC bandwidths (BW ≤ 1.2 MHz). See description for LC
Prefilter Wide.
This alignment is run for wide XTAL bandwidths (BW> 20 kHz). The 321 MHz Comb
calibrator oscillator is used to align BW, SYM, and CF for the analog IF XTAL path.
Coefficients are generated for a curve that maps BW-DAC to requested BW. Another curve
is generated which represents Gain vs. BW. Both of these curves are then used during
Prefilter and IF gain setup during a measurement. The prefilter is located on the A8 Analog
IF assembly.
This alignment is run for narrow XTAL bandwidths (BW ≤ 20 kHz). See description for
XTAL Prefilter Wide.
The ADC autoranging subsystem has several hardware components that need to be setup.
This alignment takes care of 3 DACs: 2 DACs on the A7 Digital IF assembly that set the
positive and negative thresholds for range switches; and 1 DAC on the A8 Analog IF
assembly that sets the GAIN of the autoranging detector signal. It also generates 4 gain
values to be used for 4 possible measurement setups. The 4 setups cover the on/off positions
of dither and analog IF post-filter. This is necessary since these positions produce different
relative signal levels between the main path and the autorange path.
This alignment derives rough values for the AIF Main Gain DAC to be used for each RF
band. The front end is not measured, only the IF path. Constants set in cal files specify the
amount of AIF gain desired for each band (e.g. 3 dB in band 0, 1 dB in band 1). The isolated
gain of just the AIF is measured by using a detector at the AIF input to establish
approximately -7 dBm input, and then measuring the power entering the digital IF with
standard ADC scaling processes. The AIF gain dac is adjusted up or down until the band 0
desired gain is achieved. The AIF gain dac for all other bands is predicted (not measured) by
using the band 0 value, the relative gain between bands, and the AIF gain curve (gain vs.
dac) which was established by another alignment (IF GAIN CURVE). If Option B7J is
present, this alignment switches DIF dither on and off, and performs calculation for
electronic attenuator usage. The routine uses the 321.4 MHz Cal oscillator.
Align errors of the mechanical attenuator pads at 50 MHz. If Option B7J is present, this
alignment switches through the 41 electronic attenuator steps. Performs check of relative
attenuator switching. When testing mechanical attenuators, this checks only the 0 dB,
10 dB AC Coupled, 10 dB DC Coupled, 12, 14, 16, 20, 30, and 40 dB steps.
If the error is > 1 dB the test fails.
Chapter 2