Agilent Technologies E4440A, Service Manual

Page: 51 / 436

Chapter 2 51
Overall Troubleshooting
Troubleshooting Using Auto-Align Tests
Align RF Gain
This alignment derives values for the RF Gain DAC to be used at a center frequency tuning
of 50 MHz, for various system signal path setups. The RF Gain DAC in general is adjusted
relative to these reference DAC values during sweeps to perform RF flatness compensation;
however, at 50 MHz center frequency, the RF Gain DAC should always be exactly one of the
values generated by this routine. The various system signal path setups include, but are not
limited to: nominal reference path, ditherOff, and optional hardware paths (1DS and B7J)
tested as part of the optional path RF Gain routine. Each path has a CAL FILE specified
system gain that must be achieved (e.g. 9.8 dB nominally, 12.8 dB with ditherOff, etc.).
Each path is measured using the 50 MHz AREF signal (at -25 dBm) iteratively as the RF
gain DAC is adjusted up or down, until the digital IF readings indicate the correct gain has
been achieved. Residual errors are recorded (e.g. 01 dB) to be applied as video shift. This
alignment establishes the ABSOLUTE amplitude accuracy in the reference positions (50
MHz, DC coupled, 10 dB atten, 30 kHz RBW, 75 kHz prefilter, etc.). Because of this, it must
be done near the end of the full alignment sequence, after many other system elements are
aligned.
If this alignment fails, it can be due to something in the several possible signal path setups
mentioned above. The RF Gain DAC is located on the 3
rd
converter and controls the
variable gain circuit.
Align Audio Board Gain Switches in the audio board calibration signal and measures its amplitude and DC offset. If
the alignment fails, assure there is no high level 10 kHz signal applied to the front panel
Audio Input. An alignment failure is caused by an unseated audio board or a faulty audio
board.
Wide IF ADC Image
(Options 122 or 140) The instrument uses the 50 MHz reference signal and compares the outputs of both A/D
converters on the A31 Wideband Analog IF assembly. Internal adjustments are made to
provide the best match. If this alignment fails, it can be that one of the A/D converters is
faulty or the 50 MHz reference power is incorrect.
Option Path RF Gain This procedure is the same as the Align RF Gain routine, where it derives values for the RF
Gain DAC. However it tests the optional hardware paths:
Electronic attenuator, Option B7J
Lowband preamplifier, Option 1DS
Wideband IF assemblies, Options 122 or 140
If this alignment fails, three messages are possible:
Align_RF_Gain_Eatten,
Align_RF_Gain_Preamp,
Align_RF_Gain_WBIF.
Align Variable Gain Characterizes the usable range of the RF Gain DAC setting. This allows for analog and
digital gain compensations (to improve the overall amplitude accuracy of the box), used for
RF flatness compensation.
A valid DAC/gain response curve is required for this alignment to pass (30-35 dB from
minimum to maximum). The DAC is located on the 3
rd
converter.
Align LO Nulling
A circuit on the A20 Lowband assembly feeds a portion of the LO signal back into the signal
path that is out of phase with the LO feed through. This reduces the total LO feed through
to below -65 dBm.
Wide IF Step Gains
(Options 122 or 140)
This alignment characterizes the gain errors of the IF step gains on the A31 Wideband
Analog IF assembly. The 50 MHz reference is used to make the measurements. Both filter
paths on the wide band AIF are measured. If this alignment fails, it could be caused by the
A31 Wideband Analog IF assembly. Check for other alignment failures also since this
alignment depends on non-Option 122 or 140 assemblies such as the 50 MHz reference
switch on the A14 Input Attenuator and the variable gain amplifier on the A10 Third
Converter assembly.
Table 2-2 Sequence for Auto-Align Procedures
Procedure Name Procedure Description