HP 8340A Service Manual page 50

Flatness is adjusted and tested with the RF output power at 0 dBm (RF Attenuator at 0 dB
and ALC at 0 dBm). The frequency response will normally remain the same over the ALC
range. For accuracy problems within the ALC range, refer to the ALC adjustments in Section
V and, if necessary, to the RF Section ALC Loop Theory of Operation and Troubleshooting.
If accuracy is within specifi c ations over the ALC range and is out of specifi c ations below the
ALC range, the problem is most likely associated with the RF attenuator. Note that the RF
attenuator is outside of the ALC Loop. The instrument processor programs the RF
attenuator, but there is no way for the processor to know ifthe attenuator actually stepped
properly. If the power level is off by a factor of 10 dB, the attenuator may not be responding
properly. Select a CW frequency, set the power level to 0 dBm, press
The Entry Display should indicate:
ATN: -00 dB, ALC: 0.00 dBm
Using the step keys step the RF a tten ua tor (and RF power output) to - 90 dB. The a tten ua tor
should "click" at each step and the RF power should decrease in 10 dB steps. If the attenuator
does not perform properly, refer to the RF Section overall block diagram. Determine ifthe
correct programming bits are being applied to the RF attenuator (See RF Attenuator truth
table on the A24 Attenuator Driver/SRD Bias assembly, schematic diagram, located in the
RF Section) or ifthe RF attenuator itself is not responding properly.
There is an offset and slope calibration constant correction factor for each RF attenuator step
from 10 to 90 dB. These correction values are generated in the automated RF Attenuator
Calibration Test program. A hard copy of values generated at the last calibration should be
located inside the instrument (Remove the top cover - see pocket along lef t side rail). Check
the calibration constants in HP 8340A memory against the hardcopy values. Restore the
correct values if necessary. Refer to Section VIII Calibration Constants for more informa­
tion.
If the attenuator is operating properly and the calibration constants are correct, check the RF
connections from the AlO Directional Coupler to the RF attenuator and then measure the
power accuracy at the attenuator output.
16.
SWEEP
For sweep problems, refer to the Sweep Generator - YO Loop functional group, Sweep
Generator Troubleshooting. The following information may be used to further defi n e the
symptom.
It is assumed that the instrument works properly in CW mode (no UNLK indication). Press
and check for the following sweep indications:
[INSTR PRESET]
Check front panel SWEEP LED. The LED should be blinking.
1.
The sweep LED is turned on at the start of sweep, turned off at each bandcross, and at
end-of-sweep. The LED is controlled by LSPLD (Low Sweep LeD), see A58 Sweep
Generator board Block P. The sweep LED is turned on when HSP (High Start Sweep)
from A57 Block Mis high, LRSP (Low Reset Sweep) from A59 Block G is low, and LBX
(Low Bandcross) A58 Block U and A57 Block Fis high.
The sweep LED is turned off if HSP is low, at end -of-sweep if LRSP is low, and at
bandcrossings ifLBX is low.
2. Check front and/or rear panel SWEEP OUT signal.
The SWEEP OUTPUTS come from the A57 Marker/Bandcross board Block
generated from the MKR RMP A58 Block K. For a detailed description of the SWEEP
GENERA TOR/YO LOOP Theory Of Operation under Multiband Sweeps.
If the sweep out waveform is normal, the Ramp Generator on the A58 board is working,
the WSPTM (Write Sweep TiMe) strobe (A59 Block
(Low Reset) A58 Block Pis OK.
Model 8340A - Service
Scans by HB9HCA and HB9FSX
[SHIFT] [PWR SWP].
is being generated, and LRESET
A)
and are
H
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