Chapter 12 599
RF Section
Low Band Problems
Low Band Problems
1. Disconnect all inputs from the front panel INPUT 50Ω connector.
2. Set the spectrum analyzer to the following settings:
Center frequency ..........................................................0Hz
Span .......................................................................... 1MHz
Input attenuator ...........................................................0dB
3. The LO feedthrough amplitude observed on the display should be
between −6 (above top of screen) and −30 dBm.
NOTE The marker will not PEAK SEARCH on the LO Feedthrough when in a
non-zero span. To measure the LO Feedthrough amplitude with the
markers, set the SPAN to 0 Hz and
CENTER FREQ to 0 Hz. Press MKR
ON.
4. If the LO feedthrough amplitude is within limits, but signals are low,
the RF path following the A8 mixer is probably operating properly.
5. If the LO feedthrough amplitude is higher than −5 dBm (signal will
be "clipped" at top of screen) and signals are low in amplitude,
suspect a defective A8 mixer assembly.
6. If signal amplitudes <2.9 GHz are bad, but signals >26.8 GHz are
good, check the A8 Low Band Mixer or the filter FL1. If both the LO
feedthrough and signal amplitudes are low, check the A12 SBTX.
7. Perform the steps located in “Control Latch for Band-Switch
Drivers” on page 615.
8. Check A13 second converter mixer diode bias at A14J19 pin 1. The
bias voltage should be between −150 and −800 mVdc.
9. Troubleshoot the signal path. Refer to the power levels listed on
Figure 12-11 on page 631, RF Section Troubleshooting Block
Diagram.
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