424 Chapter8
IF Section
Log Amplifier Assembly (P/O A4)
3. Set the signal generator controls as follows:
Amplitude ................................................................ +10dBm
Frequency ............................................................... 10.7MHz
4. Simultaneously decrease the signal generator output and spectrum
analyzer reference level in 10 dB steps down to −110 dBm. At each
step, the signal displayed on the spectrum analyzer should be close
to the reference level.
5. Reconnect W27 (coax 3) to A4J3 and cycle the spectrum analyzer
power. Press
STOP REALIGN when it appears.
6. On the spectrum analyzer, press
SWEEP, SINGLE, CAL, and IF ADJ
OFF.
7. The offset DAC, A4U102 pin 2, should now be at its default value of
approximately +2.45 V. The voltage at U601 pin 3 should be
approximately 0 V for a DAC output of 2.45 V.
8. If this default offset voltage is incorrect, DAC U102 is the most
probable cause.
Video Output
1. On the spectrum analyzer, press PRESET, FREQUENCY, 300 MHz,
SPAN, 100 Hz, AMPLITUDE, −10 dBm, SGLSWP, CAL and IF ADJ OFF.
2. Connect the CAL OUTPUT to the INPUT 50 Ω connector.
3. Disconnect W54 (coax 2) from A4J4. Connect a short SMB to SMB
cable from A4J4 to an SMB tee and connect W54 to the tee. Connect
a test cable from the tee to the input of an oscilloscope.
4. Set the oscilloscope controls as follows:
Amplitude scale ................................................... 200mV/div
Offset ........................................................................ +400mV
Coupling ...............................................................................dc
Sweep time .............................................................. 50µs/div
5. The oscilloscope should display a 4.8 kHz sine wave.
6. Disconnect the cable from the CAL OUTPUT and the INPUT 50 Ω
connectors.
7. Set the resolution bandwidth to 2 MHz.
8. Broadband noise should be displayed on the oscilloscope from
approximately +200 mV to +400 mV.
9. As the
REF LVL is decreased in 10 dB steps from −10 dBm to −70
dBm, the noise displayed on the oscilloscope should increase in 100
mV increments. If this response is not observed, refer to "Step
Gains" and "Video Offset" in this chapter.
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