448 Chapter9
IF Section
Log Amplifier (P/O A4 Assembly)
4. Set the oscilloscope controls as follows:
Amplitude scale ................................................... 200mV/div
Offset ........................................................................ +400mV
Coupling ...............................................................................dc
Sweep time .......................................................50µs/division
5. The oscilloscope should display a 4.8 kHz sine wave.
6. Disconnect the cable from the CAL OUTPUT and the INPUT 50Ω
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”.
10.Reconnect cable W54 to A4J4.
Frequency Counter Prescaler/Conditioner
Refer to function block Q of A4 log amplifier schematic diagram (sheet
4 of 4) in the 8560 E-Series Spectrum Analyzer Component Level
Information
.
The frequency counter prescaler/conditioner divides the frequency by
two, and then attenuates it. The circuit consists of frequency divider
(U703A) and an output attenuator. The frequency divider turns on only
when the instrument is counting.
AM/FM Demodulator
Refer to function block R of A4 Log Amplifier Schematic Diagram (sheet
4 of 4) in the 8560 E-Series Spectrum Analyzer Component Level
Information.
The demodulator circuitry on the log amplifier on A4 produces a
low-level audio signal. This audio signal is then amplified by the audio
amplifier on A4. The FM demodulator demodulates narrowband FM
(5 kHz deviation) signals. The detector (block J) demodulates AM
signals.
1. If demodulation problems occur when the spectrum analyzer is in
the frequency domain, perform the Frequency Span Accuracy
performance test and, if necessary, the YTO Adjustments procedure.
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