Agilent Technologies 856290216 service manual A3 Assembly Video Circuits, 14 ADC/lnterface Section

A3 Assembly Video Circuits

Voltages from A3JlOl to the A3 Variable Gain Amplifier correspond (approximately) to on-screen signal levels. (One volt corresponds to the top of the screen and zero volts corresponds to the bottom of the screen.) This is true for both log and linear settings except when the spectrum analyzer is in 1 dB/div or 2 dB/d’iv. In these cases the log expand amplifier is selected, and 1 V corresponds to top-screen and 0.8 or 0.9 V corresponds to bottom-screen. The spectrum analyzer can be set to zero span at the peak of a signal to generate a constant dc voltage in the video circuits during sweeps.

1.Disconnect W26 from A3JlOl and W20 from A2J4.

2.Connect W26 to A2J4.

3.Set the spectrum analyzer to the following settings:

Span . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . OHz Sweep time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20ms Resolution bandwidth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 MHz Log/division . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 dB/DIV

4.If a trace is displayed, troubleshoot the A3 assembly. If a trace is absent, connect an oscilloscope to the rear-panel BLKG/GATE OUTPUT.

5.The presence of a TTL signal (TTL low during 20 ms sweep) indicates a good A3 Interface Assembly. Troubleshoot the IF section.

6.If the BLKG/GATE OUTPUT is always at a TTL high or low, troubleshoot the A3 trigger/video gating circuits.

7.Reconnect W26 to A3JlOl and W20 to A2J4.

8.Remove the A3 assembly shield.

9.If the video filters appear to be faulty, refer to “Video Filter” in this chapter.

10.If there appears to be a peak detector problem, refer to “Positive/Negative Peak Detectors” in this chapter.

11.Connect the spectrum analyzer CAL OUTPUT to the INPUT 50R and set the controls as follows:

Center frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..300MHz Span . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . OHz Reference level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -10 dBm

12.If the spectrum analyzer works correctly in 5 dB/div and 10 dB/div but not in 1 dB/div or 2 dB/div, refer to “Log Offset/Log Expand” in this chapter. Continue with step 13 if the problem involves on-screen amplitude errors which appear to originate in the video chain.

13.Press ICALl and IF AD3 ON OFF until OFF is underlined. Monitor A3TP9 with an oscilloscope. If the voltage is not approximately +l Vdc, troubleshoot the Log Amplifier on A4. (Refer to the IF troubleshooting procedure in Chapter 9, “IF Section.“)

14.To confirm proper video input to the video circuit, set the spectrum analyzer to Log 10 dB per division and change the reference level in 10 dB steps from -10 dBm to

j-30 dBm. At each 10 dB step, the input voltage should change 100 mV. The input level should be to.6 Vdc for a j-30 dBm reference level.

8-14 ADC/lnterface Section