Video Offset

Refer to function block P of A4 Log Amplifier Schematic Diagram (sheet 3 of 4) in the HP 856OA/6lB/63A Spectrum Analyzer Component Level Information binder.

The circuit provides a programmable video offset, with a step size of 5 mV, from -300 mV to +900 mV.

1.On the spectrum analyzer, press (m), (SPAN), ZERO SPAN, (-1, 1 IGHz), [AMPLITUDE), 50 (--dBm). Turn the IF auto cal off.

2.Disconnect W27 (coax 3) from A4J3 and connect a signal generator to A4J3.

3.Set the signal generator controls as follows:

Amplitude . . . . . , :. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +lO dBm Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.7MHz

4.Simultaneously decrease the signal generator output and spectrum analyzer reference level in 10 dB steps. 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 analyzer power. Press STOP REALIGN when it appears.

6.Press [e’) and SINGLE on the spectrum analyzer.

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 (K], (SPAN), 100 (Hz), (m), 10 (--dBm), and (SGLSWP).

2.Connect the CAL OUTPUT to the INPUT 50R.

3.Disconnect W54 (coax 2) from A4J4. Connect a test cable from A4J4 to the input of an oscilloscope.

4.Set the oscilloscope controls as follows:

Amplitude scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..0 to+lV Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..dc Sweep time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5 ms/DIV

5.The oscilloscope should display a 4.8 kHz sine wave.

6.Disconnect the cable from the CAL OUTPUT and connect to the INPUT 5Os1.

7.Broadband noise should be displayed on the oscilloscope from approximately +200 mV to +400 mV.

8.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.

8-16 IF Section