10.If log fidelity is poor near the bottom of the screen or the 1 MHz resolution bandwidth is narrow, a fault might exist in the wide/narrow filter switch. Refer to function block G of A4 Log Amplifier Schematic Diagram in the HP 856UA/6lB/63A Spectrum Analyzer Component Level Information binder. Check this switch as follows:

a.Monitor voltages on A4U302 pins 1 and 7 while changing the spectrum analyzer resolution bandwidth from 100 kHz to 300 kHz.

b.If the voltages do not come within a few volts of the +15 V and -15 V supplies, U103 and U302 are suspect.

c.Disconnect the digital multimeter and reconnect W27 to A4J3.

Linear Amplifiers

Refer to function block C of A4 Log Amplifier Schematic Diagram (sheet 2 of 4) in the HP 856OA/61B/63A Spectrum Analyzer Component Level Information binder.

The Linear Amps consist of two variable gain stages, U2OlC and U201E as well as the buffer amplifier A4U201B, A4U201D, and A4Q201. The linear amplifiers provide 0 to 40 dB of IF gain in 10 dB steps. The gain of A4U201C can be increased by 20 dB by turning on A4CR201 and A4CR210 with the control line LIN_20B. The gain of A4U201E can be increased by either 10 dB or 20 dB with the control lines LIN_10 or LIN_20A respectively. The gain can be selected by setting the spectrum analyzers reference level.

IF Gain Application Guidelines (ATTEN=lO dB)

Power into A4J3 Reference Level Gain of A4U201C Gain of A4U201E Total Gain

+6 dBm

- 50

dBm

0

dB

0 dB

OdB

- 4

dBm

- 60

dBm

0

dB

10 dB

10 dB

- 14dBm

- 70 dBm

0

dB

20 dB

20 dB

-24

dBm

- 80

dBm

20 dB

10 dB

30 dB

-34

dBm

- 90

dBm

20 dB

20 dB

40 dB

Total gain can be measured by injecting the specified power into A4J3 and measuring the total gain provided by A4U201C and A4U201E. The following procedure provides a means of troubleshooting the linear amplifiers.

1.On the spectrum analyzer press (PRESET), (SPAN), ZERO SPAN , (w), 1 IGHz), (-1, 50 (--dBm], LINEAR, MORE, AMPTD UNITS , m, and (-1.

2.Disconnect W27 (coax 3) from A5J5 and connect W27 to the output of a signal generator.

3.Set the signal generator controls as follows:

Amplitude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +6dBm Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.7MHz

4.Simultaneously decrease the signal generator output and spectrum analyzer REF LVL in 10 dB steps to -90 dBm. At each step, the signal displayed on the spectrum analyzer should be within one division of the previous position.

5.If a problem exists, isolate it by comparing the actual gain of A4U201C and A4U201E with those listed in the above gain guidelines.

6.Reconnect W27 (coax 3) to A5J5.

IF Section 8-15