Agilent Technologies 856290216 service manual Log Offset/Log Expand, ADC/lnterface Section

Note	The on-screen amplitude level will probably not change as expected, since the
	video circuitry is assumed to be faulty.

15.Monitor ASTP14 while stepping the reference level from -10 dBm to j-30 dBm. If the voltage does not step approximately 100 mV per 10 dB step, refer to “Video MUX” in this chapter.

16.If the Video MUX is working properly, monitor A3TP15 with the oscilloscope and step the reference level from -10 dBm to +30 dBm. If the voltage does not change 100 mV per 10 dB step, refer to “Video Filter” in this chapter.

17.If the voltage at A3TP15 is correct, move the oscilloscope probe to A3TP17 and step the reference level between -10 dBm and i-30 dBm. If the voltage does not change 100 mV per 10 dB step, refer to “Video Filter Buffer Amplifier” in this chapter.

18.If the voltage at A3TP17 is correct, move the oscilloscope probe to A3TP6. Set the following controls to keep the ADC MUX set to the MOD-VIDEO input during the sweep :

Sweep time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 s Detector mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sample

19.Step the reference level from -10 dBm to i-30 dBm while monitoring the voltage change on the oscilloscope. If the voltage does not change 100 mV per 10 dB step, refer to “ADC MUX” in this chapter.

20.If the voltage at A3TP6 is correct, move the oscilloscope probe to A3TP8 and step the reference level between -10 dBm and i-30 dBm. If the voltage at A3TP8 is not the same as that at A3TP6, replace A3UllO.

21.If the voltage at A3TP8 and A3TP6 are equal, move the oscilloscope probe to A3TP7.

22.Change the reference level from -10 dBm to 0 dBm. The voltage change on A3TP7 should be between 630 mV and 770 mV. If the voltage change is outside of these limits, refer to “Variable Gain Amplifier (VGA)” in this chapter. The gain of the VGA should be 7 &lo%.

Log Offset/Log Expand

Refer to function block X of A3 Interface Assembly Schematic Diagram in the

HP 8560 E-Series Spectrum Analyzer Component Level Information.

The log scales are modified using a combination of amplification and digital trace manipulation. The video input to the A3 assembly is either 10 dB/div or linear. To obtain the 5 dB/div scale, the CPU manipulates the trace data from the 10 dB/div scale. To obtain the 2 dB/div scale, the video signal is amplified and offset so that top-screen in 10 dB/div corresponds to top-screen in 2 dB/div. To obtain the 1 dB/div scale, the CPU manipulates trace data from the 2 dB/div scale.

In 2 dB/div, Log Offset/Log Expand amplifies the top 20 dB of the display. This is done by offsetting the video signal by -0.8 V and providing a gain of 5 to the top 0.2 V of the video signal. The -0.8 V offset is accomplished by sinking 2 mA through R114 by current source U105/QlOl.

I. On the spectrum analyzer, press (PRESET), ISPAN), ZERO SPAN, (CAL), and IF BDJ IIFF .

ADC/lnterface Section 8-15