Variable Gain Amplifier (VGA)
See function block AB of A3 Interface Assembly Schematic Diagram (sheet 6 of 6).
The VGA provides adjustable gain in the video path. Its nominal gain of 7 can be adjusted 510%. U112 removes dc offset to keep U113 in its monotonic range. (Both U112 and U113 are set to the same value.) The DAC settings cannot be changed from the front panel.
Track and Hold
See function block AC of A3 Interface Assembly Schematic Diagram (sheet 6 of 6).
1. Press [PRESET) on the HP 8560A and set the controls as follows:
C E N T E R F R E Q . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...300 MHz
SPAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . OHz
DETECTOR MODE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sample
REF LVL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
LOG dB/DIV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..2dB/div
SWEEP TIME . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50ms
2.Disconnect any signal from the analyzer input. A full scale display of sampled noise should be present.
3.Trigger an oscilloscope on the positive going edge of HHOLD (A3U506 pin 16).
4.The waveform at A3TPlO should be random noise with an average level of approximately 4 V. The noise should have a flat spot in its response while HHOLD is high, indicating proper operation of U114.
ADC/lnterface Section