390 Chapter7
ADC/Interface Section
A16 Assembly Fast ADC Circuits (8560E with Option 007)
7. Adjust the HP 8560E Option 007 reference level to place the signal
at the top graticule line on the CRT display.
8. Measure the dc level at TP25 and TP26. The level should be +1.0
±0.25 V. If the level measured at TP26 differs from the level
measured at TP25 by more than 0.25 volts, troubleshoot U43 and
associated circuitry.
9. Disconnect the CAL OUTPUT signal from the INPUT 50 Ω
connector.
10.The level at TP26 should drop to −0.35 Vdc. If the level is less (more
negative) than −0.35 Vdc, replace voltage clamp CR4.
11.Measure the dc level of the flash video at TP27. The level should be
near 0 Vdc with the signal at the bottom graticule line (no input to
the spectrum analyzer).
12.Connect the CAL OUTPUT to the INPUT 50 Ω connector.
13.Measure the dc level of the flash video at TP27. The level should be
near +1.7 Vdc.
8-Bit Flash ADC
Refer to function block I of the A16 fast ADC assembly schematic
diagram in the HP 8560 E-Series Spectrum Analyzer Component Level
Information.
The flash ADC (U35) converts the analog video signal into 8-bit digital
values at a fixed rate of 12 megasamples per second.
NOTE When measuring voltages or waveforms on the A16 fast ADC assembly,
connect the ground (or common) lead to the ground-plane trace on the
A16 assembly. This digital ground plane is totally isolated from the
chassis.
1. Press
PRESET on the HP 8560E Option 007 and set the controls as
follows:
Center frequency ..................................................... 300MHz
Span .................................................................................0Hz
Reference level ......................................................... −20dBm
Log/division ................................................................5dB/div
Sweep time ................................................................... 20ms
2. Connect the CAL OUTPUT to the INPUT 50 Ω connector.
3. Pins 4 through 10 (ADC7-ADC1) and pin 21 (ADC0) of U35 should
all be high (logic 1), corresponding to an ADC digital count of 255 for
the analog input of +2 volts or greater.
4. Disconnect the CAL OUTPUT signal from the INPUT 50 Ω
connector.
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