384 Chapter7
ADC/Interface Section
A3 Assembly ADC Circuits
1. Set the HP 8560E/EC controls as follows:
Center frequency ..................................................... 300MHz
Span .................................................................................0Hz
Sweep time ....................................................................... 60s
Detector mode ..........................................................SAMPLE
2. Trigger an oscilloscope on HSTART_ADC (U506 pin 15) and monitor
the outputs of the SASM (U527 pins 18 and 19; U528 pins 14 thru
23). Each bit should start high and be switched low. It will either
stay low or return to a high state 1 µs later, depending on the
comparison at U512.
3. If the outputs do not exhibit this bit pattern, and the ADC ASM
checks are working properly, suspect A3U527, U528, or one of the
latches (U514/516). If the output of comparator U512 does not toggle
back and forth during a conversion, suspect either U512 or one of the
clipping diodes (CR500/CR501).
NOTE Because currents are being summed at U512 pins 2 and 3, voltage
levels at these points are difficult to interpret.
Ramp Counter
Refer to function block D of A3 Interface Assembly Schematic Diagram
(sheet 2 of 6) in the
HP 8560 E-Series Spectrum Analyzer Component Level
Information.
The ramp counter is used for sweeps with widths greater than 2.0 MHz
times N. The analog sweep ramp is compared to the digital ramp
counter. When the analog sweep ramp exceeds the DAC output
generated for that ramp counter setting, HRAMP_COMP toggles high,
indicating the end of a bucket. The ramp counter counts horizontal
buckets. There are 601 buckets per sweep, so the ramp (bucket) counter
counts from 0 to 600. The ramp counter is incremented by
HRST_PK_ENA.
1. Press
PRESET on the HP 8560E/EC and set the controls as follows:
Span ............................................................................. 5MHz
Detector mode ..........................................................SAMPLE
2. For spans greater than 2.0 MHz times N0, HODD (A3U525 pin 3) is
a square wave with a period defined by (2 × sweep time/600). For
example, for a 6 s sweep time, HODD has a period of 20 ms. The
ramp (bucket) counter will be odd every other bucket.
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