Circuit Descriptions
1502C MTDR Service Manual
5–17
The analog timebase circuits receive the driver and sampler triggers and provide
strobes to the driver/sampler. The driver trigger is delayed by an analog time delay
and amplified by a driver circuit to provide the driver strobe.
The ramp trigger is used to start a linear voltage ramp generator. A voltage
comparator detects the time when this ramp reaches the programmed voltage of the
timebase DAC (digital-to-analog converter) and signals a driver to produce a strobe
for the video sampler. The timebase DAC is programmed by the processor to
provide a voltage proportional to the portion of the 50-ns time delay period desired.
Timebase control by the processor system is shown in Figure 5–9. Each period of
the pulse rate, the processor calculates a new 33-bit digital time delay value for the
next sample to be taken. The sixteen most significant bits of this value are loaded
into the coarse delay counter, causing it to count that number of 2.5 MHz clock
periods before starting the fine delay counter.
3 BITS16 BITS 14 BITS
MSB LSB
33-BIT
DIGITAL TIME
DELAY VALUE
COURSE
DELAY
COUNTER
FINE
DELAY
COUNTER
ANALOG
DELAY
2.5 MHz
CLOCK
20 MHZ
CLOCK
STROBE TO
SAMPLER
PRT
PULSE
Figure 5–9: Timebase Control
The next three bits from the processor time delay value are loaded into the fine delay
counter. This counter starts at the end of the coarse delay, and counts the selected
number of 20 MHz clock periods (o through 7) before triggering the analog delay.
The analog delay circuit receives the 14 least significant bits of the time delay word.
A digital-to-analog conversion provides a proportional voltage, which is compared
to a linear voltage ramp to produce the programmed time delay (o to 50 ns).
The timing diagram in Figure 5–10 (next page) shows the combined effects of the
three time delays. The output of the PRT counter, waveform (a), begins the coarse
delay (b). The falling edge of this signal triggers the driver strobe (c), which causes
a pulse to be applied to the cable test output.
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