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Model 5328A
Operation
3-30.
The 5328A synchronizers operate as in Figure 3-5. The top waveshape shows a repetitive
time interval which is asynchronous to the square wave clock. When these signals are applied to
the main gate, an output similar to the third waveform results (no synchronizers). Note that much
of this output results in transitions of shorter duration than the clock pulses. DCA's designed to
count at the clock frequency are unable to accept pulses of shorter duration than the clock. The
counts accumulated
in the DCA's will therefore approximate those shown
in the fourth
trace—the exact number of counts is indeterminant since the number of short duration pulses
actually counted by the DCA's cannot be known. Since the time interval to be measured is slightly
greater than the clock period, the fourth waveshape shows that the average answer will be in
error, having been biased, usually low, because of the DCA's requirement of having a full clock
pulse to be counted.
START
START
START
STOP
STOP
STOP
CLOCK
GATE
OUT
COUNTS
ACCUMULATED
IN DCA'S
WITH
SYNCHRONIZERS
J l
j l J l
I l
j |
COUNTS ACCUMULATED
IN DCA'S
Figure 3-5. Synchronizer Operation with Time Interval Averaging
3-31.
This problem is alleviated by the synchronizers which are designed to detect leading
edges of the clock pulses that occur while the gate isopen. The waveshape applied to the DCA's,
when synchronizers are used, is shown by the fifth waveform.
The leading edges are detected
and reconstructed, such that the pulses applied to the DCA's are of the same duration as the
clock.
3-32.
Synchronizers are a necessary part of time interval averaging; without them the averaged
answer is biased. In addition, it may easily be seen that with synchronizers involved, time intervals
of much less than the period of the clock can be measured. This technique is only as good as the
synchronizers, however. The 5328A's high-speed synchronizers enable intervals as small as 100
picoseconds to be measured.
3-33.
There are occasional situations where time interval averaging cannot be performed ona
periodic signal. This problem occurs when the input time interval repetition rate is synchronous
with the internal clock. The Option 040 avoids this problem by modulating the clock with white
noise whenever the 5328A is in the Time Interval Average function, to minimize the synchronous
lock-up associated with this measurement.
3-34.
SOURCES OF MEASUREMENT ERROR
3-35.
The major sources of measurement error are the +1 count ambiguity, the time base error
and trigger error.
These are discussed in the following paragraphs.
3-36.
+1 Count Ambiguity
3-37.
Since the signal input to the main gate of the counter and the clock input are not coherent,
an inherent +1 count ambiguity exists in the count accumulated
in the decade counting
assemblies.
This is illustrated by Figure 3-6.
3-5
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