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as
1
count
+ trigger error
.
—
+ time base error
in which N is the number of intervals averaged.
Since the +1 count error is equal to the period of the
internal
oscillator,
which
is actually
10 ns for the
PM 6650, the significance of this error is considerably
reduced.
In practice also a systematical error must be added.
This error originates mainly from the rise time of the
input amplifiers and is +1 ns for the PM 6650.
Although the PM 6650 is a direct-gated counter, biased
T.l. average measurement
is prevented by a synchro-
nizing circuit.
MEASUREMENT
ERROR
£15 10°5(PM 96804)
+ Sx1019(PM 9681)
MEASURED FREQUENCY (Hz)
PERIOD AVERAGE MEASUREMENT
'1 count $trigg-error +time base accuracy
—
Accuracy obtained with
N
oscillator PM9680A (PM 6650 A)
FREQUENCY MEASUREMENT
——
Accuracy obtained with
1 count ttime base accuracy
oscillator PM 9681 (PM 6650E]
N= Number of periods averaged
Figure VIlIl-1.
Measurement error vs frequency and
period, PM 6550 A and E
MEASUREMENT
ERROR
|_Time base"{TCXO)
'accuracy (assumed
+5x10 \PM9681)
1
10
100
1k
10k
100k
1M
10M = 100M
1G
MEASURED FREQUENCY (Hz)
PERIOD AVERAGE MEASUREMENT
"Average ageing of TCXO
ttcount + trigg. error * time base accuracy
ist1=10 "month
N
——
Accuracy obtained with
FREQUENCY MEASUREMENT
standard oscitlator (TCXO)
SYcount + time base accuracy
——
Accuracy obtained with
optional oscillator PM 9681
= Number of periods averaged
Figure VIII-2.
Measurement error vs frequency and
period, PM 6650 B.
27
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