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26
Vill. OPERATION
1. General information
1.1. Switch on power
The secondary
power switch POWER
ON/OFF
oper-
ates only the DC voltages of the circuitry. The oven-
enclosed oscillator, however, is operating as soon as
the mains cable is connected to the mains. The light-
emitting diode marked OSC then turns on.
WARNING:
Check that counter is set to correct mains
voltage before POWER
ON is depressed!
(Refer to section VI-6).
1.2. Warm-up;time
Models PM 6650 A and PM 6650 E require 7 minutes of
warm-up from the moment of mains connection. If the
counter
is switched
off with
the
secondary
power
switch POWER
ON/OFF,
no warm-up time is neces-
sary next time the PM 6650 A or E is going to be used.
Model PM 6650 B needs no warm-up time unless it has
later
on
been
equipped
with
optional
oscillator
PM 9680 A or PM 9681.
In such
a case
the same
prescriptions
as for the
PM 6650 A and E apply.
1.3. External frequency standards
House standards or other frequency standards can be
used instead of the internal 10 MHz oscillator.
The frequency can be 1 MHz or 10 MHz.
If a time resolution of better than 10 us is required,
10 MHz should be used.
Proceed
as follows to set counter to external opera-
tion (refer to fig. VII-3):
— Set switch A to EXT.
— Select 1 MHz or 10 MHz using switch B.
— Apply external standard to rear input EXT.STD.
NOTE:
100 kHz can
be used
if switch
B is set to
1 MHz.
To interpret the display, shift shown
decimal point one step to the left.
1.4. Measurement accuracy
1.4.1. Basic error types
In digital counters there are three basic error types
whose significance depends on which measuring mode
is selected:
the +1 count ambiguity, the trigger error and the time
base stability.
The +17 count error is inherent in all digital counters.
It occurs because the input signal is not synchronised
with the gate operation. Its significance is reduced in
period average or time interval average measurements.
For example,
in period
average
measurement,
the
input signal frequency
time base frequency x periods averaged
The trigger error is caused by the noise superimposed
on the input signal and by the inherent noise of the
input amplifiers
and trigger circuits.
This
noise
will
give rise to a width variation of the conditioned input
signal used to operate the main gate in the period and
time interval modes. For signals with a signal to noise
ratio of 40 dB or better and at rated sensitivity, the
error is +
-3
3x1
trigger error can be expressed as
a
, Where N is
the number of periods averaged.
The trigger error is insignificant in frequency, ratio and
totalize measurements.
The error caused by the time base stability will be a
limiting factor only when
measuring
long time inter-
vals, which means that in practice the time base error
can be ignored.
The PM 6650 has a choice of three oscillators:
Model
PM6650B
has
a temperature
compensated
crystal oscillator (TCXO) having an average ageing of
+8
107/year after 3 months of initial ageing.
Model
PM 6650A
features
a high-stability
oven-en-
closed oscillator, PM 9680 A, with an average
ageing
of +1.5x 10-9/24 h.
This oscillator is also an optional accessory for model
PM 6650 B.
Model PM 6650 E is equipped with an ultra-stable oven-
enclosed oscillator, PM 9681 with an average ageing
of +5 x 10-19/24 h. This oscillator is an optional acces-
sory for models PM 6650 A and PM 6650 B.
1.4.2. Frequency measurement
The measurement
accuracy is determined by the +1
count error of the least significant digit and the ageing
of the internal or external oscillator.
The error can be calculated from the following expres-
sion in which f is the input signal frequency in Hz:
yp tePe
os
=
~~ f x gate time (s)
The measurement error for various gate time settings
of models PM 6650 A, B, and E is given in the diagrams
figures VIII-1 and VIII-2.
error =
+ time base error.
1.4.3.
Period measurement
All of the three basic errors can influence the accuracy
of period
measurement.
The
period
average
mode,
however,
reduces the significance
of the +1
count
error and the trigger error.
The measurement
error of the PM 6650 A and E and
PM 6650 B in the period average
mode
is plotted in
the diagrams figure VIII-1 and figure VIII 2.
The following formula expresses the error:
fy
eT
error =
+
Neh
ON
f,; = time base frequency counted (= 100 MHz in
PERIOD AVERAGE)
f. = input signal frequency
N = number of periods averaged
er= trigger error (=3 x 10°3/period for signals with
S/N of 40 dB at rated sensitivity)
Coxe = average time base ageing rate
(PM 6650 A = +1.510-9/24h
PM 6650 B =
+1>10-7/month
PM 6650 E = +5» 10-19/24 h)
+ €ose.
I
1.4.4. Time interval measurement
The significance of the +1 count and trigger errors is
reduced
in averaging
situations.
The
accuracy
of a
time interval average measurement
can be expressed
(
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