Specifications

Instrument Overview
14
source compensated. If the DWELL mode is
EXTERNAL, an EXTERNAL START pulse starts
counting and an EXTERNAL STOP pulse stops
counting. The counters are enabled only during
those gates which occurred between the two
EXTERNAL pulses.
COUNT MODES
The length of a COUNT PERIOD is the time it
takes counter T to reach its preset count. If N
t
is
the preset count in counter T, and R
a
and R
t
are the
mean pulse rates of the inputs to counters A and T,
then the COUNT PERIOD is
T = N
t
/ R
t
The number of counts in counter A will be
A = R
a
T = R
a
N
t
/ R
t
Rate:
If R
t
= 10 MHz, then the COUNT PERIOD is a
constant amount of time and
A = R
a
( N
t
/ R
t
) ~ R
a
and counter A measures the mean rate of its input.
Gated:
If R
t
is the gate trigger rate and counter A is
gated, then the COUNT PERIOD is N
t
/R
t
but the
"effective" time the counter is enabled is N
t
W
a
where W
a
is the width of A gate and
A = R
a
( N
t
W
a
) ~ R
a
In this case, counter A measures the mean rate
during the A gate.
Source Compensated:
If Rt is INPUT 2, then the COUNT PERIOD is
determined by the pulse rate of INPUT 2 and the T
discriminator. Count A will be
A = N
t
R
a
/ R
t
~ R
a
/ R
t
and is proportional to the ratio of the rates of
counters A and T. If INPUT 2 monitors source
intensity, then the COUNT PERIOD will adjust
for fluctuations in the source. Counter A measures
the ratio of its mean input rate and the mean rate
of INPUT 2.
Reciprocal:
If R
t
is INPUT 2, then the COUNT PERIOD is
N
t
/R
t
. If R
a
is 10 MHz, then counter A measures
A = R
a
N
t
/ R
t
= ( R
a
N
t
) / R
t
~ 1 / R
t
and is proportional to the reciprocal of the count
rate of INPUT 2. Note that each COUNT PERIOD
is determined by N
t
counts of INPUT 2, thus the
measurement time is determined by the signal
strength. Measurements of both strong and weak
signals are made to the same accuracy for constant
signal to noise ratio.
Counter B:
Counter B may be used in all of the above modes
except Reciprocal. Since counter B is completely
independent of counter A, 2 sources of counts may
be measured simultaneously. These sources may
be the same signal or 2 different signals; the
sources may be discriminated at separate
thresholds; they may be non-gated or gated; if
gated, the 2 gates may be independently set.
Almost any measurement can be made.
In addition, counter B has a preset mode called A
FOR B PRESET. In this mode, the input to
counter B is sent to counter T. The COUNT
PERIOD is now determined by the B
discriminator and the B gate. If R
a
is INPUT 1,
then counter A is source compensated by the gated
input of counter B. This allows source
compensation of gated signals and sources. If R
a
is
10 MHz, then counter A measures the reciprocal
of the rate of counter B's gated input.
OUTPUTS
The front panel can display counts up to 10
9
- 1.
Counters A and B can be displayed as separate
counters or combined as A-B or A+B. When A
FOR B PRESET is the count mode, then only A
data is displayed.