is to drive the reset circuits, thereby causing the counters to initiate a
new cycle of operation each time the gate is opened.
The gate usually
consists of a bistable multivibrator which is triggered by start and stop
pulses.
The start pulse opens the gate for a precise period of time
determined by the time base. The pulses which are passed by the gate during
count time are counted, stored, and displayed by the DCUs. These pulses, or
events are displayed as a number representing the total number of events
which occurred during the time the gate was open. Upon completion of the
display time, a reset pulse is generated which automatically resets all DCUs
to zero for the next measurement cycle.
c. Oscillator Circuit. For an accurate count, the duration of the gate
must be extremely accurate. To provide an accurate time circuit to control
the gate time, a crystal-controlled oscillator is used to develop a basic
frequency. The basic frequency (time base) is multiplied and divided within
the counter.
To maintain the accuracy required for frequency and period
measurements, the frequency of the oscillator must be kept as stable as
possible.
By keeping the temperature of the frequency determining device
constant, external temperature variations will not cause the frequency of
the oscillator to change.
This is accomplished by placing the quartz
crystal, which controls the frequency of the oscillator, in a temperature-
controlled "oven."
d. Frequency Multipliers and Dividers.
As previously stated, the
frequency of the oscillator is set by selection of a precisely ground quartz
crystal.
In order to vary the time base frequency, the basic oscillator
frequency is converted by a series of 10:1 decade dividers and frequency
multipliers.
To allow the display circuits to present the higher
frequencies, the basic oscillator frequency is sometimes multiplied, and the
period of time the gate is open is reduced by factors of ten.
Thus, the
number of events actually counted is reduced along with the required number
of display units.
While a frequency multiplier allows the higher frequencies to be
displayed, it does not allow for the display of the lower frequencies. In
order to obtain a usable display of the lower frequencies, the basic
oscillator frequency is divided within the frequency counter. The dividers
provide a simple method of obtaining longer gate times.
By adding more
dividers, gate times as long as ten seconds can be obtained. To make use of
the multiplier and divider signals from the oscillator in the frequency
counter, a switching circuit is incorporated. The switching circuit allows
the operator to select the gate times and control the position of the
decimal point.
2.
ELECTRONIC COUNTER APPLICATIONS
a. Frequency Measurement. In the frequency function, the three major
circuits used are the time base (known frequency), gate and DCUs
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