- the basics of the operation and use of the frequency counter measuring
instrument
Frequency counters and timers are widely used items of
test equipment within the electronics industry for measuring the frequency
of repetitive signals and measuring the elapsed time between events. In
particular, frequency counters are used for radio frequency (RF)
measurements where it is important to test or measure the precise frequency
of a particular signal.
The requirements for frequency counters and interval
timers are slightly different. However it is often possible to utilize the
same basic test instrument to perform both functions, and as a result
frequency counter-timers are widely available. These frequency counter
timers are more commonly found as general purpose laboratory test
instruments. Where higher frequencies are to be measured, the timer element
is not included and the test instrument is just a frequency counter.
Basis of operation
Frequency counters and timers operate by counting events within a set period
or discovering what a period is by counting a number of precisely timed
events. The time periods within which events are counted, or the precisely
timed events can be generated using a highly stable quartz crystal
oscillator. This may even be oven controlled, and in this way a very
accurate reference is obtained.
To look at the way in which a frequency counter or timer
works, it is necessary to described the two approaches separately. The two
approaches may be termed direct counting and reciprocal counting.
Direct counting
Those frequency counters that use a direct counting approach count the
number of times the input signal crosses a given trigger voltage (and in a
given direction, e.g. moving from negative to positive) in a given time.
This time is known as the gate time
It is important that the gate time is accurately
generated. This is done by having a highly accurate frequency source within
the frequency counter. Typically these will operate at a frequency of 10 MHz
and this needs to be divided to give the required gate time. Figures of
0.01, 0.1, 1, and 10 seconds may be selected. The shorter times obviously
enable the display to be refreshed more often, but against this the count
accuracy is less.
The reason that the gate time determines the resolution
of the frequency counter is that it can normally only count complete cycles,
as each crossing represents a cycle. This a gate time of one second will
enable frequency resolutions of 1 Hz to be gained, and a ten second gate
time will enable resolutions down to 0.1 Hz. It is worth noting that the
measurement resolution is a not a percentage of the measurement, but instead
it is fixed amount relating only to the gate time.
Reciprocal frequency counter timers
Another method of measuring the frequency of a signal is to measure the
period for one cycle of the waveform and then take the reciprocal of this.
Although this approach is slightly more expensive to implement than direct
counting and it is not as widely used, it does have some advantages. The
main one is that it always will always display the same number of digits of
resolution regardless of the input frequency. As a result, reciprocal
frequency counters are specified in terms of the number of digits for a
given gate time, e.g. 10 digits per second. In view of this it can be seen
that reciprocal counters give a higher resolution at low frequencies. At 1
kHz, a direct counter gives a resolution of 1 Hz (4 digits). A 10
digit/second reciprocal counter gives a resolution of 1 ?Hz (10 digits).
The other advantage is that these counters can make very
fast readings. A reciprocal counter will give 1 mHz resolution in 1 ms,
whereas a direct counter takes a second to give a reading with a resolution
of 1 Hz.
Summary
Frequency counters are an essential tool for any accurate measurements of
frequency. They may be used in development, production, repair or
maintenance.. Of the two types, the direct frequency counter is the most
common. In fact some frequency counters may be bought for a particularly low
cost as a result of the high levels of integration that are now available.
Even small handheld versions are available. However frequency counters with
much higher levels of performance tend to be contained in larger cases.
Often they will require highly accurate crystal oven oscillators to provide
the very accurate gate times required. Nevertheless they still represent
very good value.
