- an overview of resolution and accuracy for RF frequency counters, and
a definition of the difference between the two figures.
Frequency counters are widely used for the measurement of
frequency of radio frequency, RF, signals, or for that matter the frequency
of any repetitive electronic signal. These days frequency counters are
widely available for comparatively low prices and within an electronic
repair or development laboratory a wide variety may be available.
Additionally frequency counters are widely available from electronic
measuring equipment stockists. Accordingly it is necessary to choose the
best counter for a given applications.
The accuracy and resolution of frequency counters are two
important elements of the frequency counter specification. The resolution
and accuracy, although loosely linked are two completely different aspects
of the performance of the frequency counter. It is found that the resolution
and accuracy are often equated with each other, but they are two distinctly
different concepts.
In order to be able to interpret the readings of a
frequency counter, it is necessary to have an understanding of the
difference between accuracy and resolution.
Resolution
The resolution of a frequency counter is its ability to differentiate
between two signals that are close to each other. It is a measure of the
number of digits in the reading of the signal frequency. Most frequency
counters are what are termed direct reading counters, and the resolution is
determined by the gate time. This is the time for which the counter is
counting the number of pulses or transition crossings. As the number of
crossings in a second is equal to the frequency, a one second gate time will
enable frequencies to be read down to a resolution on 1 Hz. It can be seen
that for other gate times, a 0.1 second gate time will allow a resolution of
10 Hz to be achieved, whereas a 10 second gate time will enable a resolution
of 0.1 Hz to be achieved.
Thus it can be seen that for a direct reading frequency
counter, the resolution is a function of the gate time.
Accuracy
The accuracy of a frequency counter is a little more difficult to determine
as it is a function of a number of factors. These inaccuracies can be split
into two main groups:
- Random errors
- Systematic errors
Random errors: The random errors
include a number of different factors. One is the quantisation error and
this arises from the uncertainty of the final count that appears in the gate
window time. Another random error arises from false triggering. This can
arise when random noise spikes give spurious counts. In addition to this
there are short term random instabilities in the time base.
Systematic errors: Apart from the
random errors, there are also the systematic errors in the readings. These
may be though of as biases that move the measuring instrument away from the
correct reading. The major source of these errors is the timebase. As a
result the timebase is normally crystal controlled, often using an oven
controlled oscillator, and for some instances where exceedingly accurate
measurements are required a rubidium standard may be employed.
The errors introduced on the frequency counter reading
may result from the basic frequency accuracy of the timebase, the ageing
rate, temperature variation and the variation with line voltage. To ensure a
highly accurate counter, a high stability clock or timebase oscillator is
required.
Often crystal oven oscillators may be used to ensure that
the oscillator remains stable with respect to temperature. Although more
expensive than an ordinary crystal oscillator, these oscillators are
contained within a small "oven" and run at a stabilized temperature, thereby
removing most of the effects o temperature variations. These oscillators are
normally an order of magnitude better than ordinary crystal oscillators.
Crystal ageing is another feature in the accuracy of a
timebase oscillator. It is found that the frequency of a crystal moves
slightly with time as a result of what is termed ageing. Although it is not
possible to stop the process, it is found that crystal ageing is more
apparent in the early months of the operation of the oscillator. Accordingly
it is possible to pre-age crystals by running them at elevated temperatures
before incorporating them in product. In this way the movement can be
reduced and a more accurate standard produced.
Total error
The overall accuracy of a combination of the random and systematic errors.
Often these can be summed statistically to provide a reasonable figure for
the overall accuracy. This can be done because it is very unlikely that all
the errors will act in the same way at the same time.
Summary
By understanding the errors in a frequency counter, it is possible to use
them more effectively, knowing what can and cannot be achieved. It is also
possible to better define a frequency counter when one is to be bought.
