Frequency counter accuracy and resolution

- an overview of resolution and accuracy for RF frequency counters, and a definition of the difference between the two figures.

This frequency counter tutorial is split into several pages each of which addresses different aspects of frequency counter technology and operation:

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.

Further pages from this tutorial
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