RF digital frequency counter

- summary or tutorial description of the RF frequency counter, how a digital frequency counter works, and how to use one.

Digital frequency counter tutorial includes:

    •  Digital frequency counter basics
    •  Counter accuracy & specifications

Digital 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, digital 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.

RF frequency counter basics

RF 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 digital 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.

Digital frequency counters are an essential tool for any accurate measurements of frequency. RF frequency counters may be used in development, production, repair or maintenance.. Of the two types, the direct frequency counter is the most common. In fact some digital 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 digital frequency coutners are available. However RF 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 these digital frequency counters still represent very good value.

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