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 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 equipment. Where higher frequencies are to be measured, the timer element is not included and the test equipment is just a frequency counter.

an image of a typical frequency counter in this case a TTi TF930

RF frequency counter basics

RF frequency counters and timers are items of test equipment that 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

Frequency counter block diagram showing the incoming waveform is gated for a given time and the pulses counted
Basic frequency counter block diagram

Within the basic counter there are several main blocks:

  • Input:   When the signal enters the frequency counter it enters the input amplifier where the signal is converted into a logic rectangular wave for processing within the digital circuitry in the rest of the counter. Normally this stage contains a Schmitt trigger circuit so that noise does not cause spurious edges that would give rise to additional pulses that would be counted.

    Trigger levels and sensitivity are controlled within this area of the frequency counter.
  • Accurate time-base / clock:   In order to create the various gate / timing signals within the frequency counter an accurate timebase or clock is required. This is typically is a crystal oscillator and in high quality test instruments it will be an oven controlled crystal oscillator. In many instruments, there will be the capability to use a better quality external oscillator, or to use the frequency counter oscillator for other instruments. This is also beneficial when it is necessary to lock a number of instruments to the same standard.
  • Decade dividers and flip-flop:   The clock oscillator is used to provide an accurately timed gate signal that will allow through pulses from the incoming signal. This is generated fromth e clock by dividing the clock signal by decade dividers and then feeding this into a flip flop to give the enabling pulse for the main gate
  • Gate:   The precisely timed gate enabling signal from the clock is applied to one input of a gate and the other has a train of pulses from the incoming signal. The resultant output from the gate is a series of pulses for a precise amount of time. For example if the incoming signal was at 1 MHz and the gate was opened for 1 second, then 1 million pulses would be allowed through.
  • Counter/ latch:   The counter takes the incoming pulses from the gate. It has a set of divide-by-10 stages (number equal to the number of display digits minus 1). Each stage divides by ten and therefore as they are chained the first stage is the input divided by ten, the next is the input divided by 10 x 10, and so forth. These counter outputs are then used to drive the display.

    In order to hold the output in place while the figures are being displayed, the output is latched. Typically the latch will hold the last result while the counter is counting a new reading. In this way the display will remain static until a new result can be displayed at which point the latch will be updated and the new reading presented to the display.
  • Display:   The display takes the output from the latch and displays it in a normal readable format. LCD, or LED displays are the most common. There is a digit for each decade the counter can display. Obviously other relevant information may be displayed on the display as well.

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 counters

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 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 item of test equipment 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 counters 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.

By Ian Poole

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