# Frequency Counter Specifications

### RF Frequency Counter Tutorial

Like any other item of test equipment, it is necessary to specify the performance of the frequency counter, so that prospective users can match it to their requirements.

As with any item to test equipment, the specifications of the frequency need to be fully understood to enable their performance to be correctly judged to ensure they will be able to undertake the measurements that they will need to make.

Many of the frequency counter specifications are easily understood, but still have some finer points that need to be kept in mind when making any decisions. Other frequency counter specifications are specific to the type of test equipment and are not always so easily understood.

## Frequency counter frequency range specification

One of the first frequency counter specifications is the input frequency range. This is the range over the input amplifier is specified. Often the input will be selectable coupled between AC and DC. When this is the case, both must be specified separately as they will be different.

Typical examples of how this may be specified are:

 DC Coupled:     DC to 250MHz AC Coupled:     20 Hz to 100 MHz

As shown in the example above the bottom end of the coverage for the AC coupled inputs typically starts are frequencies of a few tens of hertz. Normally the input impedance will be high and therefore the coupling will not impair the low frequency response too much.

The AC coupling is useful when it may be necessary to measure signals that have a DC component on them.

## Input impedance specification

Like other instruments, frequency counters have a specification for their input impedance. This is the impedance of the signal input to the counter. There are two forms of frequency counter specification for the input:,/p>

• Standard input specification:   Most frequency counters have a high input impedance. This is specified in terms of a resistance and the capacitance that is in parallel with it. Typically this might be 1MΩ / 20pF, i.e. 1 MΩ resistance and 20pF capacitance. This form of input is normally used for frequencies up to 10 to 30 MHz or so.
• RF / microwave input specification:   Inputs used for frequencies above 30 MHz or so, especially those intended particularly for RF or microwave measurement applications only often have a matched input. This is typically 50 Ω. These are used with matched impedance systems. When using these inputs, care must be taken not to apply signals that are too large otherwise the terminating resistor may blow.

## Sensitivity specification

The sensitivity specification for the frequency counter defines the lowest signal amplitude that he counter will count. If there is a trigger level setting, it assumes that this will have been set for a value equal to the midpoint of the input signal.

The sensitivity specification is actually a measure of the level of hysteresis in the input comparator. As this may vary with frequency, the sensitivity specification may be split into two or more frequency ranges.

To look at how this frequency counter specification is derived it is necessary to look at the input and triggering of the frequency counter.

The input consists of an amplifier followed by a Schmitt trigger circuit. The signal, shown as a sine wave, enters the circuit and triggers, typically on the positive going edge causing the Schmitt trigger to change state.

To look at how this frequency counter specification is derived it is necessary to look at the input and triggering of the frequency counter.

The input consists of an amplifier followed by a Schmitt trigger circuit. The signal, shown as a sine wave, enters the circuit and triggers, typically on the positive going edge causing the Schmitt trigger to change state. Then at a point on the negative going edge, the Schmitt trigger will change state again. There is a difference between the two called the hysteresis which prevents small levels of noise from spuriously triggering the counter.

Frequency counter triggering

The difference between the two switching states for the Schmitt trigger, Von - Voff is known as the switching window.

From a knowledge of the two switching states it is possible to generate a specification for the RMS sine wave sensitivity:

If the sensitivity of a frequency counter can be set, then care must be taken not to increase the sensitivity level to far to the state that it detects very small signals. Noise and other signals such as ringing on any square waves can then be a problem, and the frequency counter may trigger on these signals giving spurious counts and as a result the readings will be false.

## Signal operating range

This specification, sometimes called the dynamic range refers to the input voltage operating range for the frequency counter.

The input stages of the frequency counter or timer will only be able operate correctly up to a certain voltage. Beyond this distortion and overload issues will arise.

Issues associated with overload may include miscounting or timing inaccuracies. Often (but not always) these will be gross inaccuracies so the problem may quickly become obvious.

There are two ways in which the allowable signal range can be exceeded:

• Signal exceeds operating range:   The operating range specification of the frequency counter is the maximum voltage level that can appear at the input terminals. Typically this may be absolute voltage levels of, for example ± 5V. if the voltage of the waveform rises above 5V or falls below -5V over part of its cycle then it could overload the input.
• Signal exceeds dynamic range:   This element of the frequency counter specification refers to the peak to peak value of the waveform. Sometimes the dynamic range may be less than the operating range. It could therefore fall outside the operational limits if it is above the dynamic range, but within the operating range.

It can be seen that a) is within specification as it falls within both the operating and dynamic ranges. Example b) falls outside the dynamic range, and although it is within the operating range, this is not allowable. Example c) falls outside the operating range even though it is within the dynamic range and is therefore not allowable.

## Frequency counter time base specification

One of the key elements of any frequency counter or counter timer is the accuracy of the measurements made.

The accuracy of the measurements are fundamentally governed by the accuracy of the oscillator used for the timebase.

There are three main types of time base or clock oscillator that are generally used for frequency counters and time interval counter-timers:

• Crystal oscillator:   A basic or room temperature crystal oscillator will only be used in the budget frequency counters and counter timers. Although very accurate by some standards, they are often not sufficiently accurate for laboratory measurements. They may be able to achieve accuracy levels of around 1 part in 10^6 or thereabouts. Typically these oscillators will be aimed at operation within a room with a reasonably stable temperature.
• Temperature compensated crystal oscillator, TCXO:   This form of crystal oscillator has compensation for any temperature variations. TCXOs can be bought as manufactured products and give a much better level of accuracy than that of an uncompensated oscillator.
• Oven controlled crystal oscillator, OCXO:   The oven controlled crystal oscillator is the most widely used in frequency counters because its specification enables it to give a level of performance that is much better than the other two forms of oscillator. The OCXO oscillator is contained within a small insulated box and its temperature is accurately maintained to ensure that the crystal resonant frequency remains as constant as possible.

Although sometimes there are forms of oscillator, these are the most widely used.

These are some of the main frequency counter specifications used to defined the performance of a particular test instrument. The resolution and accuracy parameters are addressed on the following page as they are interlinked and require a more involved treatment.

By Ian Poole

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