Column Control DTX

Choosing the Best Frequency Counter: 10 Hints

Application Notes


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Counters can be a plug-and-play instrument and seem fairly simple from the outside. You connect a signal to the input, and a digital readout tells you the frequency or some other parameter. However, to achieve the best results, whether that means speed or quality, attention to how you set up the counter measurement is important.

Choosing the Best Counter

Selecting which counter will best meet your needs is the first step. There are several related products that perform a variety of tasks at various frequencies:

• Universal counters

Both frequency and time interval measurements, as well as a number of related parameters.

• RF frequency counters

Precise frequency measurements, up to 3 GHz and beyond.

• Microwave frequency counters

Precise frequency measurements, up to 40 GHz and beyond.

• Time interval analyzers

Optimized for precision time interval measurements.

• Modulation domain analyzers

Designed to show modulation quantities, such as frequency versus time, phase versus time, and time interval versus time.

Hint 1: Recognize the difference between resolution and accuracy 

Assuming a large number of digits equates to a very accurate measurement may not be correct. It is a common mistake to equate resolution and accuracy. They are related, but different concepts. 

The resolution of a counter is the smallest change it can detect in closely spaced frequencies. All other things being equal (such as measurement time and product cost), more digits are better—but the digits you see on the display need to be supported by accuracy. Digits can be deceptive when other errors push the counter’s resolving ability away from the actual frequency. In other words, it’s possible for a counter to give you a very accurate reading of an incorrect frequency. 

Random and systematic errors both determine a counter’s accuracy. Random errors are the source of resolution uncertainties and include: 

• Quantization error 

When a counter makes a measurement, a ±1 count ambiguity can exist in the least significant digit. This can occur because of the non-coherence between the internal clock frequency and the input signal. 

• Trigger error 

Noise spikes can be triggered by noise on the input signal or noise from the input channels of the counter. 

• Timebase error 

Any error resulting from the difference between the actual time base oscillator frequency and its nominal frequency is directly translated into measurement error. 

Systematic errors are biases in the measurement system that push its readings away from the actual frequency of the signal. This group includes effects on the time base crystal such as aging, temperature, and line voltage variations. 

Compare the two counters in Figure 1. Counter A has good resolution but a serious systematic error, so its displayed result in most cases will be less accurate than those of Counter B, which has poorer resolution but a smaller systematic bias error.