The Inaccuracies of Frequency Counter are as follows:
1. The ± count error:
One of the forms of frequency counter error occurs due to the inexact timing of the gate open and closure with respect to the incoming waveform. Sometimes it is possible for the time base to open so that there is one more count than at other times. The ambiguity occurs because of the non-coherent relationship between the time base clock and the incoming waveform.
The time base gate is open for the same amount of time in each case, but in the first case only one positive going edge is counted, whereas in the second case, two are counted. The reason for this is the relative timing or phasing between the two signals.
2. Time base error:
Although the time base will usually be accurate, its actual accuracy is dependent upon the type of clock oscillator used. Its accuracy will have a direct impact on the overall frequency counter accuracy.
The oscillator accuracy is the sum of the various oscillator errors:
- Mains / line voltage variations:
Variations in the power input voltage will cause variations of the voltages to the crystal oscillator. The level of the variation will depend upon the effectiveness of any voltage regulators, but there is always some small voltage variation. This can result in small errors in the crystal oscillator voltage that will be reflected in the frequency counter timer errors.
- Temperature stability:
Temperature stability of the crystal oscillator used for the clock or time base is one of the main sources of error or inaccuracy. The short term stability is generally quoted in parts per million over a temperature range. Often when wanting to make accurate measurements, it is wise to allow the test instrument to warm up and for the temperature to stabilize over a period of a few hours before the measurement is taken. Oven controlled crystal oscillators may reach their specified accuracy sooner, but experience generally indicates allowing a frequency counter, or any other test instrument to warm up and stabilize over a few hours before making any accurate measurements.
- Long term stability:
This form of crystal oscillator frequency error occurs over time. While many high grade crystal oscillators are pre-aged, these errors still occur to some degree. Typically they are expressed in parts per million over a month. A period this long is taken because the effects of ageing are masked by the short term effects over much shorter periods of time. The errors resulting from ageing of the frequency counter timer clock can be reduced by periodic recalibration of the test instrument.
- Short term stability:
The short term variation includes elements such as the short term frequency variations including phase noise / phase jitter. If phase noise is high it can mean that the gate period could vary or jitter by an amount that could cause fewer or more pulses to be gated than the exact gate time would allow through.
3. Trigger error:
Trigger errors on a frequency counter are those errors that occur on a time interval counter as a result of noise on the incoming signal that result in the input gate being opened or closed too soon or too late. They cause the one limit of the hysteresis window of the input trigger to fire at the wrong time, thereby introducing a timing measurement error. This error can be made worse by increasing the sensitivity of the input circuitry and thereby allowing noise to have a greater effect.
4. Systematic error:
This form of measurement error on a test instrument occurs as a result of a mismatch between the start and stop channels. It may be a result of differences in channel rise or fall times, or the propagation delay differences.