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+/-1 count error, Time Base error, Trigger error, Systematic error

This page describes types of measurement errors in electronic counter viz. +/-1 count error, Time Base error, Trigger error, Systematic error. It mentions difference between +/-1 count error, Time Base error, Trigger error and Systematic error.

What is Electronic counter?

Definition: The electronic counter is a digital logic device which stores and displays specific events continuously based on its configurations and programming. Sequential digital logic circuit is common type of counter. It consists of single input line (i.e. clock) and number of output lines. The value of output lines denotes number in binary number system such as BCD. Electronic counters are implemented using register type circuits such as flipflops.

The major sources of measurement error are classified into following categories. These errors affect various measurements carried out and also affect counter timer.
• +/-1 count error
• Time Base error
• Trigger error
• Systematic error

+/-1 count error | ±1 count error

The electronic counter such as frequency counter display various digits. In this counter, output is discrete (i.e. quantized) even though the input is continuous. Even if input is stable, output readings may fluctuate. This fluctuation depends on quantization error known as +/-1 count error.

This ambiguity exists in LSD (Least Significant Digit) during electronic counter measurement. This error or ambiguity can occur due to non-coherence between clock frequency (e.g. internal) and input signal. The error is +/-1 out of total accumulated count measured in absolute terms.

Time Base error

Due to time base, the best the electronic counter can resolve is 1 part in 107 per second (i.e. 1 count in 10 MHz frequency). Though time base is accurate, its actual accuracy depends on type of clock oscillator. The accuracy of time base has direct impact on overall accuracy of the electronic counter.

Any error due to difference between actual time base oscillator frequency and (its) nominal frequency can be directly translated into measurement error. This difference is cumulative effect of all individual time base oscillator errors. It can be expressed as dimensionless factor e.g. so many parts per million.

Trigger error

This error occurs on time interval counter. It is a random error. It occurs due to noise on incoming signal as well as noise from input channels of the counter. Due to this error, gate will open or close very fast or very late. It has direct impact on random timing error for period as well as time interval measurements.

Systematic error

For time interval measurements, any slight mismatch between start channel and stop channel amplifier risetimes and propagation delays results in internal systematic errors. Mismatched probes or cable lengths introduce external systematic errors.

For time interval measurements, trigger level timing error is another systematic error which is caused by uncertainty in the actual trigger point. This uncertainty is not due to noise, however, but is due to offsets in trigger level readings caused by hysteresis and drifts. Trigger level timing error may be expressed as follows.
ΔT = trigger level error/signal slew rate at trigger point

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