Time-Frequency instrumentation - Improvements in counting techniques

The frequency of a signal is a measure of the number of oscillations in a unit of time.

Metering techniques and basic (conventional) meters were the subject of the previous report [R 686] .

To determine a frequency, all you need to do is count and store a number of pulses. It is possible to count the number of pulses of the unknown signal during a reference period, or conversely to count the number of cycles of a reference clock during a signal period. However, whatever the method used, the synchronization of the reference variable and the signal to be measured is never perfect. As a result, there are usually fractional time residuals T _a at the start and T _b at the end of the count, which are not taken into account. These two times are generally short and therefore more difficult to measure.

In this issue, we focus on measurement methods that aim to improve meter resolution and take these residuals into account. For example, more sophisticated counters include analog or digital interpolators that measure T _a and T _b .

Basic techniques using CTN time-digital converters with a low measurement span (typically between 10 and 20 ns) have much better accuracy than conventional meters. CTN accuracy is essentially determined by the differential or INL integral non-linearities of time-to-digital conversion, as in commonly used CAN analog-to-digital converters.

Methods for improving counter resolution can be classified as analog A and digital N. The most common are :

• time interval stretching (A), followed by the counter method (N);

• double conversion: time to amplitude (A), followed by conventional analog/digital (A/D) conversion;

• the Vernier method activated by two oscillators (N) ;

• time-to-digital conversion using fixed delay lines (N) ;

• the Vernier method with a differential delay line composed of two fixed delay lines (N).

The methods listed above are used in two ways.

For the first time, the method is applied without the use of an additional conventional real-time counter. The...