Counter Better Accuracy

Normal direct frequency counting

The evident method is to feed the measurement signal into a counter chip which is turned on for a set amount of time e.g. 1 second.

The counter value is actually the frequency measurement since:

f = events/time = counter value/1 second = counter value in Hz.

The problem with this method is that the solution of the counter is 1Hz and the number of digits displayed is dependent on the input frequency:

A 1Hz input gives 1 digit

A 100Hz input gives 3 digits etc.

The solution is in Hz (dependent on the gate time -which will have to be a multiple (or sub-multiple) of a second) – this is why the reciprocal counting method is better as it gives a solution in terms of the master clock frequency inside the unit that is not dependent on gate time and the number of digits is likewise not dependent on the input frequency.

Reciprocal frequency counting

The second method is reciprocal counting where rather of counting the input signal you count periods of a master clock and rather of counting the input signal edges you let the input signal commence and stop the counter.

For example the counter is started at the rising edge of the input signal and stopped at the next rising edge. Now the counter reading is genuinely the amount of time of the input signal in multiples of the master clock.

Its called reciprocal counting since you have to work out:

f= 1.0 / period

to work out the frequency.

The reason for using reciprocal counting

Reciprocal counting is more difficult as you have to use drifting point routines to work out the frequency but because the counter counts edges of a master clock the solution is fixed in multiples of that master clock and not dependent on the input signal i.e. the frequency counter will show all digits irrespective of the frequency of the input.

The solution of the measurement is better since in a normal counter the count will be out by ±1 Hz count whereas the reciprocal counter will be out by ±1 master clock cycle.

Note: This does not mean that it is more precise – accuracy depends on the clock stability of the unit.

Reciprocal counters are utile for amount of time measurements and you will find for mercantile counters that they normally have two inputs labeled A and B. You may set up the frequency counter to just measure A frequency on A or on B but you may likewise do signal amount of time measurements e.g. trigger on the rising edge of A to get the high amount of time etc.

In addition you may do measurements amongst dissimilar signals e.g. rising edge of A and only after that has triggered stop the count on the rising edge of B – that would be utile for measuring timing characteristics for a peripheral e.g. RAM access time.

You may find a project for a normal Direct frequency counter here.

You may find more selective information from the internetsite here.

Counter Better Accuracy Pic

Counter Better Accuracy Picture

Counter Better Accuracy Photo