Nuts and Volts - March 2015

because the TC’ occurs 256 pulses early and I add one clock pulse for synchronizing, so I add minus 256 plus

1 to the desired gate time. Because the gate bank starts and stops exactly on a clock pulse (because of the gate synchronizer) and the delay through the counter is very low, the gate timing is very accurate. At the end of the gate time, the PIC reads the count bank outputs to get the frequency.

Making a period measurement follows a similar path, except the gate bank and count bank inputs are reversed by U2. The PIC preloads the gate bank with a value equal to minus the number of input signal pulses minus 255. For example, to count 1,000 signal pulses, the gate bank is loaded with minus 1,255 (minus 1,000 minus 255). Period measurements are a little more complex since we have to pick the number of input signal pulses (the 1,000 in this example).

The accuracy of the measurement depends on how many internal reference clock counts are accumulated in the count bank. If we use the 1,000 signal pulses discussed here and the input signal is 1,000 Hz, then the gate will be open for one second and the count bank will record 10,000,000. This gives us our 7/8 digit accuracy.

However, if the input signal frequency is 100 kHz, the gate will be open for .01 seconds and the count bank will record 100,000 or 5/6 digit accuracy. On the other hand, if the input signal frequency is 10 Hz, the gate will stay open for 100 seconds. As we’ll see shortly, the software (available at the article link) solves this problem nicely and will force a gate time of close to one second (or whatever value we want). Five more components complete the

1. The input signal amplifier.

2. The rotary switch input circuit.

3. The PIC16F886.

4. The display.

5. The internal 10 MHz frequency reference.

The input signal amplifier (Figure 4) is used to take a low amplitude input signal with a frequency range of 10 Hz to over 70 MHz and convert it to a TTL voltage level. Every frequency counter has some sort of input amplifier and the Internet is full of designs ranging from simple two-transistor FET amplifiers to multi-component designs often using the popular MC10116 ECL triple line receiver.

The frequency range can be increased to the 300 MHz range, or even higher by using a prescaler. Again, the Internet has many designs for this. The rotary switch input circuit is simply a four-resistor voltage divider, with the voltage taps connected to the switched terminals of a rotary switch. The switch wiper terminal connects to RA0 on the PIC, and the PIC A/D converter determines the switch position by simply reading the voltage on pin RA0.