Wireless Frequency counter
This project is a wireless frequency from 25-200MHz.
The counter has a small coil which picks up the
oscillation frequency, and present it on a
display with 6 LED.
All contribution to this page are most welcome!
A really important instrument for radio builders is a frequency counter.
You can buy one for 200$ or you can take a few LEDs, a PIC, and some
transistors and build it yourself.
I did so and here is a very simple project
to begin with. This frequency counter has 6 digits and will work from 25MHz up to 200MHz.
I have not tested it at higher frequency, but it should go higher as well.
The best with this counter is that you don't need to connect it with any wires to your equipment. Wires=interference and drift in the oscillation frequency.
This counter use a small pick up coil to probe the oscillation. Just hold the pickup coil a few cm from the main oscillator coil and read the LED.
The LED has 6 digits and the resolution is set to 1kHz.
I don't think you need
better performance than that, in a portable wireless frequency counter.
This counter is perfect to check that oscillators are working and at which frequency.
Hardware and Schematic
The hart of this project is the PIC processor. To this processor is 6 LED display connected.
The info on the LED is scanned so only one LED is lightning at a time, but your eyes will
see it as all LED are lightning at the same time.
I use a sensitive prescaler which divide the signal with 64.
The output of the prescaler is then connected to a counter input of the PIC processor.
You can use almost any prescaler on the market in this construction. I have found a prescaler
called SP4633. It is a quit good one. High sensitivity and no self-oscillation.
The prescaler use a small pickup coil to probe the RF signal.
The pickup coil is made
of a wire with two turns. A small portion of the oscillator signal will be picked up by
this coil and amplified in the prescaler. The prescaler will also divide the input frequency
with 64 and output the signal to pin 6.
A NPN transistor is buffering the signal and amplify
it to square wave shape and then the signal enter the PIC-processor.
So how does it all work?
Imagin you have a 200MHz oscillator. The prescaler picks up the signal and divide it to
200*106 / 64 = 3.125MHz output signal.
Inside the PIC at RC0 is a 16 bit counter. This counter is first reset and then it counts
If the frequency was 3.125000MHz during 64mS, the counter will reach 200.000 which will be presented
to the display.
The resolution will be 1kHz which is good enough for a handheld wireless frequency
Of course one can make the counting time longer and thereby get more digits, but I seldom
need more than 1kHz resolution.
Building and Testing
It is not a difficult project to build, but before you add the prescaler you should test
the counter function. I used a external oscillator for this. See picture at right.
The pic show you a crystal controlled oscillator. I use a 3.579545MHz crystal.
Then I connected the output of the oscillator (pin 9) to the RC0 of the PIC (pin 11).
If I use a 3.579545MHz and it
simulate a division by 64 it means that the LED display should show 3.579.545 * 64 = 229.091MHz.
If you don't have a 3.579545MHz crystal you can use any one from 1-to 15MHz, you will have to calculate so the output frequency is about 1 to 4MHz of the oscillator.
Just multiply the frequency with 64 and read the display.
Simple way to test the construction.
The 64mS counting time in the processor is set by the software and is based on the crystal frequency at 18.432MHz. If you don't have this crystal you can use any crystal from 10 to 20MHz, but a small change of software must be done. I can help you with this, just mail me.
Download PIC16F870 program (INHX8M format - The hexfile is zipped)