1. Oct 20, 2009

### buzachaka

The idea has been bounced around, and i thought to see if anyone here had any guidance before i tear into the literature.

What kind of circuit is necessary to determine the frequency of an rf signal?

The situation:

An rf transmitter is pumping out waves (no modulation or data to be decoded) at some frequency in the kilohertz range. I want to know what that frequency is.

How do i go about finding it. Is there a scanner or simple circuit that could do the job?

*for the sake of argument, assume that it is the strongest signal in the vicinity

2. Oct 20, 2009

### Bob S

In order to measure frequency, you have to have a circuit that counts and displays cycles, and a frequency standard (stopwatch?) to set count time intervals. I have an old frequency counter I bought on ebay for about $50 that works well and will do the job. There are many for less than$100 on eBay now. It's a lot easier than building one.
Bob S

3. Oct 20, 2009

### buzachaka

Thank you so much! There's an arduino library for just the counter i need. Huge help.

4. Oct 20, 2009

### vk6kro

It depends how accurate you want the result to be.

So, you would just tune in the signal and read off the frequency on the dial.
If it was a silent (unmodulated ) carrier, you would get a heterodyne (whistle) in the audio and tune this until it was close to zero Hz. Then read off the frequency.

This would get you within 100 Hz or so of the correct frequency.

To do better, you need a frequency counter, a signal generator, a very nice computer program called Spectran, and a computer.

Spectran gives a moviing display of audio frequencies that looks like a waterfall.
The key to accuracy is the WWV signal on 5 or 10 MHz.
Tune to one of these.

Put your signal generator near the WWV signal and observe the two frequencies on Spectran.
Spectran gives a high resolution readout of audio frequencies, so you can tell how far away from WWV the signal generator is. So, you know its frequency to good accuracy.

Now, measure the signal generator's frequency with the frequency counter.
Calibrate the frequency counter so it reads exactly right.

Now view the signal you want to measure with Spectran and put the signal generator near it.
Measure the frequency of the signal generator with the frequency counter.
Calculate the frequency of the unknown signal using Spectran's readout.

This should get you to within about 5 Hz of the right frequency.

A much cruder method is to just put the signal generator on the same frequency as the incoming signal, zero beat it and read off the frequency of the signal generator using the frequency counter.
The signal generator is always a lot stronger than the incoming signal, so I have never had much luck with this technique, but it is always given as the classic way of doing it.
Frequency counters do drift and need to be calibrated before any serious measurement is done.

5. Oct 20, 2009

### waht

here is a guide on how to build one:

http://freenet-homepage.de/dl4yhf/freq_counter/freq_counter.html

If you are looking to buy one, I concur, ebay has a wide selection at affordable prices.

I used have a small 2.4 GHz pocket counter I got on birthday, and always carried it with me. I took it to the doctor's office once, and in the waiting room I checked up on any rf leakage. Surprisingly, there was 66 MHz or so coming from behind a TV. I came closer and saw a cheap VCR coax connected to some kind of a channel converter. It was poorly shielded and radiated rf like crazy. I told the nurse about it while she administered a shot, and then she posed having that "uuhhhhh" look. :uhh:

Last edited: Oct 21, 2009