How to *test* rf circuits

Thread Starter


Joined Aug 24, 2011

I hope this hasn't been asked too much here already: I couldn't find anything anyway.

Firstly some background (skip a paragraph if you don't care). I have a project planned which needs to send very low bit-rate data (basically just a "hello") one way over the air. The nontrivial bit is that the receiver, in particular, needs to be low power & run off a battery. My idea was that this is the same problem solved by car keys and the like, so looking around (and checking which bits of spectrum I'm allowed to make a mess of), I decided to try to use a chip like Melexis's TH72031 ( and broadcast at 869 MHz.

Now, the problem is that I don't have any serious experience of RF circuits, so I expect to make lots of mistakes (hopefully not frying too many chips). Also, 869 MHz is somewhat beyond most oscilloscopes (!), so how can I check what's going on? For example, how could I go about telling whether the FSK transmitter chip is transmitting at the base frequency I expect and modifying its frequencies as I expect? I'm wary of assuming I can build a correct receiver to test it with...

I had an idea of some counter-based approach: since the output is basically digital, you could use a counting circuit (say an 8 bit counter), and then look at the high bit, which would drop everything by a factor of 256. Maybe one could even chain these if necessary. Then you could get frequencies down to something you'd expect to plot on an oscilloscope.

Anyway, I don't know if that's the best approach or whether this has been done to death and I just didn't guess the right search terms! Does anyone have any advice?


wiskey six

Joined Sep 18, 2011
If I am correct, you could sample the outgoing frequency at the output of the second mixer or the band pass filter with a frequency counter.

Or use an external handheld frequency while the tranceiver is transmitting.