How to *test* rf circuits

Discussion in 'Wireless & RF Design' started by rswarbrick, Aug 24, 2011.

  1. rswarbrick

    Thread Starter New Member

    Aug 24, 2011
    1
    0
    Hi,

    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 (http://melexis.com/Assets/TH72031-DataSheet-4806.aspx) 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?

    Rupert
     
  2. wiskey six

    New Member

    Sep 18, 2011
    4
    0
    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.
     
  3. radiohead

    Active Member

    May 28, 2009
    474
    31
    Here's an IC that you might want to consider using.
     
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