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Sunday, 28 August 2011

Resonant or Otherwise

As I suggested yesterday, today I have been trying to test a bunch of crystals to see if I can match their resonant frequency. I have set the DDS and 'scope up as the diagram below:

So because the crystal is in parallel with the DDS frequency generator, I have to tune the DDS to get the minimum reading on the scope; this will then be the resonant frequency of the crystal. I think I need to be cautious over introducing capacitance into the circuit through test leads to get an accurate frequency, but as all I am trying to do is match crystals, rather than accurately measure their resonant frequency, I figured it didn't matter that much as they were all tested under the same conditions.

I only have 10 10MHz crystals here, and these are the results I found:

So I have quite a difference between individual crystals. If I now add a Hi and Lo line to my graph, we can see that four crystals are actually quite close to each other:

So what I propose to do now is re-make my crystal filter from yesterday using crystal numbers 1, 2, 4 and 8 and then re-test to see if my bandwidth has improved.

So after all that giggery pokery, the chart below shows the old crystal filter in red and the new one in blue:

So matching the crystals didn't make much difference, did it?

I can now calculate where the BFO in an SSB tranciever should be to get the USB and LSB passing through correctly:

So I'm going to go with these values and see how the performance looks.

Watch this space!

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