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Sunday, 19 January 2014

Yet more Minima!


Following on from yesterdays antics, I've done a little bit more this morning. Firstly I needed to make the "KISS" mixer I mentioned last time. It's a simple design using two FETs which are being switched on and off alternatively by the VFO signal. Here I have fed a 7MHz signal into the LPF which in turn feeds into the one RF port, the LO port has a 13MHz signal and the output is connected to the 50R terminated Spectrum Analyser:

So above we can see the large peak at 20MHz which is the sum of the two input signals - I adjusted the preset for the cleanest looking signal with the highest amplitude of the 20MHz sum output. Once we have the other parts of the transceiver built I can come back and fiddle with this some more.

Here we have a 7MHz signal into the LPF which in turn feeds into one RF port, the LO port has a 27MHz signal and the output is again connected to the SA:

I've tried to highlight some of the bigger peaks with markers in the image above; but I only have 4! There is the 7MHz signal and it's harmonics, the 27MHz LO signal plus all the sums, differences and other gubbins. In this example I am simply trying to see that the mixer is indeed mixing!

So here we have a 27MHz LO with a 20MHz signal to the RF port resulting in a 7MHz signal out of the 15MHz LPF:

Next to build is the crystal filter. This uses two parallel crystals at the input and output and a total of 8 matched 20MHz crystals are needed. I built a very simple test rig to enable individual crystals to be measured like this:

In this setup the frequency where the biggest attenuation takes place should be the exact resonant frequency of the crystal. You could just as easily do this with a manual signal generator and look for the smallest signal on a 'scope. Here's an example with a crystal in the test rig:

So I found a bag of 100 20MHz crystals and kept testing individuals until I found 8 that were very close to the marker frequency set in the image above. I then used these to create the crystal filter. Here's a sweep of the filter once built:

So this looks quite impressive, there's a very sharp skirt at each side - which we will need to suppress the carrier - we will have to adjust the USB and LSB BFO frequencies to get the required sideband within the filter and the carrier and the sideband we don't want attenuated.

Progress is fast and furious!

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