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Thursday, 5 July 2012

Mixing Better than Before (70MHz)

Well, following on from my efforts at a 70MHz "transmit converter" here:

I've made some of the changes I said I would do. The main two things I have done are re-tune the local oscillator for the 60MHz harmonic rather than the 80MHz harmonic that I was using previously and also re-made the board thinking about layout and symmetry where required:

So now I have a 60MHz oscillator mixed with my 10MHz frequency reference I made here:

to make 60 + 10 = 70MHz signal. As you can see from the output on the spectrum analyser, the signal is now really much cleaner than before, even the 60MHz oscillator is at least 45dB down. I always like to remember here that 6dB is half voltage, so 45dB down from the 70MHz signal is a really good result.

So what I need to do now is swap the 10MHz frequency reference for the DDS from here:

and check that the bandpass filter at the output from here is wide enough for the parts of the 4M or 70MHz band that I am interested in.

The theory here is that a 10.0 to 10.5 MHz input signal should give me a 70.0 to 70.5 MHz output signal - if (and it's quite a big if) the bandpass filter will let that range of frequencies through.

Once thats all sorted I need to think about an RF amplifier for the output and an attenuator for the 10MHz input which eventually will come from one of my HF radios. The RF amplifier will be designed around an MRF151 power FET (because I have some), so that's something to start the next round of fiddling with, I also have a neat 48V switched mode PSU that should do nicely, now maybe I can make something for 6M also....
I even managed to be here and work someone on 4M the other day using the modified FT-847; it was SP6GWB, Stainslaw. He's got a really neat site here:

but thanks for the QSO! My first on 4M....

More madness to follow I'm sure, but it's good though, egh?

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