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Tuesday, 3 September 2013

A Pixie - Really?


Been fiddling recently with some very basic CW transceiver designs, most notably the classic QRP "Pixie" design.

This is a very basic CW transceiver comprising a Colpitts oscillator, a second stage which is an RF Amp in TX and a mixer in RX plus an Audio Amp. The basic schematic looks something like this:

I stole the image above from The free information Society:

I've substituted the MPSH10 transistor in mine, but apart from that it's going to be the same circuit except for a different band.

To build one of these for a different band, it should be a case of changing the crystal frequency and also the output filter.I'd like to make one of these for the CW end of 20M; this led me to do some investigation into Low Pass output filters. Starting in the most excellent reference book Experimental Methods in RF Design, we can design a very simple Low Pass Filter like this:

Having constructed the filter above, it was clear that the roll off wasn't very sharp, so I did the maths for a 5 pole filter (above is a 3 pole). Here's the 5 poll filter constructed in a very experimental way:

I've hooked one side of the filter up to the Tracking Generator in the Spectrum Analyser and the other to the Analyser input, here's what I see:

Which means that when the basic oscillator in the Pixie is connected to this filter as its output low pass, we get this suppression of the output harmonics:

Which all looks rather good to me!

As an alternative to the "norm", here's a picture of Saki dog:

Fun, egh?

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