Wednesday, February 5, 2014

The Farhan Minima RF Mixer

I have been wanting a simple Mixer Circuit for experiments in my electronics shop, and most recently for use with my Small Signal Amplifier (see previous post).

Alan - K6ZY at the last Puget Sound QRP (pQRP) meeting mentioned he was considering building a Homebrew Farhan VU2ESE Minima Transceiver project.  After looking at the schematic for the Mimina, it appears that the Mixer Circuit would work very nicely as an stand-alone mixer for my experimental use. Very good documentation of the mixer's operation is provided by Farhan. The mixer used in the Minima has been dubbed the “KISS Mixer” by Chris Trask in his paper.

Minima Mixer Circuit in LTSpice
To play with the idea of building an experimental mixer, I created a LTSpice simulation circuit to start my understanding of it operation.

So far I have not gleamed much information from the LTSpice simulation, but maybe I have something wrong with my circuit; bias, configuration, signal levels or expected output. Or, maybe LTSpice can not deal with mixed signals correctly, but  I will continue working with LTSpice to learn more.

In preparation for my experiment and use of the Mixer Circuit, I have started the initial design of a PCB using DipTrace. Keeping with my ever present Goal of building electronic projects as small as I can, this initial design is larger than I think the final design will be, as I have currently configured it to used larger components than necessary. I think I can find small SOT-23 J310 JFET packages and smaller transformer cores. But this is a start.

Mixer Layout in DipTrace
 I have configured the PCB as a single-sided circuit as it can be produced with simple Homebrew Toner Transfer Method. But perhaps, using double-sided will be best for reducing the over all size.

This mixer as a stand-alone device will be very useful in my electronics shop - Thanks Farhan.

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