Thursday, September 23, 2010

The Subharmonic (Frequency Doubling) Mixer

This is one of my favorites, the subharmonic mixer. It sound complicated, it may be, but, to build it is not.
How, let's have a look on the options of mixing.

Mixing of a signal with a local oscillator (LO) can be done in two ways, either enable the bypass of a signal in the "rhythm" of the LO or shorten the signal in the "rhythm" of the LO. Usually this is done by a single "non linear element", e.g. diode. In the positive 0-180 degrees of phase, the diode is open, letting through the signal, in the negative 180-360 degrees of phase, the diode is closed, blocking the signal. Alternatively, shorting the signal to ground using the diode would provide the same results, a sum and a difference of the two frequencies.
So far so good...

But where's the doubling?!
Well, here it comes. Assume that two anti-parallel diodes are used as a mixer. Given that the LO is adjusted to the correct level, the first of the two diodes is open for one 1/4 of the period (90 degrees) on e.g. 50% of the positive half-period of the LO and the second of the two diodes is open for another 1/4 of the period (90 degrees) on e.g. 50% of the negative half-period of the LO. This would correspond to the mixer's diodes being open during the phase angles 45-135 degrees and 225-315 degrees. Compared to the 0-180 degrees a single diode would be open, the frequency is effectively doubled.

There are some advantages to this approach. In a direct-conversion receiver, the LO is far of the receiver's front-end, the preamp would therefore be unaffected. Also, a lower frequency oscillator is less critical in design. Any variation of the LO will have double impact on the operating frequency... this has pros and cons, e.g. VXO.

I like using sub-harmonic mixers, try 'em out for yourself!

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