Monday, December 9, 2013

How to tune the suburban sub-harmonic receiver

Some help to adjust the balance and drive of the sub-harmonic receiver used in my grabber (dashed lines indicate 0.7V cut-in voltage for Si-diodes, V is mixer drive voltage).

1) Mixer correctly driven: Within one period a single diode opens for exactly 90 degrees (red). Within the same period, the other diode opens for 90 degrees (blue), while the rest of the time, the diodes are both closed (yellow). As you see, the first diode opens at 45 degrees, that is very important! You also see, the entire mixer is open twice within one period; that’s the trick of the frequency doubling. Any off-balance shift will produce harmonics, which you can’t get rid of.

2) Let’s have a look at the mixer being extremely out of balance. In this example, -0.7V, which would open the second diode, is never reached, hence, this diode will never open. Consequently, the mixer will open once in a period, thereby not doubling the frequency. Still, due to under-sampling, some signal will be received, however, this is essentially a DC-RX for the LO-frequency. Actually, one can make use of this, e.g. for a 14MHz sub-harmonic receiver, which, when adding the correct off-set, will turn into a 7MHz DC-RX (either with a second crystal filter, or non at all).

3) And now, the mixer in balance, but over-driven. This will let a lot of signal through and will actually also double the frequency. However, the phase of the mixer is now totally off, i.e. asymmetric. This will produce a lot harmonics, so the receiver will receiver on many multiples of the local oscillator.

Actually, it takes a bit of patience to find the sweet spot in such a receiver. For me, it works well, for many years by now.