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On 4/11/2014 3:11 PM, R. R. (Robby) Robson wrote:

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See, also,? (the discussion on sub-harmonic sampling)

Hi Robby,

That actually has it wrong.? The statement:

• The two quadrature signals from the Divider stage are fed into the mixer stage. The 4.6825 MHz quadrature signals from the divider in the 20m option are rich in harmonics. The third harmonic (14.0475) is what is actually used in the mixer stage.

It is true that the 4.6825MHz from the divider is rich in harmonics (all odd becuz of square wave), but only the fundamental freq is used by the mixer (since it is a digital chip and only responds when the signal crosses the threshold).? The mixer itself is what allows the sensitivity to harmonics, becuz it is a sampling type.? A common analogy is a strobe light on a fan....you will see the same thing when strobing for one revolution, also for two revolutions, also for three, ...

The mixer is sensitive to the fundamental and all harmonics of the mixer sampling freq, but the transformer (with 180deg phasing) and the hookup of the mixer cancels even harmonics of the freq; leaving only the fundamental and odd harmonics.

Just as you would see with a strobe light the signal will be reduced (over the same time period).? The reduction for the 3rd harmonic is 1/3 the voltage on the holding cap.? That give a overall 20log(1/3) = -9.54dB loss in the signal compare to receiving the fundamental.? This is the drawback to sub-sampling; i.e. a reduction in sensitivity.? It does allow the mixer to switch at a much lower rate which is why it is used.

The front end filter rejects the fundamental and also the 5th and higher harmonic products so you are left with only the third.

The harmonic rich LO is often misstated as why the receiver works with harmonics.? If the LO was a sine wave it would still work fine (maybe more phase noise, but basically the same).

73,? Mike Collins?? KF4BQ