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On 3/3/23 21:28, Shirley Dulcey KE1L wrote:

The Softrock uses a crystal that is approximately four times the frequency
you want to receive. It has to be a fundamental mode crystal; the
oscillator circuit is not designed for overtone crystals. That is divided
by four by the divider circuit that generates the quadrature signals. The
30 and 20 meter versions, and some of the IF versions, use 1/3 subharmonic
sampling because fundamental mode crystals for the necessary frequencies
were not readily available at the time.

Is there a write-up somewhere describing how this works?

So far as I can tell it would only work if the sampling interval were in the 17 ns range[1], which neither the quadrature mixer nor any sound card would appear to be capable of. Further, Softrock Lite II Home - Softrock Lite II RX <> contains by way of explanation:

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.

which would appear not to be subharmonic sampling at all, but the normal operation of the quadrature mixer taking advantage of there being enough power in the crystal's third harmonic to operate as usual with a 14.0475 MHz clock.

Have I missed something?


- Roland 9V1RT


1: At 30 MHz, the Nyquist rate is 60 MHz or ~1/(17 ns).

On 04/03/2023 02:53, Roland Turner via groups.io wrote:

which would appear not to be subharmonic sampling at all,

Roland,

/g/softrock40/message/76494

73 Alan G4ZFQ

On 4/3/23 16:14, Alan G4ZFQ wrote:

On 04/03/2023 02:53, Roland Turner via groups.io wrote:

which would appear not to be subharmonic sampling at all,

Roland,

/g/softrock40/message/76494

Thanks Alan, that makes a little more sense.


- Roland 9V1RT