[Lf] the best RX for 136 kHz?

[Andre' Kesteloot]

Klaus von der Heide wrote:

> Klaus, DJ5HG wrote:
>
> > > After an input filter a low-noise operational amplifier
> > > is followed by an analog to digital converter. The receiver
> > > structure, the demodulation, noise blanking etc... is entirely
> > > programmed on a DSP or on one of the new Million-Gates FPGAs.
>
> Alberto, I2PHD wrote:
>
> > I substantially agree, with a precisation :
> > depending on the type of demodulation you wish to do on the signal,
> > having the I and Q components is of great help. So, why not mix down
> > the 137 kHz signal to baseband, using a couple of mixers fed with
> > an oscillator that produce two signals in quadrature ? Some DDS have
> > this capability, so the +- 90 degrees shifting could also be done
> > digitally, reducing the analog part of the receiver to the bare minimum.
> > Then the two I and Q components could be fed to the left and right
> > inputs of a stereo sound card, and all the processing done on a Pentium,
> > which nowadays rivals in processing speed with DSPs that only a few years
> > ago were considered state of the art.
> > Just food for thought...
>
> Yes, that is a good design if the hardware effort of the whole system
> is to be minimized. Also, if you want to use one of the well-known
> DSP-cards as the DSP56002EVM that is the best solution.
> The DDS-module of your external front end should be clocked
> synchronously with the sampling rate of the analog to digital
> converter. That is a little technical problem when you use the
> sound card. The A/D-converter of the EVM-board can be clocked from
> an external clock.
> The second point in favour of the direct way of sampling the 136 kHz
> is the better linearity of the digital mixer.
> That are the reasons why I prefer to do the DDS and quadrature mix
> down to baseband by the DSP. Your design in principle is the same
> as mine, only the selection what is done in hardware or software
> is different with the result of minimum extra hardware in the case
> of the sound card, nice!
>
> 73 de Klaus, DJ5HG