[Fwd: LF: sound cards...well amybe if.....]

[Andre Kesteloot]

Stewart Nelson wrote:

> Hi all,
>
> An S&H with 1 us sample time and 100 us hold time is very easy for
> today's parts.  The state of the art is several orders of magnitude
> better for both parameters.  Take a look at this A/D with S/H:
> http://products.analog.com/products/info.asp?product=AD6645 .
> It can sample at up to 105 MHz, and with decimation, a.k.a.
> undersampling, works with band limited IF centered at up to
> 200 MHz.
>
> Clocking from the sound card is also no problem.  Just tap its
> internal sample clock and divide by 256 (or other oversample
> factor).  The phase will be arbitrary with respect to the
> output samples, but that should not matter.  Better, remove
> the sound card's crystal and feed in your own stable main clock.
>
> But it is much easier to design these systems when you can
> set the sample rate to a value suitable for the input
> frequency.  With a sound card, you may not have that option.
> Paul's example, with one A/D channel, would be unable to
> distinguish 143 kHz from 145 kHz; they would both produce
> a 1 kHz audio signal, since the third harmonic of 48 is 144.
> With stereo recording (using two S/H 90 degrees apart), you
> can reject the image, and ideal filtering would give you up
> to 48 kHz bandwidth, but maybe not the 48 kHz band you want.
>
> IMO, the linrad approach is much cleaner and will perform better.
>
> 73,
>
> Stewart KK7KA
>
> > On Tue, 29 Oct 2002 23:08:21 -0000, "Alan Melia"
> > <Alan.Melia@btinternet.com> wrote:
> >
> > >Hi Alberto, we are all jumping on you for your over-enthusiasm........but it
> > >occurs to me that this is a 24bit card....than may be more significant. It
> > >has the potential to give the level of s/n we might need.
> > >
> > >Could not the dreaded aliasing be used to benificial effect, or am I missing
> > >some subtle point. It occurs to me that a 96ksps sampler will 'fold back'
> > >136kHz  to 40kHz....so if any anti-aliasing filter could be disabled (I
> > >think, where used, these are normally passive rather than active ?).....it
> > >might be possible to have a software 136kHz RX !!
> > >THERE is a CHALLENGE for you software gurus !!
> > >I will await my idea to be shot down in flames, before I conside buying one
> > >of CL new audigy units !!
> > >Cheers de Alan G3NYK
> > >alan.melia@btinternet.com
> >
> > One could think about using decimation, in which the band is limited
> > by passive filters to say 125 kHz to 145 kHz, but as far as I
> > understand, in order to use decimation, you need a sample & hold
> > circuit ahead of the ADC with the sample time less than a half cycle
> > time at 135kHz (or about 3 us, preferably even less), while the hold
> > time should be the same as the ADC conversion time.
> >
> > In this example, even 48 kHz ADC sample rate would be sufficient, but
> > how do you control an external sample&hold circuit from a sound card ?
> >
> > Paul OH3LWR
> >
> >
> >