[linrad] Re: RX2500 and audio cards sampling at 192 Khz

[Leif Asbrink <sm5bsz.com; leif@xxxxxxxxxxxxxxxx>]

Hello Pierre and all,

> The Oxygen HD CMI 8788 chipset from C-Media allows sampling at 192Khz and 
> seems to be supported by OSS.
> A review of a soundcard using this chipset can be found at 
> http://www.elitebastards.com/cms/index.php?option=com_content&task=view&id=278&Itemid=27&limit=1&limitstart=0
> This card is sold for 70 pound in the uk by
> http://www.wired2fire.co.uk/product_info.php?cPath=9&products_id=15720
> 
> Before I give it a try I would like to get your advise about the modification 
> that would be required  in the RX2500.
I once started to modify a unit to fit the Lynx-Two soundcard:
http://www.sm5bsz.com/linuxdsp/rxiq/rx2500lynx.htm

As it turned out, the Lynx-two is completely useless for an
SDR because of the extremely high phase noise on the
sampling clock.

I did not continue the modifications because I have no
good 192 kHz soundcard. The one you point at only has
two channels for the input. To use this card I would
have to use two of them and they would have to be 
synchronized.



> I assume that the audio section of the RX2500 willl require some modifications 
> in the anti-alias filter  (and maybe in other parts of the receiver also ).
Yes.

> Could you confirm this assumption and if so, give us some guidelines as how to 
> proceed.
> Is it possible to make these modifications without the help of sophisticated 
> measurement instruments?
No. But yes:-) You will need a computer in which you
can run Linrad. Depending on what you want to achieve
you also need one or a few signal sources - and if you
want to verify sideband noise performance you will need
a signal source that is a bit better than the -155 dBc/Hz
that one might guess the ADC will provide.

A better soundcard will de designed for the same max
signal level, but it will have a lower noise floor. Perhaps
by 10 dB. This means it would be nice to reduce the gain
of the RX2500 by 10 dB or perhaps a bit less depending
on whether you want optimum sideband noise or optimum IP3.

Lowering the gain by as much as 10 dB is not easy however.
Most of the gain is in the mixer and it uses a feedback with
a voltage divider using 340 and 47 ohms. One can not reduce 
gain by making the 340 ohm resistor smaller, the op-amp
can not drive a smaller resistor. It is possible to
increase the 47 ohm resistor, but that will cause a degradation
of the noise floor due to the thermal noise in a bigger 
resistor so it would be a compromise.

The op-amps that do filtering have voltage gains
2.0 * 1.47 * 1.47 => 12.5 dB so here is where gain should be
decreased. Filter performance will be degraded, but that
will probably be OK. Since gain is reduced by X dB, signals
above the Nyquist frequency will need X dB less attenuation. 

The details of how the anti-alias filter of the soundcard is
designed will determine whether the notch filters that the 
RM6 cores provide will still be needed.

The 2.5 MHz RF filter can easily be made wider. The 22 pF
coupling capacitors have to be increased and coupling
(1+1.5 nF in parallel) has to be decreased. Another way is to
increase L and decrease the tuning capacitors.

                          --  o  --

I will make the mods as well as I can as soon as I know a
soundcard that can provide 4 channels at 192 kHz with
24 or 32 bit drivers for Linux that conform to the original
soundcard API. (ALSA does not provide more than 16 bit and
will not support more in the future as far as I know. I have 
no idea how to use ALSAs own API....)

73

Leif / SM5BSZ






> 
> Thanks in advance and best 73
> 
> Pierre /ON5GN
> 
>   
> 
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