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RE: [linrad] re: CS5396
Chris Albertson wrote:
> Have you looked at the Maxim ADC chips. Prices in the range
> of US$60 for one unit but they will ship free samples if you
> ask. They make ADC specifically for sampeling RF or IF
> frequency. The "MAX1200" for example does 16-bits at
> one megasample rate. If you can go with less bits (8) you can
> get sample rates at 60Mhz. see www.maxim-ic.com
16 bits at 1MHz is not quite good enough. It corresponds to
14 bits at 16 MHz and not even the Analog Devices chip which
samples 14 bits at 65 MHz provides enough dynamic range
for a good EME system on 144 MHz. The new 16 bit 65MHz ADC
should be fine:-)
> I have worked on a few radar projects (which now that I think of
> it) were actually software radios. We sampled in quadarure at
> the tens of megasample rate at 8-bits and performed synthethic
> aperature algorithum in software.
Yes but as amateurs we face far more difficult dynamic range
problems. The desired signal is easily 110dB weaker than
a signal which is only a couple of kHz separated in frequency.
The ADC must handle these 100 dB plus an additional 20 dB
degradation of the dynamic range that comes from the need to
place the mast mounted preamplifier noise 16 to 20 dB above
the noise floor of the ADC itself.
> I thought it might be possible to home brew a software radio
> that decoded most modes. I looked around and found this project.
> I also found the softmodem drivers built into Linux. Both projects
> are built around sound cards. The problem is sound cards have,
> and no surprize, audio bandwidths. I think sound cards will
> always have limited bandwidth. So I thought about looking into
> digital video then found that the chip makers already are thinking
> about supporting software radio and the component prices are not
> unreasonable either.
I am sure we will have PC cards for software radios in the
future. I guess they will be for the mass market so they will not
quite give the performance I am looking for, but for anyone in
a good location where high dynamic range is not needed this
will surely be a fine low cost solution.
As soon I have such a card and the appropriate Linux driver
I will make sure it works with Linrad:-)
Leif / SM5BSZ