[hpsdr] Mercury

[Philip Covington]

On 9/24/06, jeff millar <wa1hco at adelphia.net> wrote:
> Lyle Johnson wrote:
> >
> > Since these are flip-flops and not MUXes, I don't understand their use.
> >   Can't we just run the ADC output directly to the FPGA and to the DDC?
> >
> ADC performance depend critically on minimizing the digital switching
> currents flowing on the digital pins (to minimize noise). Designs always
> buffer the output unless the lines run straight to a single device (that
> has low capacitance inputs).
> > Do we need a larger/different family FPGA for more resources or speed to
> > implement a DDC?  I'm concerned that the ADI part may not have a long
> > life.  And of course, it is BGA...  If we are prepared to use BGA, we
> > can get a very fast, very dense FPGA.
> >
> If an ASIC can do the job, then it will have less size and power
> consumption than an FPGA from the same technology era, because ASICs
> have more optimized designs. The gap is about 10:1 in base cost. Another
> way to say it...It takes 5 years of Moore's Law advancement for an FPGA
> to equal an optimal ASIC in size, power and base cost. The FPGA has an
> advantage when no ASIC exists for the available ASICs don't fit the
> requirement very well.
>
> The AD6636 has 5 channels and most of today's SDR applications only
> require one, so an FPGA gains some advantage by implementing just what
> the SDR needs.

It actually has 4 or 6 depending on the version.  We want to be able
to use multiple channels (for example a rx on 40 meters and a
simultaneous rx on 20 meters) so it causes the FPGA to have to be even
larger to support the same functions as the AD6636.  Just doing one
channel and maintaining the dynamic range we want pushes us into a
FPGA in a BGA unless we use an older device family.  Any which way it
is more expensive to use the FPGA than the AD6636 for equivalent
function.

> Maybe a different part in the AD66xx family that costs less, such as the
> AD6620??

The AD6620 is not fast enough.  The AD6624A would be if we limited the
LTC2208 clock to 100 MHz.  Since there is a LTC2209 coming soon with
the same pinout as the LTC2208 we would be best off using the AD6636
at 150 MSPS.

> jeff, wa1hco
>
>

73 de Phil N8VB

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