[linrad] Re: Linrad via Ethernet

[<leif@xxxxxxxxxx>]

> I've had a brief look over the respective datasheets; is there really
> anything you can do with the GC401x that cannot be done by an AD6620
> + either another AD6620 in cascade or post-filtering on a PC ?
Hmmm, maybe one could use a pair of AD6620s. The problem is that it
does not have a resampler so one has to use minimum decimation on
the filters.

With an AD6644 sampling at 66.666MHz (as in SDR-14)
one could set Mcic2 and Mcic5 to 2 for an input to the
FIR filter of 16.666 MHz. By setting Mrcf to 4 one could
get the output at 4.1666MHz but the filter required to get
a decent alias suppression would be much narrower than
4 MHz. At this data rate only 8 taps are available for the
FIR filter. A second AD6620 with Mcic2=1, Mcic5=2 and
Mrcf=2 could probably reshape the filter to a reasonable 1 MHz
bandwidth by using 32 taps in the rcf filter. Still the signal
would have to be a bit undersampled to allow good enough alias
suppression.

Maybe it would be better to use some general purpose gate array
for the second filter. I do not like the idea of allowing the PC
to receive oversampled signals because any spare processing power
it might have can be put to bettert use:-)

With the SDR-14, the optimum AD6620 parameters are Mcic2=3,
Mcic50, Mrcf=4. This allows 180 taps for the fir filter at
an output data rate of 185 kHz. The CIC2 aliasing spurs are at
-80 dB only but cic5 and rcf aliasing are well suppressed
(all according to the data sheet)

The problem with the AD6620 is that it does not allow many taps
in the FIR filter at the high data rates needed. I actually do
not know for sure that the GC4016 is much better. It seems more
flexible - but also much more complicated to use;-)

In any case, the AD6620 is easy to use and it is sufficiently good
when the bandwidth and dynamic range is limited to 185kHz/16 bit
by a USB interface.

Some more bandwidth is highly desireable (to fight static rain well.)
Two channels (provision for syncing several units) is required for
use with Linrad on a X-yagi array for EME. More channels would
be extremely valuable on HF bands. (On HF less bandwidth is needed
so the PC will have no problems)

The SDR-14 with an ethernet interface, a digital filter in a gate
array and a better sampling clock would be one solution:-)

73

Leif / SM5BSZ






> Regards,
>
> JDB.
> --
> LART. 250 MIPS under one Watt. Free hardware design files.
> http://www.lart.tudelft.nl/
>
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