[linrad] Re: Linrad via Ethernet

[<leif@xxxxxxxxxx>]

> I've been pondering these things for a while - thought you might be
> interested.
> My receiver design uses: OCXO -> AD6644 -> HSP50016 -> AD-537
> Blackfin DSP -> TCP/IP over ethernet
What is the AD-537? I found a voltage to frequency converter
but nothing at Analog Devices. The HDSP50016 gives a 1.5 shape
factor so the useful bandwidth would be 75% One more digital filter
and another step of down conversion might be worthwhile - is that
what the AD-537 is doing?

> My board uses a 40MHz OCXO which Vectron sampled for me.  I
> used this because it divides down easily to 10MHz, which I needed
> elsewhere.  It might be hard to find an OCXO at 65MHz, though it
> would be nice since you could use more decimation...
Yes, it would improve the dynamic range:-) Making a very good
oscillator with a crystal is not too difficult.

> The design allows for 7 boards to be daisy-chained together for
> diversity/phased array reception.  The boards occupy eight
> addresses - address 0 programs all of them simultaneously.  The
> clock is distributed down the chain by LVDS chips driving ethernet
> patch cords.  The idea is that these can be trimmed to length to
> allow precise 0 degree clock phase shifts from board to board.  Of
> course, jitter will accumulate down the chain, but since the CIC
> filters in the HSP50016 are averaging the data together, some phase
> dithering may not be such a problem!
Oooh! I would not accept phase jitter to accumulate. It makes the
system useless at high frequencies because the sideband noise that
it is equivalent to will degrade the dynamic range. The SDR-14
for example is nearly useless on 144 MHz just because the clock
has phase jitter - but on low HF frequencies it is perfectly ok.


> I used the LVPECL option on
> the AD6644 datasheet to drive the sampling clock.  This gave me the
> differential signals for the clock, and I found I could interface
> LVDS easily to LVPECL with off-the-shelf parts.
OK. Did you check the noise level close to a carrier above 100MHz?
Maybe you just want to use this system for 3.5 MHz and below? Then
it is not critical.

> Two other LVDS daisy-chains are used to program the HSP50016's and
> to receive the downconverted signals.  These are taken care of by
> the serial ports (SPORTs) on the Blackfin.  I plan to use the
> multichannel mode of the SPORTs to de-multiplex the phased array
> data, but I haven't come anywhere close to evaluating this yet.
>
> The current problem is the TCP/IP code from Analog Devices.  The
> Blackfin hardware talks OK to my boards, but I'm having difficulty
> with sending packets from it to a host PC down the ethernet.  In my
> application the received signal is in the form of echoes, ie not
> continuous but bursty, so it lends itself to being packetised.
> After a few packets the code dies!  I'm wondering if the lwIP code
> that's bundled with the Blackfin (BF-537 EZKIT-Lite - $350 from
> Analog Devices) is really ready for the marketplace...  or is it my
> programming??
>
> The Blackfin may seem like overkill for the ethernet link, compared
> to an FPGA solution, but I also plan to use it for waveform and
> control purposes in other parts of the application - if it can hack
> it!
This is too much computing/networking for me to be able to have
any opinion about. My focus is in the radio related hardware and
software....
It seems to me you are not doing amateur radio. Sounds more like MRI
or something scientific:-)

73

Leif / SM5BSZ




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