Ethernet SDR Interfacing

wb4jfi wb4jfi at
Mon Sep 13 14:43:28 CDT 2010

  On 9/13/2010 4:57 AM, Patrick Strasser OE6PSE wrote:
> wb4jfi wrote on 2010-09-11 22:16:
>> I'm thinking of adding an Ethernet interface capability to the
>> Charleston SDR receiver. This would also allow me to learn Ethernet
>> interfacing with some of the newer designs. I'm looking at the Wiznet
>> chips and modules, available from Sparkfun. They seem to offload all the
>> Ethernet and PHY stuff into the chip, allowing the FPGA to concentrate
>> on the SDR part.
> Have you head a look at the Digilent Pmods, namely the PmodNIC? It is 
> a stand alone 10Mbit/sec transceiver with SPI interface and interrupt 
> line for receive for 30$.
>> I'm concerned with USB interfacing, and how (at least with Windows)
>> everything is getting more restrictive.
> What do you mean with restrictive? Driver support, data rate, 
> flexibility, line length?
> For datarate, USB has gross bandwith of 480MiBit/sec, which boils down 
> to a sustainable net datarate of 32MiByte/sec or even a bit more.
>> Plus, Firewire is not as easy to
>> use as it once was. Using E'net and UDP would move this into a more
>> "stable" environment.
> I guess you are looking at something similar to RFSpace netSDR or SDR-IP?
> 73 de Patrick
Sorry, I was being purposely obtuse about the "restrictive"....

The Digilent NIC PMOD (I do own one) only does ethernet at 10Mb, and 
uses serial SPI for communicating.  Not nearly fast enough to handle the 
high-speed IQ data from an SDR.  For 192k sampling, the data samples 
alone will be at over 6Mb/s.  At 384k, they are 12.3Mb/s, and 480k over 
15.3Mb/s.  This does not account for ANY overhead or separate 
command/status data.  (192k samples per second of separate I and Q 
16-bit data samples = 6,144,000 bits per second).  So, I wasted a few 
dollars on the Digilent NIC PMOD (at least for SDR usage).

You hit it right, my main concern with USB is the drivers for whatever 
hardware chip is used.  I believe the Cypress FX2 drivers for Vista and 
Win7 are now finally OK, but that fiasco pointed out how susceptible we 
are to having Microsoft change something and having hardware 
manufacturers lag behind on getting "approved" or signed drivers for 
their older products.  An even worse example is what any Flex user has 
had to put up with when using their 5000 or 3000 with Vista or Win7.  
Hit F8 at boot, and ignore the unsigned driver warning.  Every time.

I used to like Firewire (a LOT), but between the driver issues, and the 
cost of embedding those devices, no thanks.  Plus, try to buy a laptop 
computer with Firewire for under $1,000...

The most ubiquitous interface out there (next to USB) seems to be 
ethernet, so that's what I'm looking at.  No more hardware-specific 
drivers AT ALL!!  If it connects, it just runs.  And, UDP seems reliable 
enough over a hard wire, so no fancy-schmancy protocol handshaking or 
different modes like USB.  In addition, ethernet is made for distributed 
processing, so one computer can do some processing, then hand off the 
data to others.  100base-t is fast enough to handle present SDR data 
rates, and gig-e gives much higher speeds (with different hardware).  
Yes, ethernet wires can be much longer than USB as well.  (I have a USB 
extender that uses ethernet wiring between the devices, but that costs 
another $30-$100).

One major issue is that most computers have only one ethernet 
connection.  So, if you want to use the e'net connection for both the 
SDR and Internet, a small switch must be used.  Or, wireless can be used 
for the Internet (on a laptop).

I'm not worried about USB speeds or interface issues right now.  The 
existing Charleston board works fine, even at 480k, on a fast enough 
computer.  So, I'm more looking into the future.  I see James Ahlstrom's 
work, new openHPSDR designs, and SDR-IP all going to ethernet.

I've looked at some of the other low-cost ethernet interfaces, and have 
tentatively chosen the Wiznet hardware.  I'm looking for others that may 
have used these devices, and what their experiences were.  They have 8 
and 16-bit data paths, and direct or indirect register addressing.  
These options dictate how many FPGA pins are required.  I'm wondering 
about the various trade-offs with these options.

Thanks for your response Patrick.  I may be totally wrong in my 
thinking.  I'm just getting worried about where Windows is going, and 
want to avoid any future problems, if possible.  It seems like ethernet 
would do that.

Terry, WB4JFI

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