Comparison between GNU Radio and FlexRadio

Hi guys,

I’m going to be doing some research on digital comms on HF bands, and
I’m
having trouble deciding between the GNU Radio and the FlexRadio as a
platform. The GNU Radio seems more general-purpose, and the FlexRadio
more
ham-oriented but perhaps more ready-to-go.

So I’m just wondering if there’s a comparison between the two somewhere?

Maybe it’d be a good question for the FAQ: “How does GNU Radio compare
to
other SDRs?”

Thanks,
Ben.

Considering the hardware, I think that for ham usage, as you said,
FlexRadio is more ready-to-use, but usrp, equipped with the correct
hardware, allows you to do much more things, and also it can work with a
much wider bandwidth.
But I don’t own a FlexRadio, and don’t know about it’s intrinsics.
I’d also like to hear more about a USRP/FlexRadio comparison too.

Rafael D.

On Thu, Nov 19, 2009 at 11:33:56AM +1300, Ben Hoyt wrote:

other SDRs?"

Thanks,
Ben.

One thing to remember when talking about both GNU Radio and FlexRadio
is that they are each a combination of separable software and hardware.

For HF work, FlexRadio has a nice line of HF hardware and they provide
the PowerSDR software that goes with it. It’s possible to talk to
their hardware (last time I checked) using other software. I’m pretty
sure the linux dttsp stuff can use it. There’s no reason in
particular that GNU Radio doesn’t talk to it. It’s just a matter of
programming for somebody that wants the two working together.
Matt’s USRP offerings (USRP1, USRP2) don’t provide a high-performance
out of the box (narrow band) HF transmitter and receiver. If you wanted
to
experiment with signals wider than say 150kHz, then the USRP* would be
useful. I think the QSD in the FlexRadio h/w has an upper bandwidth
limit in the neighborhood of 120 kHz. I’m sure a Flex expert can set
me straight on that part.

Eric

For HF work, FlexRadio has a nice line of HF hardware and they provide the
PowerSDR software that goes with it. It’s possible to talk to
their hardware (last time I checked) using other software.

Yes, I’m currently steering towards a FlexRadio running custom DttSP (or
perhaps GNU Radio) software. PowerSDR looks like it’s basically a nice
GUI
on top of DttSP.

Also, I just thought I’d relate my stream-of-consciousness findings
after
digging into the FlexRadio a bit more (probably most of this is old-hat
to
folks here):

  • The FlexRadio does “direct down conversion” using a QSD (quadrature
    sampling detector), rather than traditional radios which go via an
    intermediate frequency, see this FAQ answer:
    http://www.flex-radio.com/Products.aspx?topic=faq#q5-genSDR

  • Out of that you get two base-band signals (I and Q or I/Q, for
    in-phase
    and quadrature or orthogonal, basically a complex number).

  • The FlexRadio samples those signals at fairly low base-band
    frequencies.

  • So you can do this kind of thing with a sound card (e.g., the
    Softrock)
    but it’s fairly low quality – the FlexRadio receiver is basically
    higher
    fidelity and uses higher bandwidth ADCs (192KHz), but still not
    MHz-frequency ADCs like the USRP (32MHz bandwidth).

  • As the page at http://www.flex-radio.com/Products.aspx?topic=faq says,
    “Once the I and Q signals are digitized, DSP algorithms perform all of
    the
    demodulation and signal enhancement eliminating the analog (and sometime
    digital) circuitry found in traditional radios which provide the same
    functions. Transmission is just the reverse of the RX process. There are
    no
    multiple IFs in a SDR.”

  • Here’s a block diagram of the FLEX-3000:
    http://support.flex-radio.com/Downloads.aspx?id=266

  • This document, written by the FlexRadio folks back when they were
    starting, has a fairly good explanation of the maths behind QSD in the
    section “From RF to a PC’s sound card”:
    http://www.flex-radio.com/Data/Doc/qex1.pdf

So the FlexRadio is ready-to-go for HF radio transmit and receive, but
basically is only good for HF a bit above (up to 60MHz carrier freqs).
The
USRP is much more general purpose and uses much higher bandwidth ADCs
(and
has an FPGA), but isn’t nearly so ready-to-go for HF stuff.

Cheers,
Ben.

Also, a couple more questions about the USRP daughterboards (showing my
radio ignorance here!):

The BasicRX (from the schematics) looks like it’s just a transformer and
a
300MHz low-pass filter. Is the transformer for impedance matching, or
does
it serve other purposes as well?

All “the literature” notes that you should have an RF front end before
the
BasicRX board. There’s an example at
http://www.gnuradio.nl/pascal/BasicHFfront.html put together by the
gnuradio.nl folks.

Is this kind of RF amplifier/filter required so that you’ve got some
decent-sized voltages to sample in the band you’re sampling? The reason
I
ask is because I read somewhere that the purpose of SDR is to sample “as
close to the antenna as possible”, and at least for HF frequencies, an
RF
front end seems like an unnecessary analog step.

Thanks,
Ben.

Thanks, Eric.

One thing to remember when talking about both GNU Radio and FlexRadio

is that they are each a combination of separable software and hardware.

Yeah, that’s be a helpful distinction.

Matt’s USRP offerings (USRP1, USRP2) don’t provide a high-performance

out of the box (narrow band) HF transmitter and receiver.

(Disclaimer: I’m a software developer rather than a radio guru.) Can you
tell me what specifically you mean by “high-performance” in this
context?
Would it need to be narrow band to perform well? It seems that several
people are using the BasicTX/RX boards to do HF stuff – what would be
the
limitations with that?

Thanks heaps,
Ben.