[time-nuts] Ideas for a long-wave receiver sought

[John Miles]

It's not necessarily inexpensive, but here's one data point: an EVAL-AD7760
board is $150, and a Nexys2 FPGA board is $129.  This combination can
acquire DC-1 MHz with 100 dB SNR and 120 dB SFDR.  There is no need for a
hardware or FPGA-based DDC, as any modern PC can do host-based processing at
these rates without breaking a sweat.

In the setup I have been playing with, the FPGA is used only to transfer the
AD7760's output to the USB 2.0 chip.  With a 16 KB block-RAM FIFO in the
FPGA and overlapped reads from the CyUSB driver, my test app has run for 2-3
days at 10 MBytes/sec without missing any samples.  At 20 MBytes/sec,
sending every 32-bit sample twice as a test, the FIFO overruns every couple
of hours.

Considering that you can spend $279 on a high-end sound card without trying
very hard, the AD7760-based USB digitizer isn't too unreasonable, at least
for single-channel applications.  I'm still debating whether to spin a PCB
that'll put the FX2 USB chip, FPGA, and AD7760 on one board... it's
tempting, because I'd really like a two-channel digitizer, but it won't
necessarily be much cheaper than the Nexys2/EVAL-AD7760 combo.  The Nexys2
is a real bargain.

I'm not quite ready to call the Verilog and ADC API code "done," although
anyone who wants to play with it is welcome to it, just write me offline.

TI also has a new 24-bit sigma-delta ADC called the ADS1675 that digitizes
at 4 MS/s and uses about half the power of the AD7760.  You can buy the
chips from DigiKey but there appears to be no official evaluation board yet.
It's hard to compare the specs on an apples-to-apples basis, but my
impression is that the overall performance isn't too far behind the AD7760
at 2.5 MSPS.

Re: the SDR-IQ, yes, it comes with a public interface spec.  It's based on
ASCP (Amateur Station Control Protocol), which I'm not too familiar with.
Great little box, usable as a general-purpose digitizer for narrowband HF
applications that don't need to go all the way down to DC.

-- john, KE5FX

> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
> Behalf Of Lux, James P
> Sent: Monday, June 22, 2009 9:13 AM
> To: brooke at pacific.net; Discussion of precise time andfrequency
> measurement
> Subject: Re: [time-nuts] Ideas for a long-wave receiver sought
>
>
> 200kHz is a bit tricky for the top end.. That probably puts the
> standard music recording A/D out of the picture (although they
> have very high performance A/Ds in them, and because of large
> production volume, they're relatively inexpensive).
>
> Almost any PC these days has enough processor to take a 400 ksps
> stream of samples and filter/decimate it.
> Maybe the boards from the HPSDR folks might serve?
>
> Do you need continuous stream of samples? One of the eval boards
> for high performance A/Ds with a USB might work for you, but last
> time I checked, they were more oriented to "capture a buffer,
> then analyze" sorts of approaches.
>
> Your best long range approach might be to use a high quality A/D
> with a small FPGA behind it that implements a digital down
> converter, feeding the (lower rate after
> downconversion/filtering) samples through USB to an application
> like DL4YHF's spectrum lab.  DL4YHF is very interested in VLF
> receiving, and he might have good ideas on inexpensive approaches.
> http://freenet-homepage.de/dl4yhf/spectra1.html
>
>
>
>
> > -----Original Message-----
> > From: time-nuts-bounces at febo.com
> > [mailto:time-nuts-bounces at febo.com] On Behalf Of Brooke Clarke
> > Sent: Monday, June 22, 2009 7:18 AM
> > To: Discussion of precise time and frequency measurement
> > Subject: Re: [time-nuts] Ideas for a long-wave receiver sought
> >
> > Hi Marc:
> >
> > The SDR-IQ has the ability to record everything between 500
> > Hz and 190 kHz when used with a fast enough PC.  This is the
> > best of the SDR series for use below 200 kHz.
> > It can be used with SpectraVue or Winrad.
> > Works with I2PHD WINRAD, SM5BSZ LINRAD, HOKA and DRM software.  See:
> > http://www.rfspace.com/SDR-IQ.html
> > http://groups.yahoo.com/group/SDR-IQ/
> >
> > Have Fun,
> >
> > Brooke Clarke
> > http://www.prc68.com
> >
> > Marc Balmer wrote:
> > > Hi
> > >
> > > I want to build a small, cheap, yet precise long-wave
> > receiver which
> > > can be tuned from the computer in the 2KHz - 200 KHz range (the
> > > intended use is to receive various time signal stations).
> > >
> > > Does a chip for such a receiver exist?  Should I take the SDR route?
> > > I designed a DCF77 receivers some years ago, but I need
> > something more
> > > flexible (and a bit more modern...)
> > >
> > > - Marc Balmer
>
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