TAPR High Performance Software Defined Radio

Frank Gentges fgentges at mindspring.com
Wed Jan 7 21:57:50 CST 2009


I am impressed with the newer hardware and how we can get this 
hardware/software in the home as hams/experimenters.

First, the 130 dB dynamic range you quote is a good number for HF, at 
least in the 1970s when was looking at that.  The number of 500 kW 
broadcast stations created a number of high level signals to deal with. 
  Essentially, the front end had to not overload for even an instant 
(sample period?) to realize all the dynamic range.

However, if the clipping can be limited to just a few samples, you can 
let them happen if you are not digging down to the 130 dB down stuff.

Now with Radio Moscow pretty much gone and the fact we will be using our 
receivers on this side of the Atlantic, we can ratchet down out top end 
peaks that would have caused the clipping.  Right now I would be happy 
with a 66 MHz sample rate at 16 bits and put up with a few clipping 
points. Put that same receiver in Europe and all the strong signals and 
it is not so great.

But lets step back to an earlier day.  Consider the older vacuum tube 
receiver front ends and IFs.  With a Hammarlund SP600 you would get two 
tuned stages plus a tuned mixer before the first IF.  As you moved away 
from the tuned frequency you did not have to be far before the front end 
would start seriously rejecting the signal enough that the IF would 
never see enough of the signal to be a problem.  As you got even closer 
to the tuned-to frequency the IF would have several stages to again 
reject the signal.  The problem was as much with the first RF amplifier 
overload and clipping as further down in the chain.

With the R-390A they found it was better to eliminate that first RF 
stage.  It lost the tuned circuit but it also lost the amplifier and its 
clipping on really strong near in signals.  That turned out to be a 
favorable trade off, especially when cost was a factor.

Now we find ourselves with a wide open front end getting our dynamic 
range with more bits, and therefore linearity, in the A/D and accuracy 
in processing.  Rather than tune the front end to the exact frequency we 
now are putting a bandpass filter like a 1/2 octave wide to reject off 
frequency signals.  Nearby signals that pass unattenuated through the 
bandpass filter present a problem with overload and its clipping.  A VGA 
will avoid clipping at the expense of noise figure and weak signal 
signal performance.

I also want to agree with your comment earlier that oversampling has 
processing gain.  As faster A/Ds become available, we can choose to 
sample faster as an alternative to the number of bits of precision. 
This is often overlooked as we seek to get the fastest sampling rate 
available and costs increase exponentially.  Newer hardware may present 
inexpensive alternatives.

Well, here I go on and on and on.  An interesting and insightful email 

Frank K0BRA

WB4JFI wrote:
> Please stand by, a long message follows:
> I received my Mercury board just after XMAS.  Now, I'm getting ready to fire
> it (and Penelope exciter) up shortly.
> I've ended up with two HPSDR setups: one Atlas/Janus(sound card
> replacement)/Ozy, and one Atlas/Ozy/Mercury/Penelope.  The first one is used
> for SoftRock and other QSD/QSE experimentation.  The second boardset will be
> built up as a stand-alone rig.
> I firmly believe that while QSD/QSE rigs are low-cost and fun, but as the
> cost of A/D converters that can do RF continues to drop and performance
> increases, we will eventually switch to direct digital up (DUC) and down
> (DDC) conversion SDR radios.
> The 16-bit, 135Ms/s A/D on Mercury is the same one that Phil Covington uses
> on the QS-1R.  With the processing gain that down-sampling and other DSP
> techniques provide, we are already very close to the complete dynamic range
> necessary for very good HF radio that is almost all digital.  Both the QS-1R
> and the Mercury are proving to be excellent performers at first blush.  If I
> remember, Joseph Mitola III recommends about a 130dB dynamic range for HF,
> which translates to about 22 bits.  Throw out a bit or two (as the A/D LSb
> is usually not reliable), and a 24-bit system is about the minimum.
> If you look at it from the perspective that you need 130dB dynamic range,
> and you can get about 90dB from the 16-bit A/D, you need another 40dB of
> dynamic range without playing with front-end Variable Gain Amplifiers, or
> switching pads and/or preamps in and out.  That's a tall order for the DDC,
> but is close to feasible.
> There's an oft-quoted formula for processing gain:
> 10log(Nyquist_BW/output_BW) (my words).  In a note regarding the SDR-14
> (14-bit A/D DDC receiver), which samples at 66.666MHz (Nyquist=33.333MHz)
> and output bandwidth of 100kHz, the example is 10log(33.333MHz/0.1MHz) =
> 25.2dB of processing gain.  Narrower bandwidths provide even more gain.  So,
> 90dB of dynamic range from the A/D itself, plus 25dB of processing gain gets
> us to 115dB overall gain.  Not too bad for a receiver covering a 100kHz
> swath of a band.  And yes, as both the QS-1R and the Mercury are proving,
> you can design RF front ends to approach or achieve the 16-bit noise limit.
> However, the DDC/DUC radios cannot compare to the cost-effectiveness of a
> SoftRock.  For limited funds (or for great fun), get a SoftRock from Tony
> Parks.  I LOVE SoftRocks.  For a higher-end, build-it-yourself, HF rig that
> will be the wave of the future, get either the HPSDR boardset, or Phil's
> QS-1R and the upcoming preamp board and matching transmit exciter (QS-1T?).
> Someone here in Charleston has also built-up a direct-digital conversion
> receiver using a 80Ms/s, 12-bit A/D, preceeded by a VGA, and followed by an
> ASIC that can control the VGA plus do some digital down-sampling.  A
> Digilent Nexys II FPGA demo board does the rest of the down-sampling and
> interfacing to the host PC.  He has it receiving both 40M (directly) and 2M
> (via subsampling).  The software he is using is Linrad I believe.  THe cost
> should be less than the HPSDR or QS-1R.  While the A/D is only 12-bits, a
> combination of band-pass filters before the board, and the control of the
> VGA, should be able to reduce problems with overload.  More on this as it
> progresses.
> Penelope boards are not available right now, however.  There is some
> movement to push for either another run from TAPR, or another source.
> THe HPSDR group has already released software for the new boards, a variant
> of PowerSDR.  It includes source and the FPGA code as well, if you are
> interested in playing with it, in an SVN.  Improvements are being made as
> moe boards are fired up and USED ON THE AIR FOR REAL QSOs.
> There are also other DDC and DUC boards available.  The SDR-14, SDR-IQ from
> RF Space, Perseus from Europe, and a recent QEX  article on building a DUC
> somewhat similar to Penelope are just a few.
> I am slowly building up the SDR and DSP pages on the AMRAD web site
> (www.amrad.org).  I plan to add sections on specific hardware and software,
> along with some stuff from my experiments.  Please check it out
> periodically, and feel free to offer suggestions.  But, I can't let this get
> in the way of the true fun, experimenting!!
> Join the SDR revolution!!  It's great fun, an interesting challenge, and you
> will learn a lot.
> 73s
> Terry
>> -----Original Message-----
>> From: tacos-bounces+wb4jfi=amrad.org at amrad.org
>> [mailto:tacos-bounces+wb4jfi=amrad.org at amrad.org]On Behalf Of
>> fgentges at mindspring.com
>> Sent: Monday, October 20, 2008 1:35 PM
>> To: Tacos
>> Subject: TAPR High Performance Software Defined Radio
>> The TAPR group has been working on a high performance SDR.  Now they
>> have asked for a show of hands of people that would be interested in
>> buying their Mercury receiver module.  You would need to pay by 15
>> November and receive the board around mid-January 2009.
>> This receiver shows real promise as the front end has a 16 bit A/D and a
>> sample rate to support up to 65 MHz.  That is over twice the high
>> frequency limit of the SDR-IQ and two more bits of A/D resolution.  By
>> upping the resolution the dynamic range is improved.  This could be an
>> important advance.
>> To run this board, two other boards are needed, a backplane named ATLAS
>> and the USB to computer interface named OZY.
>> So, to end up with a working receiver you will need the following:
>> TAPR Membership, $20
>> ATLAS, Kit, $25
>> OZY, $137
>> MERCURY, $329
>> Total, $511
>> The ATLAS board is only available as a kit and is quite reasonable to
>> assemble yourself.  It is mainly a backplane board and the individual
>> board connectors.
>> The OZY and MERCURY boards can be bought bare but for our purposes here
>> we are listing the assembled and tested option.
>> There are some other boards to fill out the backplane into a full
>> transceiver and you can see and learn more at
>> www.tapr.org at
>> http://hpsdr.org/ and ordering details at
>> http://www.tapr.org/kits_merc.html
>> And yes, by paying extra, you can buy the boards without joining TAPR.
>> Note that this is just the hardware.  Some software is available but a
>> lot of work is expected on improved and expanded versions.  I am not
>> sure a Mercury package is ready yet.
>> Terry, WB4JFI has some of these and can provide some hands-on experiences.
>> Frank K0BRA
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>> Tacos at amrad.org
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