SDR comparison tests at KFS show little difference between SDRs when the RF they are fed is properly conditioned

[Rob Robinett]

I have just posted a brief description of comparison test of four SDRs I have been running at KFS for several months at:  https://groups.io/g/wsprbeacon/message/294

Many of you have already heard my conclusions, but if not you can find them in that post.

73,

Rob

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Rob Robinett

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r...@robinett.us

mobile: +1 650 218 8896

[Phil Erickson]

Hi Rob,

Very nice comparison. 

Can you describe the RF path in a bit more detail, in particular the “high pass above 20m” part for the RX888’s benefit? Trying to understand whether you combine below and above 20m before feeding the SDRs.

Also, has someone now explicitly measured the noise figure of the RX888, and what is it? Apologies if I am behind the curve here.

Thanks -

73

Phil W1PJE 

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[David G. McGaw]

A nice demonstration that good receiver design has the front-end amplifier have (just) sufficient gain to overcome the following stages's noise.

As Phil asked, could you please give details on the front-end filtering and amplifier.

73,

Daivd N1HAC

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[Lester Veenstra]

To be specific, The noise from the first stage output should be 15 to 20 dB over the noise of the next stage.

At 15 dB delta in noise, the succeeding stages are degrading the input S+N/N by about 0.1 dB, an insignificant amount.

At 20 dB delta in noise, the succeeding stages contribute insignificant  degradation of the input S+N/N by.

Above 20 dB delta there is no advantage, and the risk ove overloading the following states (making them nonlinear).

 

Lester B Veenstra  K1YCM  MØYCM  W8YCM   6Y6Y W8YCM/6Y 6Y8LV (Reformed USNSG CTM1)

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[Nicholas Hall-Patch]

Thanks for this, Rob.

Which of SDRplay's selection of SDRs were you using please?

73,

Nick

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[David G. McGaw]

Yes, good question.  The RSPdx is said to have the best front-end, RSPduo second.

David N1HAC

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[Dana Whitlow]

Do any of the tested SDR models have any (or preferably all)

of the following features:

>  External 10 MHz reference input?

>  A specified tune frequency error?  (if other than zero, a means for finding out the error amount?)

>  The ability to stream I&Q data to disk?

>  Selectable IF BW?  If yes, what is the available range?

Thanks,

Dana Whitlow

--

[Rob Robinett]

At KFS the feed from the TCI-530 antenna is amplified by a 23 dB gain 3.8 dB NF QB-300 LNA which is described here:  http://ka7oei.blogspot.com/2020/05/a-quick-look-at-qb-300-amplifier.html

A high pass shelving filter described at: https://ka7oei.blogspot.com/2020/08/revisiting-limited-attenuation-high.html.preceeds the QB-300 and that LNA feeds an 8-way passive splitter with about 10 dB loss.

The net gain from the TC-530 feed to the SDRs SMA inputs is thus about +13 dB at 30 MHz, which means the system noise figures are degraded by less than 1 dB by the roughly 15 dB NF of the Kiwi and RX888.

WD continually records overload events reported by the Kiwi and RX888, and those logs show that gain structure ensures that overloads do not occur often enough to influence the beacon counts.

[Rob Robinett]

See  my responses below

On Fri, Nov 29, 2024 at 9:48 AM Dana Whitlow <k8yum...@gmail.com> wrote:

Do any of the tested SDR models have any (or preferably all)

of the following features:

>  External 10 MHz reference input?

The KIwi and RX888 are GPSDO by Bodnars, so the frequency accuracy is determined by their 10^-11 ppm long term accuracy.

The SDRPlay RP1a and the Airspy HF+ run on their internal clocks. 

Your question stimulated me to check the 10m frequency reports for the GPSDO beacon WW0WWV which showed all four SDRs report spot frequencies within 10 Hz of each other on 10m

 

>  A specified tune frequency error?  (if other than zero, a means for finding out the error amount?)

The Kiwi and RX888 are accurate to the precision of the Bodnars which clock them.   Look at the spec sheets for the RP1a and Airspy

 

>  The ability to stream I&Q data to disk?

Yes, WD is recording and archiving several months of 16000 sps IQ wav files on all 10 of the WWV/CHU signals.  Each day WD creates from  those recordings  24 hour / 10 hz wav files which are uploaded to the PSWS server where you can see the daily spectrographs.

I can give access to the 16000 sps records at KFA and dozens of other sites to anyone who could use them in their research.

 

>  Selectable IF BW?  If yes, what is the available range?

There is no 'IF'.  This is a direct conversion SDR system which can in addition to its current recordings make IQ, AM ... USB, etc. recordings of almost any modulate and bandwidth at one or more frequencies in the 2-30 MHz band.

 

Thanks,

Dana Whitlow

On Fri, Nov 29, 2024 at 9:47 AM Rob Robinett <r...@robinett.us> wrote:

I have just posted a brief description of comparison test of four SDRs I have been running at KFS for several months at:  https://groups.io/g/wsprbeacon/message/294

Many of you have already heard my conclusions, but if not you can find them in that post.

73,

Rob

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mobile: +1 650 218 8896

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[Rob Robinett]

The RP1a doesn't have an external clock input, however it seems quite accurate for non-radio science use.

The WSPR reports and the WWV/CHU recordings uploaded to the PSWS server come from the RX888 which is clock by a GPSDO.

The performance of the RP1a and the Airspy discourage me from investing in the SDRPlay models which include external clock inputs.

I am not aware of an Airspy HR SDR which supports external clock input, and the HF+ performance also discourages me from making such an investment.

My study is not intended to disparage the SDRPay, Airspy and KiwiSDR products.  T

hey are excellent SDRs and provide features and SW compatibility which may make them a better choice for many users.

But for our HamSCI multiband WSPR and WWV/CHU reception, I think the RX888 provides the most features and best performance

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[Dana Whitlow]

Thanks, Rob.

First, two comments, followed by my "sob story".

1.  If an SDR can provide I & Q sample values for recording,

     then there is so an IF.  It's just a "zero IF", which is no bother

     at all.  This IF frequency would range from -IFBW/2 to +IFBW/2.

     This makes life simpler almost all the way through, and avoids

     any need tor a guardband to assure that one's IF stays positively

     all on the same side of zero. 

2.  For my interests, a 1 Hz error referred to 10 MHz is a whopping

     big error (100,000 PPT)

     I'd like to stay well-within 100 micro-Hz referred to 10 MHz, which

     is only 10 PPT.  With care, I believe that the availability of GPSDO's,

     combined with inexpensive Rb standards to serve as flywheels, can

     make this a reality for amateurs.

Now for the horror / sob story...

My interests began with sim[ly making sensitive phase variation measurements,

with the idea of detecting Earthquakes by detecting small relative displacements

of my receiver from some stable UHF transmitter.  But I soon rediscovered A/C

Doppler-shifted reflections, so the 'quake detection scheme went right down the

drain.   But by this time I had come to realize that I had a pretty sensitive tool on

my hands and that it had the potential for really precise frequency measurement.

As a test, I split the output of a fairly stable 10 MHz source into two parts, and fed

the strong part into me SA44B receiver's Ext Ref port, and the weaker part into the

RF port.  I set the tune frequency to 10 MHz.  With this arrangement the IF frequency

should be exactly zero even if the 10 MHz source frequency were somewhat off or

drifting around.  Instead, careful measurement revealed an IF frequency of about

33.5 micro-Hz, which was reproducible over and over, and substituting a different

receiver of the same model revealed the same number.   I do not have a means of

generating a wide range of other RF test frequencies except for a few submultiples

of 10 MHz.  Those lower frequencies led to even larger errors in excess of 500 

micro-Hz.  My methodology also supports accurate measurements by measuring

the phase slope of the IF and compensating for it in the final result.  I went back to

a 10 MHz RF input, but this time I tried tuning the unit to frequencies a way on either

side of 10 MHz.  Tuning on the high side led to the same 33.5 micro-Hz error, but

tuning on the low side of 10 MHz led to much larger errors.  All this tells me that

there is a design flaw in the SA44B.   I contacted the engineering dept of the manu-

facturer (Signal Hound) to ask about this.   Now please understand that I fully

realize that no cynthesizer of finite complexity can be tuned to arbitrary frequencies,

and I tried to explain that I could easily  live with this limitation if only I knew what

the "theoretical tune frequency" is for each tune frequency request.  Signal Hound

has a delightful means of communicating "metadata" along with each IQ datafile,

and so it should be trivial to include the theoretical tune frequency along with the

other metadata.  But all this fell on deaf ears, so here I sit on the edge of doing some-

thing really neat at fairly low cost, but am blocked by somebody's tunnel vision.   Grrrr!

This is why I try to keep my eyes open for a receiver with the features I listed in my

earlier email.

Thanks for "listening".

Dana

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[Gwyn Griffiths]

Dana

Courtesy Rob, Dave Swartz W0DAS, WWV Amateur Radio Club and NIST WW0WWV/0 an RX888 WsprDaemon Grape receiver with Leo Bodnar GPSDO reports daily to the PSWS database at https://pswsnetwork.caps.ua.edu

The receiver and a vertical antenna is 7 km true line of sight from WWV. I attach time series graphs and a histogram of difference from 10 MHz from the MIT digital_RF 10 Hz passband zero IF signal downloadable from the PSWS database.

On the left is a co-located Grape 1 receiver with a horizontal antenna. Note likely NVIS skywave contamination even at 7 km line of sight.

The following are my notes for this slide:

Through the commitment and effort of Dave Swartz W0DAS and colleagues we have results from a Grape V1.2 DRF reporting as W0DAS 1 on the left, and a Grape RX888 WsprDaemon reporting as WW0WWV/0 on the right.

On the left, the frequency shift from exact 10 MHz for the Grape 1.2 at about -11 milliHertz is most likely from an uncorrected sound card oscillator. Dave uses a horizontal dipole for this receiver, and I suspect the greater variation in frequency shift, and bump in frequency spread were likely from multipath near vertical incidence skywave – despite being only 7 km distant.

On the right we have the Grape RX888 – only dependent on a GPSDO for its frequency accuracy and stability. And we have impressive results: a mean offset of 0.28 milliHertz and an rms variation of 0.16 milliHz, that is 1.6 parts in ten to the power eleven. There is no evidence of NVIS multipath, probably because the antenna here is a vertical.

This reference site close to WWV reconfirms the Grapes as high precision scientific instruments: for the very highest performance – use an RX888 and GPSDO and seek to avoid multipath.

Gwyn G3ZIL

[Rob Robinett]

Early in the WD project we compared the Kiwi to an Apache (Flex-class) transceiver and found no significant difference in WSPR decoding performance.  Lend me a Flex and I expect to confirm that its WSPR receive performance differs little from the Airspy and other SDRs.  WD users report that such comparisons correlate well with SSB and AM sensitivity.

In designing the KiwiSDR John Seamons appropriately designed the KiwiSDR for the widest audience, which inevitably optimizes its performance for the 'plug-and-listen' listener.  It is fairly easy and inexpensive to optimize the RF feed system for the unique characteristics of a radio quiet QTH, but it requires awareness and interest by the user.  The limitations of the RX888 are different and they appear to not be the result of deliberate choices of the designer, but it too will.almost always benefit from such an optimization procedure.

More important than SDR performance is optimizing the complete antenna + feedline + SDR system, especially to minimize the ingress of ground loop induced RFI.  That is not a simple project but can result in a 10-30 dB increase in receiver sensitivity, far more than can be gained by changing receivers.  This is a deep subject which Glenn has discussed in pages at his web site, e.g.: http://wsprdaemon.org/ewExternalFiles/Improving_Station_Noise_Performance04.pdf

I believe my tests at KFS show that when the antenna + feed system is optimized for *all* the receivers, there is little difference in performance between them.  Investing your time in finding and eliminating RFI is likely to result in much more dramatic improvements than changing the receiver.  The 0-30 MHz spectrum view of the Kiwi is an invaluable tool for finding a suppressing RFI.  I wish that KA9Q-web could give us as good a view.

Rob

On Fri, Nov 29, 2024 at 3:22 PM Philip Barnard via groups.io <phil=perit...@groups.io> wrote:

Coincidentally Roland has recently published an AirSpyHF+  versus a Flex 6600

"Airspy wins by up to 22%!"

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[msobr...@gmail.com]

For the ones interested in investing on an SDR, note that there were two evolutions of the RX888, first the RX888 mark II and then web-888.

 

Although a little more expensive (~260USD) – the new web-888 packs some improvements that might be interesting:

 

Specs
Hardware Specification
Dimension (100mm*71mm*25.2mm) exclude SMA
Active Cooling Fan (40mm)

16-bit ADC DDC architecture SDR

61.44 MHz real-time bandwidth
built-in all-constellation GPS module
dual antenna inputs
Gigabit Ethernet
reference clock input/output
24.576MHz, 0.5 ppm TCXO
8 expandable IOs for antenna switching

 

Software Specification
Alpine Linux 3.20 with Linux 6.6 Kernel
WebRX-based browser SDR experience
KiwiSDR websocket protocol for applications
WSPR, FT8 skimmers, and other decoders
13 RX channels and 13 spectrum channels simultaneously online
Binary updates with alpha and stable channels
Read-only root partition to prevent SD card corruption
FPGA DMA controller for efficient data transfer without CPU involvement. Detailed Design on FPGA
In additional, Web-888 offers Red Pitaya Notes compatibility repo which hosts several SDR related projects that are single ADC applications.

 

Sources:

WEB-888 SDR Mini Review : r/sdr

WEB-888 A New High Performance Web SDR With HF & VHF

 

In particular the GPS module works as the GPSDO needing only the antenna to work. The price is a bit more than the RX888 mkII and it brings more value in my opinion.

 

73,

 

Mauro.

 

From: ham...@googlegroups.com <ham...@googlegroups.com> On Behalf Of Rob Robinett
Sent: Friday, November 29, 2024 5:22 PM
To: ham...@googlegroups.com
Cc: wsprd...@groups.io
Subject: Re: [HamSCI] SDR comparison tests at KFS show little difference between SDRs when the RF they are fed is properly conditioned

 

The RP1a doesn't have an external clock input, however it seems quite accurate for non-radio science use.

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