[Lowfer] analog vs sdr vlf/lf comparison

pbunn pbunn at matrixei.com
Mon Nov 26 13:05:30 EST 2012 

Doug,

I fully understand. I just mentioned it because I have a NDB that is 7 miles or so away that is very loud. 

Overload performance is important and most Ham receivers don't do all that well except for the recent higher end ones. My K3 is much better than anything that I have ever used before and makes my IC718 sound awful.

Pat

-----Original Message-----
From: lowfer-bounces at mailman.qth.net [mailto:lowfer-bounces at mailman.qth.net] On Behalf Of Douglas D. Williams
Sent: Monday, November 26, 2012 12:51 PM
To: Discussion of the Lowfer (US, European, &amp, UK) and MedFer bands
Subject: Re: [Lowfer] analog vs sdr vlf/lf comparison

Hi, Pat.

As usual, your advice is absolutely true.

My comparison is only in my current listening environment (on top of a hill in East Tennessee), with both receivers using the same antenna, followed by a "brick wall" 500 kHz low pass filter, so I very much doubt either receiver was subject to any "close in" strong signals. Probably the strongest signal either receiver had to deal with was WWVB on 60 kHz (+40 dB over S9 on each receiver) and good old NAA on 24 kHz (again, +40 db on each receiver).

I will say that I am completely convinced that the R75  is a much better VLF/LF receiver with the addition of a good quality (non "Mickey Mouse") converter such as the AMRAD or Doug DeMaw/Jay Rusgrove designs, than it is barefoot.

But no, I am not willing to take this test further by remote operation in strong signal environments. I will leave that stuff to those who have more free time. :-)

73,
Doug KB4OER



On Mon, Nov 26, 2012 at 12:35 PM, pbunn <pbunn at matrixei.com> wrote:

> Sensitivity is not much of a parameter for examining receiver performance.
>
> IMD performance is where the weak receivers fail. The AMRAD front end 
> has a very strong mixer and should perform well in a nasty RF environment.
> Adding a preamp in front of it may degrade its performance unless the 
> preamp has better IMD performance.
>
> The mixer is where many receivers fail. A NE602 mixer is plenty 
> sensitive- but is a lousy performer in a strong RF environment.
>
> I'd suggest testing both radios in an environment with strong close in 
> signals.
>
> My barefoot R75 does a very poor job when my beacon is running. With 
> my up-converter, using the R75 as an IF, things are much better. The 
> up-converter uses a Mini Circuit +17 dBm BNC terminal mixer. I am 
> using a Trimble 10 Mhz OCXO from Ebay (removed from a Thunderbolt) 
> with an amplifier as a LO and a diplexer plus a  lossless FB post amp. 
> It is a little old fashion but it gets the job done.
>
> Pat Bunn
> N4LTA
>
> -----Original Message-----
> From: lowfer-bounces at mailman.qth.net [mailto:
> lowfer-bounces at mailman.qth.net] On Behalf Of Zack Widup
> Sent: Monday, November 26, 2012 12:19 PM
> To: Discussion of the Lowfer (US, European, &amp, UK) and MedFer bands
> Subject: Re: [Lowfer] analog vs sdr vlf/lf comparison
>
> I was wondering about that. I have two Softrock Ensemble II receivers, 
> one built for HF and the other a VHF unit built for 144 MHz. Both work 
> quite well and the HF unit seems to be almost as sensitive as my TS-850.
>
> I am going to get another Ensemble II for use on LF. I have heard that 
> the filters leave something to be desired on 1000 meters, so I 
> designed new filters with sharper skirts. But thney will have to be 
> built on a second pc board and connected to the Softrock with small 
> coax. I'm really curious how it will do.
>
> 73, Zack W9SZ
>
>
> On Mon, Nov 26, 2012 at 10:24 AM, Douglas D. Williams 
> <kb4oer at gmail.com
> >wrote:
>
> > A couple of months ago I posted that I would, throughout this 
> > listening season, be comparing an analog receiver (Icom R75 + AMRAD 
> > LF
> > converter) against an SDR (Winradio Excalibur Pro) using a "splitter"
> > and the same antenna (Clifton Labs active antenna) for simultaneous 
> > reception comparisons at VLF/LF frequencies.
> >
> > I have owned the R75 and AMRAD converter (constructed by Todd 
> > Roberts with an OCXO for stability) for at least one listening 
> > season prior to this and the combination has given excellent 
> > performance, so the Excalibur Pro was up against some stiff competition.
> >
> > In order to use the same antenna for simultaneous operation on both 
> > receivers, I purchased a (special order) 1.5 kHz to 2.0 MHz 
> > "splitter/combiner" from Kiwa electronics. This is a completely 
> > passive unit that has the unavoidable side effect of instering a 6 
> > dB loss into the receive chain for each receiver. In the event that, 
> > because of the 6 dB loss, I needed some extra gain, I purchased a 
> > VLF/LF 20 dB preamplifier from Advanced Receiver Research, and a 
> > variable attenuator off eBay. It turns out that the preamplifier was 
> > not necessary as the Clifton Labs antenna produces sufficient signal 
> > that the 6 dB loss introduced by the splitter is inconsequential, 
> > and is actually helpful with the Excalibur Pro receiver, which, 
> > without the splitter inline, is sometimes driven into "clipping", 
> > which would necessitate the use of one of the Excalibur Pro's 
> > built-in attenuators. I plan to use the AAR preamp for some other 
> > purpose.....probably as an amplifier for a ferrite loop experiment 
> > later
> on.
> >
> > The Winradio Excalibur Pro is the first SDR I have ever owned or had 
> > any experience with. It is quite expensive compared to many other 
> > SDRs on the market, but World Radio and TV gave it a glowing review, 
> > calling it (I
> > paraphrase) one of the best (if not the best) receivers they have 
> > ever tested. I figured "go big or go home", so I bought it. The unit 
> > itself is a quite small (6"x4"x1.5") aluminum box enclosed by a 
> > clear plastic shell. It comes with a "soap on a roap" 12 volt power 
> > supply. The only connection to your computer is via a USB port (no 
> > sound card connection). Trying to eliminate some of the clutter on 
> > my desk, I asked Winradio if it would be ok to power the unit with 
> > my Astron 13.8 volt power supply (that I use to power every other 
> > 12-14 volt device in my shack), but they advised against it.
> >
> > The Winradio software that comes with the unit was fairly easy to 
> > learn, as far as the basic functions of controlling the radio, 
> > changing frequency, filter widths, etc. The DDC bandwidth (simple
> > explanation: amount of frequency data that the unit sends to your 
> > computer for processing) can be anywhere from 20 kHz to 50 MHz. 
> > Since all I am interested in is VLF/LF, I tend to leave it on the 
> > most narrow setting, which is 20 kHz. The wider you make the DDC 
> > bandwidth, the more processing power your computer will require. 
> > When in CW mode, the audio filter can be continuously adjusted from 
> > 10 Hz to 20 kHz (at a 20 kHz DDC bandwidth). I find this to be 
> > extraordinarily amazing, since I am used to dealing with fixed width 
> > crystal or mechanical IF
> filters in analog receivers.
> >
> > My biggest concern was how well can the Excalibur Pro detect weak 
> > VLF/LF signals as compared to my analog setup? After doing 
> > comparisons of NDBs and QRSS signals in the Lowfer (160 - 190 kHz 
> > band) and the Part 5 band (137 kHz) band, as well as military MSK 
> > signals in the VLF band, I have yet to find a signal that I could 
> > copy with the R75 + Converter that I could not copy with the Excalibur Pro, or vice versa.
> > If one receiver could detect the signal, they both could.
> >
> > With that said, the ability to continuously vary the filter width on 
> > the Excalibur Pro sometimes gave it the edge on the readibility of 
> > Argo screen captures when there was a strong PLC inside the 
> > bandwidth of the R75's narrowest crystal filter (250 Hz).
> >
> > Please note that, in order to use third party software, such as 
> > Argo, with the Excalibur Pro, one needs some sort of "virtual sound card"
> > software, which sends the Excalibur Pro's audio stream to your 
> > computer's sound card in order to make it available to software such 
> > as Argo, Spectrum Lab, or what-have-you. I happen to use the one 
> > available from Winradio, but I understand there are others (possibly
> free).
> >
> > One other thing needs to be mentioned, and that is frequency 
> > stability. In order to decode the slower QRSS modes, such as QRSS 60 
> > or slower, both the transmit and receive systems must be very stable.
> > I installed the OCXO option in my R75, and Todd Roberts installed a 
> > very hefty OCXO in the AMRAD converter. The Excalibur Pro, as 
> > opposed to the less expensive Excalibur, claims a 0.5 PPM frequency stability.
> > My test for frequency stability was simple, I tuned both receivers 
> > to WWVB on 60 kHz, let them warm up for an hour, and then ran Argo 
> > set on
> QRSS 120 to see how much drift was detected.
> >
> > Here you can see the results:
> >
> > Excalibur Pro:
> >
> >
> > https://dl.dropbox.com/u/33457409/R75%26Winradio%20Comparison/WINRAD
> > QR
> > SS120%282%29.jpg
> >
> > R75 + Converter:
> >
> >
> > https://dl.dropbox.com/u/33457409/R75%26Winradio%20Comparison/R75QRS
> > S1
> > 20%282%29.jpg
> >
> >
> > Note that the entire Argo screen, from top to bottom, only covers 
> > about 3 Hz, and about three hours elapsed from the left to the right 
> > side of the screen capture.
> >
> > I also believe that both receivers had not fully completed warming 
> > up before I started Argo.
> >
> > As you can see, the Excalibur Pro comes out the winner in this 
> > comparison, but the R75 + Converter prove to be entirely stable 
> > enough for QRSS 120,  or perhaps even slower modes. In actual 
> > practice these days on the LF bands, most QRSS operators use QRSS 60 or "faster".
> >
> > So what do I think about the Excalibur Pro? Well, I'm very impressed.
> > I love the ability to quickly jump around in frequency, the "waterfall"
> > display that shows the user a large portion of the band and allows 
> > them to just "mouse click" on interesting signals, and the 
> > continuously variable filter widths.
> >
> > What don't I like?
> >
> > I miss spinning the "big knob" like on analog receivers. I also 
> > dislike the fact that the receiver turns itself off when you exit 
> > the Winradio software, thus necessitating another "warm up" period 
> > when you start the software again.
> >
> > Am I going to sell or consign my R75 to the closet? Nope. I like 
> > having both options, as well as the ability to monitor two different 
> > frequencies at the same time.
> >
> > -Doug KB4OER
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