[Lowfer] analog vs sdr vlf/lf comparison

Douglas D. Williams kb4oer at gmail.com
Mon Nov 26 12:51:11 EST 2012 

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/WINRADQR
> > SS120%282%29.jpg
> >
> > R75 + Converter:
> >
> >
> > https://dl.dropbox.com/u/33457409/R75%26Winradio%20Comparison/R75QRSS1
> > 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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