[Lowfer] analog vs sdr vlf/lf comparison

Mon Nov 26 12:18:33 EST 2012

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/WINRADQRSS120%282%29.jpg
>
> R75 + Converter:
>
>
> https://dl.dropbox.com/u/33457409/R75%26Winradio%20Comparison/R75QRSS120%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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