[Lowfer] analog vs sdr vlf/lf comparison

[Douglas D. Williams]

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