[Lowfer] analog vs sdr vlf/lf comparison

pbunn pbunn at matrixei.com
Mon Nov 26 12:35:05 EST 2012 

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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