[Lowfer] Analog VS SDR LF comparison

[Douglas D. Williams]

Todd Roberts pointed out to me something that I had forgotten, and I
believe I recall John Davis writing about this a few months ago.

It wouldn't do much good for me to test the frequency stability of the R-75
sans converter by running an Argo screen capture at some slow QRSS speed
directly on 60 kHz WWVB. The reason being is the lower you go in frequency,
the more the receiver's internal timeclock is divided down, and the more
stable it becomes due to any frequency drift errors being divided down
also. When you use a converter, such as my AMRAD model (built by Todd), you
are actually tuning the receiver through the converter's output, which, in
my case, is 3 MHz + the frequency you wish to tune to. The net effect is,
since I am tuning 0 - 500 kHz by tuning the R-75 (with the converter
inline) from 3.0 - 3.5 MHz, the R-75's internal clock is not being divided
down as much as if I were tuning 0 - 500 kHz directly, with no converter
inline. Got that? Whew!

So, a better test of a "barefoot" (i.e. no converter) R-75's (or any other
HF receiver's) frequency stability would be to tune WWV on a higher
frequency, say 10 MHz, and let Argo chew on that at QRSS 30 or 60 for a
couple of hours and see how much, if any, it drifts.

But, in the end, it really does not matter. What actually matters (to
me), from a frequency stability standpoint, is the ultimate stability of
the entire system at VLF + LF, which is the receiver, converter, and
computer. The 60 kHz WWVB tests are the best I can come up with at this
time to test the frequency stability of my two VLF + LF receive systems.

So why not just use the R-75 with no converter to tune the VLF + LF band
directly, since there is a probability that would be more frequency stable
than the R-75 + converter (even a high stability converter, like Todd's)?

For the answer to that I refer you to John Reed's review of the Icom R-75
in the November, 2009 issue of the Lowdown, and his subsequent review of
the R-75 + Todd's modified AMRAD converter in the November, 2010 issue.

Stated very simply, the R-75, like most HF receivers that just happen to
cover VLF/LF as an afterthought, is a much better VLF/LF receiver with the
addition of a high quality upconverter, such as the AMRAD model.

Please note that I am only speaking of analog HF receivers. Software
defined receivers (SDRs) are the great unknown to me, and since I just
purchased one recently, that is the main point of me making these
comparisons of my newly purchased SDR VS my tried-and-true analog receiver.
Since so much has been written on the HF and higher virtues (or
compromises) of the SDR offerings, I am only going to comment on the LF and
VLF performance, since that is all I personally care about anyway.

Todd also pointed out that, for even better (much better) frequency
stability than the CR-282 optional OCXO, one could lock the R-75 to an
external GPS reference, and he has done this with his own R-75.

As for me, for now, I am content with the frequency stability of my R-75 +
converter. It is perfectly adequate for any QRSS work up to and including
QRSS120.

Like I said earlier, I have no sophisticated test equipment or technical
training in radio engineering. All I can offer is "real world" reception
comparisons and observations. Most of what I will post will be simple A/B
reception comparisons at various VLF/LF frequencies between what I consider
to be a very good analog setup VS one of the better SDRs available to the
general public at this time. I will make absolutely no comments or
observations above 500 kHz.

One thing I will comment on right now is my dislike of the fact that the
Winradio SDR powers down whenever I exit the software. This causes the unit
to start cooling down, which necessitates another warm up period whenever I
restart the software. I wish there was some way (and perhaps there is and I
don't know about it) to leave the unit "powered up" even when the software
isn't running. The hardware unit seems to put itself into some sort of
diagnostic mode when the software isn't running, but that does not seem to
generate the heat required to prevent "warm up drift" when the software is
restarted. All part of the "keep everything green" mindset, IMO.

Another thing I miss with the SDR is "THE BIG KNOB". Yes, there are several
ways to change frequency with the Excalibur Pro, but, IMO.....nothing beats
twirling a big weighted knob. ;-)

Enough for now.

-Doug KB4OER









On Sun, Oct 7, 2012 at 1:36 PM, Douglas D. Williams <kb4oer at gmail.com>wrote:

> Next comparison will be frequency stability. *(SNIP)*
>
> Not having any sophisticated test equipment, my methodology is simple. I
> tune to WWVB and watch the signal on Argo for any frequency drift. *(SNIP)
> *
>
> Once the one hour warm up was complete, at approximately 14:32 on the
> screen captures, it becomes quite obvious that the Winradio receiver is
> more stable. With that said, both systems are quite stable enough for
> QRSS120. The Winradio has almost no discernable drift on the screen capture
> after the initial warm up period. The R75 + converter seems to only drift
> about a tenth of a Hz up and down over the three hour period, after the
> initial warm up.
>
> Note that the R75 has the CR-282 high stability oscillator option, and the
> AMRAD converter also has a large OCXO. In my opinion, most of the drift of
> the R75 + Converter is from the R75 itself, rather than the converter. I
> could confirm this by testing the R75 without the converter, and may do so
> later on.  *(SNIP SCREEN CAPTURES)*
>