Topband: Re: Weak Signal Reception (EME view)
btippett at alum.mit.edu
Fri Jul 18 12:17:31 EDT 2003
I've been thinking about this weak signal issue after
reading SM5BSZ's EME website and have come up with a
few thoughts regarding applicability to 160 and K6SE's tests.
1. Most noise at VHF is galactic white noise, which is very
different than the typical impulse noise we have on the low
bands (e.g. thunderstorm QRN, clicks, etc.).
2. Following Leif's experience, reception of EME signals
boils down to making extremely very narrow DSP filters
which work quite well for signals in white noise.
3. The limiting problem appears to be the ringing of such
narrow filters. Leif uses one filter which is ~17 Hz wide and it
has very noticeable ringing. I would estimate the CW signal
is being keyed at ~20 WPM, and I recall an approximate
estimate for keyed CW bandwidths is about 4 times the
keying speed. This would imply a BW of ~80 Hz for such
signals which explains why you hear definite ringing through
his 17 and 25 Hz filters. Leif comments that the keying is
a little fast for EME work. This is the same reason that the
QRSS mode uses a speed of 0.8 words per hour. Such very
slow-speed CW signals have keyed bandwidths at sub-Hz,
which allows DSP programs such as Spectrascan to use
filters in that range for visual display on a CRT. Even if the
audio were available, I'm not sure my mind would be patient
enough to decode 0.8 word per hour CW (.013 WPM) and I
don't see QRSS as ever being practical for actual QSOs.
4. Could we use Leif's narrow DSPs on Topband? Yes...BUT...
best under white noise conditions. If signals with the rise times
of 20 WPM CW pulses cause ringing, just imagine what would
happens when you hit a narrow DSP filter with impulse noise
like lightning-strike QRN or key clicks...RING-city!
5. This gets me to a few more thoughts about K6SE's test.
Earl is listening to a continuous carrier on a quiet, unpopulated
band. Such a signal has effectively zero bandwidth due to keying
(of course it still has phase noise variations but let's ignore that).
This means his test may be simply a test of which radio has the
narrowest BW. Why? Because for each octave reduction in BW
you gain 3 dB in S/N due to the narrower noise BW. If true, I
suspect Earl's rankings are primarily an indication of which
radios have the narrowest filters. Let's test that assumption
against the top radios in his list:
1. K2/DSP - 50 Hz DSP (per Earl)
2. IC-746PRO - 50 Hz DSP (Icom spec)
3. FT-1000MP/Inrad IF mod - 60 Hz DSP (Yaesu spec)
4. K2/KAF2 - 80 Hz audio filter (Elecraft spec)
5. FT-1000D - ?? Hz APF filter (very effective per K1ZM)
I'm not going to look up all the others but there seems to be
some consistency with Earl's ranking and minimum BWs.
Where does Orion fall in the BW list? Orion's lowest selectable
DSP is supposedly 100 Hz, however it may actually be 150-175
Hz according to measurements by two different users I know of.
Here's one published by N4PY:
Filter 6dBBW 60dBBW Shape Factor
100 150 440 2.9:1
250 240 470 1.95:1
500 510 820 1.6:1
1000 980 1160 1.2:1
1600 1580 1880 1.2:1
2400 2360 2500 1.06:1
3000 2930 3270 1.12:1
4000 3960 4280 1.08:1
The discrepancy between Orion's claimed 100 Hz BW and
measured 6 dB BW of 150 Hz is out of line compared to
the other DSP settings (which appear to be correct). I
asked TenTec to check out this anomaly which could be
an error in their DSP firmware. Note that shape factors
are automatically widened as BW goes down which may be
done to minimize ringing effects in narrow filters. If
the above measurements are correct, Orion's ~150 Hz DSP
gives away ~5 dB in S/N to radios with 50 Hz bandwidths,
simply because of lower noise BW (10 log 50/150).
6. TenTec should certainly address this issue, but frankly it
may not make much difference to me personally. I've used an
MP since 1997 and can probably count on the fingers of
one hand the number of times I've set the DSP to 60 Hz. I only
found one setting of DSP/NR to be marginally effective and
that was with the DSP set to 240 Hz. (See comments here
but note that I neglected to mention I had DSP set to 240 Hz:
http://www.albany.net/~bg/Yaesu/MPTips.htm ) Why have I
not used narrow DSP? Simply because I find it is ineffective
with the static bursts, clicks and other types of impulse noise
I typically experience on 160.
7. We all hear differently, have different noise conditions, and
use our radios differently. In a contest, I almost never set my
BW even to 500 Hz if I can get away with it, in order to hear guys
calling off-frequency. The CQ 160 CW is the usual exception to
this rule because of extremely close-spaced strong signals, but
even then I seldom set my MP crystal filters below 500 Hz. Using
any kind of very narrow filters is counter-productive to maximizing
score IMHO. Just for the record and to prove I'm not totally nuts,
I've won the CQ 160 CW for USA twice in the past 5 years and
also won 15 of 17 major DX contests entered on 10 meters since
1999...and I never used the MP's DSP filters in a single one, with
the exception of SSB auto-null for CB carriers.
8. Orion's greatest strength will be in contests like the CQ
160 due to its exceptional front-end. 13-28 dB more dynamic
range (see 5 kHz spacings below) is very compelling. BTW,
I'm curious what these same measurements will be for signal
spacings of ONE kHz when Orion's optional 500/250 Hz roofing
filters are used. I hope W8JI or someone who has the equipment
to test this will publish measurements for 1 kHz signal spacings.
I expect Orion's 5 kHz performance below will hold for ~1 kHz
signal spacings and the other radios will degrade even more.
ARRL 5 kHz measurements (14 MHz, 500 Hz BW, preamp off):
Radio IMDDR3 (dB) IP3 (dBm)
Orion* 101 +24
K2 88 +1
MP MkV 73 -5.2
756Pro2 76 -18.8
*TenTec measurement, ARRL methodology, 1 kHz roofing filter
and 500 Hz DSP selected.
9. Is Earl's test valid? I'm sure it is under the conditions he chose.
But let's just summarize what they are and then ask ourselves if
they are representative of what we normally experience on 160:
a. dead band...no strong signals present.
b. white noise...no impulse noise like clicks, thunderstorm QRN, etc.
c. continuous carrier...no ringing issues due to keyed BW.
While Earl's test conditions favor what narrow DSP is best
for, they are unfortunately not typical at my QTH...I only wish they
were. ;-) What do I feel is the most valid 160 receiver test around?
Very simple answer...the CQ 160 CW Contest. It will be very
interesting to see how all these new radios perform next January.
I would not be too quick to draw too many conclusions until the
results of that test are in.
73, Bill W4ZV
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