Mal
The answer is simple in many cases.
Why not?
I will explain.
You can use a narrower filter if you like, it would have to be 200Hz wide to
allow the reception of all WSPR signals in the range of the software. CW
filters tend to be too narrow for that, SSB filters are standard on most
rigs, so it is easy to use this mode. It is also mandatory to use SSB mode
for transmitting, and on some rigs it is difficult to transmit on SSB and
have the CW filters in. A good CW 50Hz filter would probably work OK, given
the very narrow bandwidth of the individual WSPR signals, with a small
number of WSPR transmissions on the band you could use it. It simply isn't
needed though, and it is desirable to allow the software to see the entire
200Hz segment, even if it is just to allow for drift or decoding other
signals.
For some modes the use of narrow conventional filters is bad. They
intrinsically have poor group delay response and cause distortions to the
signal not often directly audible, but that can degrade data is a way that
reduces decoding margins. This effect is worst when the wanted signal is
relatively wideband (occupying the whole bandwidth of the filter) or near
the edge of the passband. DRM, DAB and DTT signals that are OFDM based are
vulnerable.
All the WSPR operation that the MF band would need, could be squeezed in a
segment of the band less than 200Hz wide, probably 100Hz would be more than
ever needed. WSPR also time division multiplexes, so more than one station
can occupy the same frequency. No one is proposing WSPR signals all over the
band, that is self defeating as the software cannot analyse more than a
200Hz chunk of bandwidth.
The key point is that just because the receivers have a wider passband than
actually needed does not usually render the mode ineffective in the presence
of signals that are outside the decode passband.
Like any system, reducing the bandwidth of any part of the system, means
that subsequent stages are less affected by adjacent signals.
The antenna itself acts as a filter because it is tuned, your low pass
filter removes many big broadcast signals, but no one is advocating a
tuneable 50Hz wide bandpass filter implemented at 500KHz, or anything
approaching this, for CW or any other mode. It is simply not necessary in
most cases, and a disadvantage in most cases.
Almost all modern conventional receivers have roofing filters that are quite
wide in early stages, followed by narrow mode specific filters. Just because
the filter before is wider does not unduly degrade performance of the
receiver after the more narrowband filtering.
Interestingly, my grabber receiver has only very simple roofing filters,
many hundreds of KHz wide and very low Q, they stop almost nothing 'out of
band'
All the rest of the filtering is done in software after an A to D converter
(soundcard) which has a remarkably high dynamic range. There is no AGC, it
is not needed, and the software filters which operate on the same principals
as the WSPR decoding system, allow me to listen to a weak signal with an
unwanted signal over 70dB higher, less than 100Hz away. I can actually still
read the stronger stations on the band whilst transmitting low power on the
main antenna, only a few tens of feet away, and a few hundred Hz away.
Ironically I get more problem when I transmit on 6m, though the RX antenna
is only about 2 feet away from the 6m beam.
I hope this helps your understanding of the principals behind WSPR and other
FFT based systems. They are not so different. If that is not what you are
looking for then please clarify.
I have to say that the immunity of WSPR to interference is not as good as I
would expected, if an unwanted signal comes up in the 6Hz bandwidth even for
a short time, it can corrupt the decode. It is pretty good at ignoring
strong stable signals that are reasonably spaced away. last night and this
morning a small number of decodes of Jim's beacon were lost due to my
'wandering sprog'. The presence of the big CW and QRSS signals in my
receive passband (pre WSPR processing) does not affect decode. BHZ and DI2AM
are often 40dB above noise, and they are only a few hundred Hz away from
Jims WSPR signal, and presented to the WSPR decoder unattenuated by any
filtering.
I find the levels required to decode WSPR to screen about the same as my
ability to read QRSS comfortably, and significantly better than my ability
to read weak CW.
as an EME operator, I can read weak CW quite well. The slow QSB on 600m is
a problem for both WSPR and QRSS. and indeed there may be instances where a
quick burst of normal CW would be better than the slow modes. WSPRs big
advantage is that it automatically gathers propagation data. The ability to
observe and accurately quantify the varying signal strength over time is
extremely useful.
Mark GM4ISM
----- Original Message -----
From: "mal hamilton" <[email protected]>
To: <[email protected]>
Sent: Wednesday, December 24, 2008 11:28 AM
Subject: Re: LF: wspr
I did not ask a question about filtering !!!!!!!!!!!!!!!!!!
What I said was why use SSB mode which is 3 khz wide for a 6hz/200 hz wide
signal when one could take advantage of the receiver narrow CW filter, and
of course this is filtered further by manipulation of the soundcard by
software. This is old hat technology and not new.
WSPR was engineered for VHF/UHF with plenty of frequency spectrum
available
and not MF/LF squeezed into a 3 khz slot along with other more robust
modes
I am not opposed to any particular transmisson MODE but merely pointing
out
that the advantage claimed by some for WSPR is not justified in some cases
and my recent observations indicate that I could have read the transmitted
signal had it been ON/OFF CW, instead I had to wait ages for the signal to
improve before text printed. This was the case last night with WE2XGR
where
the 2 minute interval trace was good enough to be read in on/off mode CW
but
not strong enough to print most of the time due to slow fade(QSB)
I might even research WSPR further for comparison purposes but I cannot
imagine that I will get a print out first before I see a trace.
G3KEV
----- Original Message -----
From: "Scott Tilley" <[email protected]>
To: <[email protected]>
Sent: Wednesday, December 24, 2008 5:00 AM
Subject: Re: LF: wspr
If you could read you would do some research at K1JT's wonderful website:
http://www.physics.princeton.edu/pulsar/K1JT/
Joe has laboured for many a year on similar projects and has written
much about his application of the art. What you will find is that
research into communication theory that started with CW has taken us
here...
To answer your specific question the filtering is done in software. The
DSP is done in your PC, thereby making filtering in the radio somewhat
redundant unless you have strong neighbours in the passband. So using a
wide SSB filter and the radio in USB makes for easy math in ones head.
Yes, we digital types use our heads from time to time.
Often with modes like JT65 used on EME and now quite popular on HF one
wants as much bandwidth as possible in the receiver so you can monitor
up to the entire band in real time. So lots of raw bandwidth into the
computer is a good thing...
All BS aside, you may find the technology very interesting to study and
you may find that what you discover is that the spirit of the CW
operator of old is alive and well just evolving with the times.
CW will never die as it has a rich history but it shouldn't be allowed
to impede the growth of new modes and technology.
You should build yourself a Softrock SDR receiver or even a small
transceiver kit and witness a true revolution in radio technology. My
little 40/30m rig allows me to watch the entire band of either in real
time. With some new software you can monitor all of the CW QSOs at
once... Pretty cool.
Not to mention you can plug a key into the little box and do what you
love most and work'm.
73 Scott
VE7TIL
mal hamilton wrote:
If recent published info is correct, this specifies a bandwidth of 6hz
why is USB with a bandwidth of 3 khz necessary to receive this
transmission.
Surely it would be obvious that CW mode was more appropriate where
one could use a narrow filter and dsp processor of a few hertz.
I have asked this question before but never got an answer.
g3kev
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