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LF: Xmas tests

To: rsgb_lf_group@blacksheep.org
Subject: LF: Xmas tests
From: "James Moritz" <j.r.moritz@herts.ac.uk>
Date: Wed, 02 Jan 2002 19:41:38 +0000
Reply-to: rsgb_lf_group@blacksheep.org
Sender: <majordom@post.thorcom.com>
Dear LF Group,

Have returned to work this morning after the Christmas break, and have now waded through most of the 400 or so e-mails that had accumulated. I'm sure I have accidentally deleted a few, so if you don't get the response you expected, please e-mail me again.
Thanks for the reports on the 7FSK "measles" mode - well, you have to admit 
it livened things up a bit. For the record, I transmitted the following 
signals:
25/12, 2230-0712, 135.923k, 30s dots, 0.2Hz tone spacing, 1W ERP
27/12, 0000-0640, 135.923k, 30s dots, 0.2Hz tone spacing, 1W ERP
27/12, 2230-0656, 135.923k, 60s dots, 0.1Hz tone spacing, 0.4W ERP
29/12, 0000-0715, 135.924k, 30s dots, 0.2Hz tone spacing, 1W ERP
29/12, 2245-0715, 135.925k, 30s dots, 0.2Hz tone spacing, 1W ERP
30/12, 2110-0730, 135.923k, 30s dots, 0.2Hz tone spacing, 1W ERP

The message read CQ_M0BMU_M0BMU repeated 4 times an hour (or twice when sending 60s dots), together with 2 x 6wpm CW ID's per hour
Not being in touch by E-mail over the period meant that I just selected 
frequencies which were not in use before I started; sorry if it was a 
frequency you were planning to use.
It was nice to see lots of activity on the transatlantic front. The 
attached jpeg from last night shows, from the top, G4FTC (60s QRSS), 
DJ2LF(60s QRSS), G3YXM(60s QRSS), DF6NM(300s DFCW), IK5ZPV(30s QRSS) and 
G3AQC(60s DFCW) - I also copied OM2TW and G3KEV at various times.
The 7FSK generator here is based on a 13MHz TCXO module I found in the junk 
box. It has a pin to connect to an external frequency trimming pot. 
Applying a variable voltage to this pin gives a reasonably linear frequency 
shift up to about +/- 20ppm. Providing the temperature does not change 
rapidly, stability of a few parts in 10^8 is obtained. The tuning voltage 
is supplied by a simple D/A converter consisting of a string of resistors 
and an analogue switch IC, followed by an adjustable gain amplifier to set 
the frequency shift. I settled on 0.2Hz spacing for 30s dots, and 0.1Hz for 
60s because when viewed on a spectrogram, these spacings gave an easily 
identified separation between the tones, allowing for some drift in TX and 
RX. The 3 bit digital input at the moment comes from a suitably programmed 
EPROM, clocked through it's addresses with a long cascade of digital 
dividers. To get to 136kHz, the output is divided down to 1MHz and fed into 
the external reference input of my Racal 9084 synthesiser. Some gates allow 
the 7FSK output to be overdriven by a CW identification signal obtained via 
ON7YD's QRS software. I'm hoping someone will write a 7FSK program soon, so 
I don't have to blow a new EPROM every time I want to send a different message!
It seems several people are missing the point of 7FSK - the idea at this 
stage of development is not to improve the signal to noise ratio of the 
received signal, but simply to increase the speed of signalling that can be 
achieved with given signal strengths, as Rik has pointed out. From this 
point of view, it seems to work quite well, with the 7FSK covering the same 
distances under the same conditions, yet being a lot quicker. Trying to 
have even a minimalist QSO with 30 or 60s QRSS takes hours, and is rendered 
very difficult by fading. The majority of stations that have so far been 
detected across the Atlantic have had to resort to these long dot lengths 
in order to be detected at all, so anything that can speed things up has to 
be taken seriously. For beacon signals, the speed of signalling is not very 
important, since it is only necessary to transmit enough information to 
identify the beacon, rather than to have any actual exchange of 
information. So for beacons, QRSS is quite satisfactory, while for QSOs it 
isn't.
I think those who say that 7FSK is an inefficient use of bandwidth have not 
really thought it through. Compared to QRSS, or aural CW for that matter, 
7FSK uses 7 tone frequencies, whilst CW uses only 1. Therefore it needs 7 
times the bandwidth when using the same dot length. For random 
alpha-numeric characters, I reckon morse code averages about 11 dot lengths 
per character, whilst 7FSK uses 2 dot lengths for all characters - so for 
the same dot length, 7FSK is 5.5 times quicker on average. So the relative 
efficiencies of this mode in spectrum usage, measured in 
characters/second/Hz or however you want to express it is about 5.5/7, or 
about 0.8 times that of CW - not very different, really. If you wanted to 
send the same messages using CW and 7FSK in the same length of time, you 
would have to use CW dots 5.5 times shorter than the 7FSK dots, so the 
bandwidth would be 5.5/7 = 0.8 times that of the 7FSK signal. So from the 
viewpoint of spectrum usage, it is swings and roundabouts - QRSS does use 
less bandwidth than 7FSK, but only when it is sending information more slowly.
I am slightly amazed that 1.4Hz is considered by some as an excessive use 
of bandwidth. Over 1000 stations could operate on 136k simultaneously in 
this mode with the same parameters I have been using, even allowing for a 
generous guard band between them - many times more than could use, say, SSB 
on 80m. Bear  in mind that most people are happily using normal hand CW on 
136kHz - if you look at this on a spectrogram, you will see it takes up 
50Hz or more, but I have never heard the band anywhere close to being full 
up - most of last year people complained about there being too few signals 
around.
Most of the problem with frequencies seems to be due to the 3 1/2 Hz 
bandwidth that Argo has when used in 60s dot mode, which is a bit 
restrictive. I have been using Spectrum Lab, which allows you to set any 
bandwidth you like within the limits of the FFT. I usually use 10 - 20Hz - 
with the window expanded to fill the screen, this gives plenty of 
resolution. It also allows you to pan the frequency or alter any of the 
other parameters "on the fly", immediately showing the effect on the 
previous few minutes' signals, a very useful feature when a number of 
different modes are in use. I was actually transmitting 30s dots most of 
the time, so the displays people were getting were in many cases not 
optimum - with Argo in 60s mode the signal looks very wide, and causes the 
dots to overlap (I was not transmitting 2 frequencies at a time as someone 
suggested!), which makes the signal harder to copy and gives them the 
"measles" appearance. All the same, the improvement in speed over the 
neighboring QRSS sigs is obvious.
At the moment, I'm trying to catch up on a bit of sleep, but if anyone 
would like to try experiments with this or other modes, please let me know.
Happy new Year to all,

Cheers, Jim Moritz,
73 de M0BMU







JPEG image

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