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PA2AGA > HDDIG 20.07.00 05:13l 130 Lines 6789 Bytes #-9310 (0) @ EU
BID : HD_2000_194C
Read: GUEST
Subj: HamDigitalDigest 2000/194C
Path: DB0AAB<DB0SL<DB0RGB<DB0ABH<DB0CWS<DB0AIS<DB0NDK<DB0ACH<PI8JOP<PI8ZAA<
PI8HGL
Sent: 000719/1404Z @:PI8HGL.#ZH1.NLD.EU #:62415 [Den Haag] FBB $:HD_2000_194C
From: PA2AGA@PI8HGL.#ZH1.NLD.EU
To : HDDIG@EU
Date: Wed, 19 Jul 00 14:26:37 MET
Message-Id: <hd_2000_194C>
From: pa2aga@pe1mvx.ampr.org
To: hd_broadcast@pa2aga.ampr.org
X-BBS-Msg-Type: B
be
> spread about 200hz or so apart and at lower strength as you go out from a
> "center" signal.
>
> 2. I can understand not getting a contact if I'm not on freq. , but I must
> wonder if I am putting out a signal at a the appropriate frequency. I
have
> called CQ a few times and I seem to get a reply slightly to the side of
the
> selected freq. Any idea as to the cause of this?
>
> 3. Any other info would be gladdly accepted.
>
> Thanks!!!
>
> 73 de Virgil, KU4TP
In response:
1. If the "multiple tracks" are what your description suggests to me, they
may be spurious sidebands caused by an overdriven signal. A clean PSK31
signal should look like a pair of lines (a single set of railway tracks) on
your waterfall display. A signal which is overdriven may display sidebands
at the 3rd, 5th, ... harmonics of the modulating frequency (31.25 Hz). These
look like weaker rails on both sides of the main railway track Overdriven
signals are not that uncommon, unfortunately.
It may be possible that these spurious sidebands could be generated by
overloading in your receive system, but that should be very easy to check.
Just reduce the sound card's input audio level and the radio's receive audio
level a bit, one at a time, and if the spurious signals are locally
generated, they should drop in strength much faster than the main signal as
the "culprit" audio level is reduced. If all of the signals drop in strength
together, then the spurious sidebands most likely originate elsewhere, i.e.
they are not your fault.
These spurious signals contribute nothing to the readability of the signal,
and cause QRM to other QSOs. You really don't need to squeeze every last
watt out of your transmitter to make QSOs. Besides, as many RTTY operators
have found out, running your finals flat-out in a digital mode like PSK31
can be injurious to their health. In most amateur transceivers, the finals
are rated for intermittent service, not key-down digital signals, so in
digital modes you are well-advised to keep your output power lower than you
would for CW or SSB.
The basic solution to the overdriven signals problem is to reduce the audio
drive to the transmitter, and to avoid any kind of audio processing or
compression. Some of the best advice I am aware of on this is in the help
file for G3PLX's original PSK31 program (file psk31sbw.hlp, which is
contained in p31sbw108.zip), under sound-card setup. There is also useful
info in the help file for WinPSK (or WinPSKse). You can find both of these,
and lots of other good stuff related to PSK31, at www.psk31.com.
There is another type of spurious signal which can be generated by
overdriving in the audio stages of a transmitter. If your central audio tone
is at, say, 1000 Hz, one or more harmonics of the audio tones may lie within
the bandpass of the transmitter's IF filters and, if generated at
significant levels, can be transmitted along with the main signal and
various other mixing products. The end result is that there can be several
copies of the same signal at various locations on the band, at separations
on the order of 1 or 2 kHz, rather than 62 Hz. This may be cured not only by
reducing drive levels, but also by increasing the frequency of the audio
tones coming from the sound card so as to take full advantage of the IF
filtering in the transmitter. Dick Stevens, N1RCT, used to have a very good
explanation of this phenomenon on his web pages (much better than I am able
to give), but unfortunately it seems no longer to be there.
There is an interaction with your radio's ability to use narrow CW filters
in SSB mode. For example, on my radio I can switch in my 500 Hz CW filters
in SSB mode, with a centre frequency of 1360 Hz, so that's the audio
frequency I tend to use for PSK31 and sound-card RTTY. If I were using the
radio's 2.7 kHz SSB filters, I would be tempted to push the audio frequency
up, perhaps as far as 2210 Hz, which also happens to be where the narrow
filters are centred in FSK mode on my radio, and therefore where my TNC is
set up to decode and encode RTTY; this is the North American standard for
RTTY, and as you might guess, the choice of such "high" tones as a standard
is no coincidence.
2. It is indeed possible that your transmitter is not on the same frequency
as your receiver. If replies to your CQs are consistently off your frequency
in the same direction, it is quite likely that you have this problem. It may
be that your transceiver is not quite in alignment (RIT and passband tuning
are possible culprits). This appears to be quite common, judging from the
number of hams on CW, and lately on RTTY as well, who don't seem to be able
to zero-beat with the other guy (and no, we're not talking about
split-frequency operation here). You can check this out with the help of a
couple of ham friends. One of them transmits a CW or PSK signal to you; you
then attempt to reply on exactly the same frequency; and the third listens
to you both and, perhaps with the help of a frequency display like the one
in Digipan, tells you how far off frequency you are.
3. As far as interfaces are concerned, I may have been unusually lucky, but
I have been able to operate successfully with no attenuator and indeed no
isolation between the radio and the computer. In fact, my computer is
interfaced to the radio's control port via a serial cable and a
level-shifting circuit, to a TNC (which is connected to the radio) via
another serial port, to the radio's CW key input via a parallel port and
keying transistor, and via connections from the sound card to the radio's
audio input and output, all with no DC isolation, and surprisingly enough
without ground loop problems. You may not be so lucky, of course. If not,
for digital control lines the best solution is probably opto-isolators, but
for audio lines the first thing to try is audio isolation transformers
(Radio Shack 273-1374 seem to work fine). I have used these as well, but in
my own case I found no observable difference in system behaviour with and
without them.
If you don't have an attenuator in the transmit audio line, you have to be
careful with your sound card audio output levels to avoid overdriving, but
depending on your radio's audio sensitivity, it may be quite possible to
work this way. If not, you need a simple two-resistor attenuator, or maybe a
step-down isolation transformer might do the trick.
Again depending on your radio, you may be able to use VOX rather than wiring
up a PTT interface. It works just fine for me. Mind you, I don't use VOX on
To be continued in digest: hd_2000_194D
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