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Asunto: Psk31 Digest V1 #323
Fecha: miércoles 23 de febrero de 2000 4:25
Psk31 Digest Wednesday, February 23 2000 Volume 01 : Number 323
In this issue:
[psk31] Clarification of Average and Peak Power
Re: [psk31] SOUNDCARD PROBLEMS
Re: [psk31] SOUNDCARD PROBLEMS
Re: [psk31] SOUNDCARD PROBLEMS
Re: [psk31] SOUNDCARD PROBLEMS
Re: [psk31] SOUNDCARD PROBLEMS
Re: [psk31] SOUNDCARD PROBLEMS
Re: [psk31] SOUNDCARD PROBLEMS
Re: [psk31] SOUNDCARD PROBLEMS
Re: [psk31] Clarification of Average and Peak Power
See the end of the digest for information about psk31-digest
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Date: Mon, 21 Feb 2000 20:41:40 -0800
From: "Scott Prather" <sdprat@worldnet.att.net>
Subject: [psk31] Clarification of Average and Peak Power
I'm a bit confused regarding the use of the term "measuring output power at
idle". When a transmitter is in the "tune" mode (and therefore transmitting
a CW signal) the instantaneous peak power is twice (3 dB above) the average
power, but we don't care about instantaneous peak power. Instead, we
measure peak envelope power, which equals the average power when
transmitting a CW signal (0 dB peak-average ratio).
If memory serves, the QPSK modulation mode in PSK-31 has a peak-average
ratio which will not exceed about 4 dB 99.9% of the time. Consequently, if
a radio is rated to provide 100W PEP, the average power during QPSK
modulation must be reduced to about 40 watts, assuming that the radio's PA
is sufficiently linear to meet the requisite IMD at its full rated PEP. BPSK
can probably get away with less than 2 dB backoff, again, assuming the
linearity and ALC action is sufficient to support the rated PEP. The
average-reading wattmeter used with the radio will read the average power of
the selected modulation mode, and this power level must be low enough to
keep the PEP at or below the transceiver's rated power (by backing off the
average power according to the peak-averege ratio of the selected modulation
mode).
In practice, many radios employ an ALC circuit which may cause unacceptable
IMD before the radio's rated PEP level is reached. Unless a spectrum
analyzer is available to verify the IMD level, the typical user should
probably use quite a bit of backoff (say, 6 dB) from the radio's rated PEP.
That should be more than sufficient for starters.
Scott
N7NB
------------------------------
Date: Tue, 22 Feb 2000 10:16:22 +0000
From: "Julian" <julian@tech-pro.co.uk>
Subject: Re: [psk31] SOUNDCARD PROBLEMS
OK, but what practical difference does -30db IMD make over -23dB, compared
with the disadvantage of running half the power? -23dB is still 400 times
down. If we are looking for culprits for bad signals, don't forget, the
input audio tones are generated by some cheap piece of consumer electronics
whose manufacturer gives no specification at all for output linearity or
distortion.
- --
Julian, G4ILO
http://www.tech-pro.co.uk/
"Gary McDuffie, Sr." <mcduffie@scottsbluff.net> wrote:
[snip]
Over 50W, mine
runs about -23db IMD or worse. Below 50W, it just keeps getting better
and better with -30db down in the 25-30W area.
------------------------------
Date: Tue, 22 Feb 2000 07:37:53 -0500
From: "Bob Lewis" <aa4pb@erols.com>
Subject: Re: [psk31] SOUNDCARD PROBLEMS
> If we are looking for culprits for bad signals, don't
forget, the
> input audio tones are generated by some cheap piece of
> consumer electronics whose manufacturer gives no
specification
> at all for output linearity or distortion.
>
With the exception of Creative Labs, I have a hard time
getting any kind of specification. I sent an e-mail off to
WinBook asking for pin outs, impedance, levels, etc. The
answer was that their lap tops used the "industry standard"
1/8-inch phone plug connection.
As several have stated before, don't forget that the receive
chain can add to the IMD as well.
------------------------------
Date: Tue, 22 Feb 2000 08:50:05 -0600
From: "Bob Hicks" <w5tx@airmail.net>
Subject: Re: [psk31] SOUNDCARD PROBLEMS
Warblers,
Haven't measured all the sound cards out there but I suspect they all exceed
50-60 db IMD performance. My measurements so indicate.
Bob
------------------------------
Date: Tue, 22 Feb 2000 10:30:40 EST
From: Charles R Greene <cgreene7@juno.com>
Subject: Re: [psk31] SOUNDCARD PROBLEMS
On Tue, 22 Feb 2000 10:16:22 +0000 "Julian" <julian@tech-pro.co.uk>
writes:
> OK, but what practical difference does -30db IMD make over -23dB,
> compared
> with the disadvantage of running half the power? -23dB is still 400
> times
As it appears on the screen, it is only about 14 times down (20 log
E1/E2). That's pretty bad. Another way to look at it is that at 23 dB,
the 3rd IMD product is about 4 S units down, and is an S5 on an
S9 signal. There's no reason why using an average sound card,
one shouldn't get at least -27 dB IMD. Anything greater than that
is due to nonlinearity in the final or a real bad card. We
should strive to be as good as we can be within reasonable
costs and complexity limitations. Given existing QSB conditions,
the difference between a 25 watt signal and a 50 watts signal is
indiscernible.
------------------------------
Date: Tue, 22 Feb 2000 08:39:14 -0700
From: "Gary McDuffie, Sr." <mcduffie@scottsbluff.net>
Subject: Re: [psk31] SOUNDCARD PROBLEMS
On Mon, 21 Feb 2000 15:38:33 -0600, Bob Hicks wrote:
> Lots of good comments on subject.
Gee, I only received this message 4 times!
Good comments though.
Gary
ag0n at arrl dot net
- --
------------------------------
Date: Tue, 22 Feb 2000 08:39:25 -0700
From: "Gary McDuffie, Sr." <mcduffie@scottsbluff.net>
Subject: Re: [psk31] SOUNDCARD PROBLEMS
On Tue, 22 Feb 2000 07:55:46 +1100, Ken Jobling wrote:
> Could power supply regulation at higher loads have some bearing on the
> purity of the signal at higher power levels?
Not likely, unless you aren't running a good supply in the first place.
I'm assuming (probably wrongly so in some cases) that everyone would
cover the obvious points like a good power supply before even getting
started, especially on a mode that runs a continuous signal like rtty,
psk, etc. I run a 55 amp supply on mine.
Now that I re-read your question, I'd have to say SURE it could. But if
it is, then you either have a problem in your supply, or it is
under-rated for the job. C'mon people. This isn't rocket science. We
need to have a supply that is up to the task.
Gary
ag0n at arrl dot net
- --
------------------------------
Date: Tue, 22 Feb 2000 08:44:07 -0700
From: "Gary McDuffie, Sr." <mcduffie@scottsbluff.net>
Subject: Re: [psk31] SOUNDCARD PROBLEMS
On Mon, 21 Feb 2000 14:54:00 PST, Peter Klein wrote:
> It also depends on what you mean when you say "50 watts." 50 watts of what?
> Peak power? Average power? PSK idle? "Tune" button single-tone carrier?
I'm (maybe wrongly) assuming we are all talking about peak power. My
meter on the rig is peak power, and my meter in the tuner is set to peak
power (but probably not as accurate) when I'm on "sound card modes". I
show about 50 watts on the radio and 40 on the peak mode in the tuner.
If I go back to "normal" mode on the tuner meter, I show maybe 25 watts
or less.
Gary
ag0n at arrl dot net
- --
------------------------------
Date: Tue, 22 Feb 2000 08:46:39 -0700
From: "Gary McDuffie, Sr." <mcduffie@scottsbluff.net>
Subject: Re: [psk31] SOUNDCARD PROBLEMS
On Tue, 22 Feb 2000 10:16:22 +0000, Julian wrote:
> the
> input audio tones are generated by some cheap piece of consumer electronics
Didn't someone post some numbers recently for direct measurements out of
some sound cards?
Gary
ag0n at arrl dot net
- --
------------------------------
Date: Tue, 22 Feb 2000 13:50:26 -0500
From: "Robert Lay" <w9dmk@crosslink.net>
Subject: Re: [psk31] Clarification of Average and Peak Power
Dear Scott and fellow Warblers,
Your confusion may relate to the fact that when PSK'ers refer to "idle" they are
referring to that condition wherein the two tones are being transmitted but
there are no characters. being transmitted. That condition generates one half of
the power output that would obtain in the "Tune" position.
QSL?
73 de Bob Lay, w9dmk@arrl.net, King George, VA
http://www.qsl.net/w9dmk
http://www.fortunecity.com/meltingpot/albania/784
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End of Psk31 Digest V1 #323
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