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Author Topic: Strong PSK-31 Signals  (Read 3146 times)
KC7CRR
Member

Posts: 61




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« on: February 27, 2002, 01:48:18 PM »

Hi All,

I've been using PSK-31 for a month now and am having a blast!! Am working stateside and DX and getting about as many QSOs as I do on SSB. My station includes a Kenwood TS-870, Dell PC, RASCAL Interface, DIGIPAN, and various antennas. I do have one question (actually I have many, but this one is at top of my list).
I live in the Pacific Northwest (Kennewick, WA). Several times on 10 and 17 meters, I've seen very strong signals come on which wipe everyone else off the screen. I'll click on it to see if it's someone fairly local, and read something like this:
"Hi Sam, name here is Fritz and my QTH is Somewhere, NY. Power out is 10 watts." Looking at Fritz's signal, I'll see that it is accompanied with a multitude of sidebands (I've read that this is BAD). Finally Fritz goes back into receive mode and all the other traffic reappears.
OK, is this a characteristic of PSK-31 combined with great band condition, or might Fritz be transmitting with a bit more power than 10 watts?
Would appreciate some feedback.

Thanks and 73s,
de KC7CRR (Bob)
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K0HZI
Member

Posts: 470




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« Reply #1 on: February 27, 2002, 08:09:21 PM »

The person with the wide signal has the transmitt audio into the transceiver set up incorrectly if you see more than one track on your waterfall display.  If you have a IMD tool in the digital software and measure the IMD any thing less than -20 dB is considered bad.  IMD readings that go more negative are better.
Here is a web site to set up the sound card so you don't generate poor signals:
http://www.waypoint.com/users/~discobay/section_7.htm
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NB6Z
Member

Posts: 550


WWW

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« Reply #2 on: February 28, 2002, 04:24:50 PM »

It is very possible that the other station does have an IMD problem from overdriving his MIC input circuit. But that can be noticed best when the signal is NOT also overloading your receiver, causing a similar issue. You indicated that the signal was strong and blocked out the other "nominal" sigs on the band. This would be the case if any strong sig were to come on and your receiver had the AGC set to ON. (Your RX gain is automatically lowerred to match the strongest signal.) This feature is very usefull for voice operation but will cause the disappearance of some otherwise readable sigs from the waterfall display. This is especially true if you like to view the entire PSK31 band in one wide display (3Khz).
I operate my FT-920 in the digi modes with AGC off and I select the J-FET pre-amp with an additional 6 dB of attenuation. This combo work well for me... A strong signal can cause harmonic and IMD distortion all over the waterfall, but if that happens, I quickly turn off the RX pre-amp, or I could also dial in more attenuation. In most cases, a strong signal will look ugly but still decode fine and I can still see other nominal and weak sigs on the waterfall.
To take full advantage of the waterfall and weak signal operation, you should try to operate the digi modes with your AGC off. The down side might be that your S-meter will no longer function. (Most rigs use the AGC voltage to run the meter.) This is no big deal because your s-meter is actually getting energy from all sigs on the waterfall and not just the one you want to report on. My approach to this problem was to experiement a little with sigs that were alone on the waterfall and come up with a waterfall color scheme that would color each signal according to how they were seen on my s-meter during the testitng. (MixW and Digipan are 2 prorgams I know of that will allow you to do this.) This method only works with the AGC off and works best with receievers that have good dynamic range.

GL and have fun... ;-)
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KF6IIU
Member

Posts: 299




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« Reply #3 on: March 04, 2002, 04:27:31 PM »

I check out bad signals with my -25 db attenuator. I have yet to see a bad signal that is a result of overloading my front end (I have an FT-101 which is very good about that.)

Most ops with bad signals are overdriving their audio, which causes all these excess sidebands. Most seem to have their sound card setings up too high. Any setting near the top or bottom on the audio range may cause problems - I mean which circuit do you trust to behave more linearly - the audio stages in your $1000 rig or your $10 sound card?

Some ops are also transmitting with 60Hz in their signal. These signals are *really nasty*, sometimes 1000 Hz wide - the sidebnds are every 60hz of course.

I have both my master and wave volume settings at 5 1/2 and I leave them there. I have a Tigertronics interface which provides some attenuation, and I make minor adjustments with the Mic gain. Audio is fed to the Mic pins. Usually my Mic gain is at or very near the same settings I use my microphone for SSB. I receive consistently good signal reports.

As for the ops who have their power cranked up too high, it's only a matter or time before they fry their rigs. So we don't have to worry about them :-)

The web site at watpoint.com mentioned above has a good algorithm for setting output power - increase your drive (not your soundcard audio) to the point where the ALC needle just begins to waver, then reduce output power to 1/2 of that. That's a good starting point.

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K5CFW
Member

Posts: 5




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« Reply #4 on: March 28, 2002, 04:36:23 PM »

As has been pointed out, it's best to operate with AGC turned to "OFF."  I've had good luck at masking out offensively strong signals through use of my transceiver's "notch" filter.  Notch filters are not normally used for PSK31, but it does a great job of attenuating about a 100-Hz wide chunk of spectrum.  This seems to also get rid of some of those offensive harmonics and sidebands, too.

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