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Author Topic: Weird receiving problem on PSK31  (Read 3014 times)
WB0AXN
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Posts: 90




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« on: March 14, 2008, 09:58:14 AM »

Made my first contact with PSK31 this morning and a problem occurred.  
I sent out a CQ and a station called me back.  His text was all garbled and then I noticed his transmitting frequency was a hair off from mine.  So, I moved my pointer on top of his transmission and the test was perfect.  
However, on his next transmission, he said he missed half of my transmission because he had to retune.  We continued the QSO with me shifting my pointer back and forth between transmissions.
This frequency change problem reminds me of the old tube days....when transmitters would drift and everyone had to change freqs....which accounted for "leap-frogging" eith each chasing each other up or down the band.
So, my question is:   Has any experienced user of PSK had or heard of this problem and know what causes it and how to correct it??
I was told by my first contact that my signal looked good and was solid copy.
I am using Digipan software on a 2.4 ghz computer with 2 gig of RAM.
My rig is an Icom 746Pro with the power reduced to about 25 watts and the ALC almost at zero level.


Roland
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G0GQK
Member

Posts: 634




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« Reply #1 on: March 14, 2008, 04:09:02 PM »

Check that your RIT is switched off. This occurs when your RX is a slightly different frequency than your TX.
If your RIT is switched off then the fault is at the other end.

G0GQK
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AA4PB
Member

Posts: 15067




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« Reply #2 on: March 14, 2008, 05:21:30 PM »

One of you should turn off the Tx Lock (various names for this are used) on the PSK31 software. With lock turned on, your Tx frequency will jump to match your Rx frequency each time you key up. If one station has a slight offset between his Rx and Tx frequency and both have lock turned on then the two of you will "walk" up or down the band with each change over as the lock tries to keep correcting the Tx frequencies.
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Bob  AA4PB
Garrisonville, VA
WB0AXN
Member

Posts: 90




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« Reply #3 on: March 15, 2008, 07:41:18 PM »

Thanks for the suggestion to solving my frequency shifting problem.  I re-wired my home brew interface and that seemed to help a lot.  Had a very long QSO with a guy in Canada and he said my signal was rock solid the entire time.   He said he has noticed a few other users....their frequency would jump on occasion too.  He suggested to try another software besides Digipan.  I have DM-780 downloaded, but have not installed it.  Saw some say it was too complicated for them and they went back to Digipan.  This is only my 3rd day on PSK31.....so what do I know.  LOL
I spent an hour this evening....reading other transmissions and I noticed the pointer jumping around slightly...but not all the time.  
I plan on keeping everyone posted as I solve my problem...maybe I can help someone else sometime...that is just starting out.
Roland
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AA4PB
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Posts: 15067




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« Reply #4 on: March 15, 2008, 08:08:05 PM »

Your long QSO would indicate to me that it was the other station on the earlier QSO that had the Tx/Rx offset problem. Again, if you turn the LOCK off after you make the initial contact then your transmit frequency will stay put even if you or your AFC has to re-tune your receiver to match his Tx frequency. When you Tx again it will be on the same frequency as last time since you had LOCK off and didn't move it. That will anchor him to your Tx frequency so that he will always come back on your Tx frequency +/- the offset. The two of you will be using two slightly different frequencies but at least they will remain constant instead of walking up/down the band.

The "walking" problem only occurs when one or both of the stations has a Rx/Tx offset. This can be caused by the radio or by some sound cards. When both stations have their Rx and Tx frequencies exactly aligned then there will be no "walking" problem regardless of what you do with the LOCK control.
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Bob  AA4PB
Garrisonville, VA
KF6IIU
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Posts: 0




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« Reply #5 on: March 17, 2008, 02:07:31 PM »

I used to work PSK31 with my old FT-101E. It actually used separate circuit components to set receive and transmit frequency. You could never get it exactly in line, and the RIT was always on.

Using Digipan, my TX Lock and AFC were always on, and I would tweak the RIT a tiny bit one way or the other after each QSO to get it as close as I could.

I made a lot of contacts that way, and something like 20 of my DXCC countries were bagged with that contraption.
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WB0AXN
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Posts: 90




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« Reply #6 on: March 17, 2008, 02:35:37 PM »

KF6IIU,

I see you found a way to use Digipan successfully.
As you noticed,  I'm new at this Digital mode, been at it for a week.  I really enjoyed Digipan and hope to continue using it.  Just for the heck of it,  I downloaded HRD+DM780 and after I got it set up, I have made nine contacts....with no complaints from anyone about frequency shifting.  Not sure if this means anything, but while receiving a station, I notice the pointer thingy moves slightly back and forth as need to stay exactly in the center of the signal.  
However, my problem is....DM780 is more program than I need or like.  After reading thru the help info, I believe it is primarily for controlling the rig and many other things.  I am not set up for that.
Another thing I have noticed, not only with stations I've contacted, but many of those where I read their transmissions is.....about eight out of ten uses were using MixW software.  It has been suggested I download a trial version and use it for 15 days.  After I read up on what it does, I may do the trial.

One ham suggested I try out EasyPal, so I checked their web site.  Picture quality is fantastic, compared to another SSTV program I tried six years ago.


73,
Roland
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AA4PB
Member

Posts: 15067




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« Reply #7 on: March 17, 2008, 04:00:26 PM »

You'll find the differences in software is features like what different modes it will work, will it control the radio, and how many stations it can copy at one time. There isn't much, if any, difference in their ability to copy PSK31 or their stability. In fact, many of the programs use the same PSK Core software to do the encoding/decoding so there can't possibly be any performance difference between those. Everybody has their personal idea about what is the "best" software but as far as performance DigiPan is as good as any of them. Because it doesn't have all the features of some others, it is a whole lot simpler to use.

The sound card reference oscillator is crystal controlled so if there is any drift it is most likely the radio. It is normal for the indicator to move back and forth a little as the AFC attempts to track the the received frequency through the noise level.

There are sound cards that have a separate reference oscillator for incoming and outgoing sides and those can indeed cause a difference between Rx and Tx frequencies. The difference will remain constant however - they won't "drift".
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Bob  AA4PB
Garrisonville, VA
WB0AXN
Member

Posts: 90




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« Reply #8 on: April 08, 2008, 10:11:23 AM »

Hi everyone,

On March 14th,  I posted the following message:

>>>Made my first contact with PSK31 this morning and a problem occurred.
I sent out a CQ and a station called me back. His text was all garbled and then I noticed his transmitting frequency was a hair off from mine. So, I moved my pointer on top of his transmission and the test was perfect.
However, on his next transmission, he said he missed half of my transmission because he had to retune. We continued the QSO with me shifting my pointer back and forth between transmissions. <<<<

I have been using other software and don't have a frequency shift problem.   A local ham friend asked me to have a QSO with him....using Digipan, so he could see if my frequency was shifting.   Sure enough, it was....but he didn't have an answer either.

Well, guess what!!! I re-read the help topics and found the following:

>>>>Sample rate If there always seems to be an offset between the received frequency and the transmitted frequency, try different values in PPM for TX or RX to minimize the offset. Typical values range from 0 to about 7000 ppm.<<<<<

So, now I will use Digipan and start entering different values....hoping I will find one or more that will solve the frequency shift problem.
Wish me luck...!!!
Oh, if anyone here has done this, let me know if it worked...and what number I should start at.

Roland (WB0AXN)


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