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Author Topic: Use of filter on PSK?  (Read 4874 times)
K4EZD
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Posts: 90




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« on: December 09, 2012, 10:24:05 AM »

My rig has filters for ssb and cw but I was wondering if it would be possible to use a filter on PSK when there are strong adjacent signals that are covering yours. At times some signals wipe out most of the band and make your transmission not able to be copied.
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WN2C
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« Reply #1 on: December 09, 2012, 10:40:53 AM »

There is a way but you have to use both vfo's working split. One set for usb for xmit and the other to cw for rcv. It may also depend on the rig you are using if it will work that way. I think most modern rigs will. I found some thing on the ts-2000 yahoo groups that explains it. Haven't tried it yet but it looks like it would work w/ my 2000. If you have a good email listed I will send it to you.

73 de wn2c  Rick
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AA4PB
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« Reply #2 on: December 09, 2012, 10:57:46 AM »

Rigs like the IC756PRO that have IF DSP filters can be set to place a narrow SSB filter (as narrow as 50Hz) in the center of the waterfall (1500Hz). You can then tune the VFO to place the desired signal at 1500Hz in order to reduce all of the adjacent signals. No need for working split with two VFOs if you have a radio with IF DSP.

Other options (not using narrow filters) is to use passband tuning, IF shift, or a tunable notch filter to reduce the strength of one or a few unwanted signals.
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STAYVERTICAL
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« Reply #3 on: December 09, 2012, 12:53:19 PM »

On my TS430S without any narrow filters I use the I.F. shift and tuning to remove those pesky varmints.

If your rig has I.F. shift, try it and see how far it shifts the passband.
You can test this by watching the waterfall display as you move the I.F. shift knob.

Lets say, like on the TS430S it will shift it about 1.5Khz.
Then you adjust your main tuning to bring your signal close to the cutoff point, and hopefully attenuate the strong adjacent signal.
It is a bit of manipulation, but if you don't have other filters, it works fine.
This system is also useful in narrowing the passband when working wider digital modes, such as olivia 32/1000 for example.
This improves the signal to noise ratio, since the passband is now narrower.

Good luck, 73 - Rob
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N4CR
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« Reply #4 on: December 10, 2012, 03:55:25 AM »

My rig has filters for ssb and cw but I was wondering if it would be possible to use a filter on PSK when there are strong adjacent signals that are covering yours. At times some signals wipe out most of the band and make your transmission not able to be copied.


Yes, you can use filters, but a wide signal is a wide signal. You can't make what the other guy transmitted any cleaner than what it is.

When you see a signal that is obliterating half the waterfall, you can't filter that out.

Now, if your rig is suffering from front end overload, you can do a few things.

1. Turn off any preamp.
2. Turn down the RF gain or turn on an attenuator.
3. Turn on your AGC. (probably already on)
4. Turn on a filter

When I used an Yaesu 857D, I was constantly battling front end overload. I added a narrow 300hz CW filter and that helped a lot.

After enough frustration with that radio on receive, I got an IC-746Pro. With this radio, I can turn the filtering down to 50hz which is just wide enough to slide a PSK signal through.

Any radio that does IF DSP should allow you to change the filter widths and slopes. That can give you any size filter you need to adjust your passband and eliminate everything outside of what you are trying to hear.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
AA4PB
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« Reply #5 on: December 10, 2012, 05:17:34 AM »

Normally, it's not "front end overload" that you experience with PSK31. It's just the normal operation of the AGC circuit which reduces the receiver gain in order to reduce the strength of any strong signal(s) in the IF filter passband. For example, if you have a 2.8KHz IF filter selected ANY strong signal in that passband will cause the AGC circuit to reduce the gain of the receiver which will reduce the strength of the strong signal along with all the other signals in the passband.

In this case, using a narrow IF filter allows you to place the desired signal inside the passband and the unwanted strong signal outside the passband. The filter then reduces the strength of only the unwanted signal so that it does not impact the AGC and the overall receiver gain. The desired signal (inside the filter passband) comes through full strength.
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W8JX
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« Reply #6 on: December 10, 2012, 07:37:26 AM »

My rig has filters for ssb and cw but I was wondering if it would be possible to use a filter on PSK when there are strong adjacent signals that are covering yours. At times some signals wipe out most of the band and make your transmission not able to be copied.


You did not mention what rig you were using but basically the answer is yes. I do it all the time. I normally use a 500 hz CW filter. Some will tell you not to bother and the use the "magic" of the waterfall  software and simply select the signal you want. Sadly though this is mostly a pipe dream as signals are usually of widely varying levels in strength. This is why some "complain" about others maybe running too much power because the front end of their receiver is too weak/wide to reject strong digi signals close together. I tried a narrower 270hz filter which worked okay for some narrow modes but it was too tight for RTTY and Olivia and Domino so I settled on 500hz mentioned above.  Simply using IF shift is not a reliable fix as you are just moving bandpass "window" around and when may let another signal in when you push one out.
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AC4RD
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« Reply #7 on: December 10, 2012, 09:14:40 AM »

Rigs like the IC756PRO that have IF DSP filters can be set to place a narrow SSB filter (as narrow as 50Hz) in the center of the waterfall (1500Hz). You can then tune the VFO to place the desired signal at 1500Hz in order to reduce all of the adjacent signals. No need for working split with two VFOs if you have a radio with IF DSP.

Other options (not using narrow filters) is to use passband tuning, IF shift, or a tunable notch filter to reduce the strength of one or a few unwanted signals.

I'm using all of those with an FT-950 for PSK and RTTY; works great!   Narrow filtering and notch filters can pull good QSOs out amazingly!
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KT4EP
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« Reply #8 on: December 12, 2012, 07:10:36 AM »

Sometimes I use the ATT on my Yaesu 840, but its easier to turn the volume down on whatever you are using to receive and process the signals, particularly when someone is way over-driving their signal or using a beam or something and have a super strong signal.   

Believe it or not, I talked to a fellow in northwest Canada who was xmitting 500 watts psk31, he said?  weird
KT4EP
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WA7NCL
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« Reply #9 on: December 20, 2012, 01:15:27 PM »

I can use the CW filtering on my TS870 down to 50Hz on PSK31.  The bandwidth of PSK31 is very narrow.  I have to use RX TX split because there is an 800 Hz shift for the CW beat note.  I use some software I wrote to provide the automatic offset and mode change (USB on TX CW on RX) for PSK31.  Works great and i can copy signals down to the noise level.

Some more modern DSP rigs allow narrowing the passband on SSB RX down to very narrow values.  Then you don't need the offset and mode change.

This is the best way to do PSK31.  With narrow RX filtering you will stop being one of those grumpy old guys complaining about excessive power.
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W2AEW
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« Reply #10 on: December 23, 2012, 02:28:42 PM »

There is a way but you have to use both vfo's working split. One set for usb for xmit and the other to cw for rcv. It may also depend on the rig you are using if it will work that way. I think most modern rigs will. I found some thing on the ts-2000 yahoo groups that explains it. Haven't tried it yet but it looks like it would work w/ my 2000. If you have a good email listed I will send it to you.

73 de wn2c  Rick

I described this method in a short Hints & Kinks contribution to QST in 2003 - here's a link to a copy of that:
http://www.qsl.net/w2aew/psk31filterhk.html
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N1ZZZ
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Posts: 160




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« Reply #11 on: December 26, 2012, 03:34:40 AM »

I have 2 Icom radios, a 746, and a 706.  I have a 450 Hz filter in the 746 and a 500 Hz filter in the 706.  To use them in SSB modes, I tell the radio that it's a wider SSB filter so it allows me to switch the filter on in SSB mode.  I then use the IF passband to "move" the filter up and down my waterfall.  It works very well to reduce strong adjacent signals in RTTY and other narrow digital modes.

73
Jeremy N1ZZZ
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W6UV
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Posts: 536




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« Reply #12 on: January 02, 2013, 12:02:55 AM »

Not only is it possible, but it's very desirable to use narrow filters for PSK. Would you try operating in a crowded CW environment (during a major contest, for example) with a 2.8 kHz filter? I certainly wouldn't.

Many digital applications have a button to move your VFO so the PSK signal you're monitoring is tuned right in the middle of the passband (typically 1500 Hz). Hit the button and then enable the narrow filter (I use 150 Hz for PSK31) and lots of issues, such as AGC reducing your receiver's gain go away.

People using PSK with their receiver "wide open" are the primary source of the complaints you hear about how others are running way too much power on digital modes and that we all should use 25 watts (or some such), when it's not a power issue but a receiver bandwidth issue.
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W8JX
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« Reply #13 on: January 02, 2013, 05:22:28 AM »

People using PSK with their receiver "wide open" are the primary source of the complaints you hear about how others are running way too much power on digital modes and that we all should use 25 watts (or some such), when it's not a power issue but a receiver bandwidth issue.

Well said. I have always used a filter when available and the "steer" the filter bandpass around waterfall with IF shift.
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