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Author Topic: What power for PSK31  (Read 7149 times)
VA3KBC
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« on: April 29, 2012, 05:30:04 AM »

I was wondering what most run for power when operating PSK31?

I have run from 25 to 80 watts.

Ts590 running Digital Master 780

VA3KBC

Don
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KX5JT
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« Reply #1 on: April 29, 2012, 05:54:09 AM »

I think 25 watts or less is what most people run from my experience and watching brag files.  Only very marginal condx would require more to make a contact and then you'd be better off with a more robust mode like OLIVIA. 

There is a popular MISCONCEPTION that running a bit of power splatters.  I know I'm about to open up a can of worms here, but the truth is that if your signal is CLEAN (i.e. no ALC kicking in from too much audio) then you CAN run a little power without splattering.

Think of it this way.... station A is running a clean 10 watts into a dipole on 20 meters. There's the signal on your waterfall and it's average.  Then station A switches to a 4 element yagi pointing your direction.  He just gains 12 db (or whatever) and the signal becomes much brighter.  Given the SAME audio setup for that station, do you think he suddenly splatters the waterfall? NO HE DOESN'T. 

So upping the GAIN either through more watts or a HIGH GAIN antenna does not cause splatter.  TOO MUCH MODULATION (i.e. audio) no matter WHAT the signal strength is what causes splatter.

All that being said, rarely is a lot of GAIN actually needed except when conditions are fading and marginal.

John KX5JT
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VA3KBC
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« Reply #2 on: April 29, 2012, 08:29:51 AM »

Thanks John.
I leave my power on the radio at 80 watts and set the soundcard for 25 or so.

VA3KBC
Don
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AG6WT
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« Reply #3 on: April 29, 2012, 10:03:03 AM »

I was wondering what most run for power when operating PSK31?

I have run from 25 to 80 watts.

Ts590 running Digital Master 780

VA3KBC

Don

I set my rig's power setting to 100 watts (the max level) then I adjust the modulation on my Signalink so that the power as read on an external meter is 20-30 watts. When the conditions are poor and the band is not crowded I can increase the modulation so that the external meter reads +60 watts without any ALC. I don't have an IMD meter but others have commented that they don't see splatter in my signal even at that high output.

A common misconception is that since PSK31 doesn't require alot of power to make contacts, you need to turn down the power setting on your rig. This is wrong because if you do this you are more likely to splatter! Instead, set your rig to max power and adjust output with the sound card level. This will keep your signal clean at higher power.
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WN2C
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« Reply #4 on: April 29, 2012, 11:23:24 AM »

What KJ6AMF said plus if you don't keep the drive level of the audio down to the radio so it is running about 25 to 30 watts out, then you may burn up the finals on a long key down. If you don't believe it then see how long it takes for the fan to come on running 80 watts vs 25.
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AA4PB
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« Reply #5 on: April 29, 2012, 11:50:30 AM »

High power does NOT necessarily equate to "splattering" if things are adjusted correctly. However, on the receive end any strong signal in the waterfall will activate the receiver AGC and reduce the sensitivity causing weaker signals to disappear. For this reason it is generally best to run a reasonable 25-30W of power output and be a good neighbor. On the other hand, it is quite possible for a 5W station to be the strongest signal on the band if the propogation is right.

Using a narrow IF filter in the receiver is a great way to reduce the strength of unwanted signals on the band - if your receiver has that capability.
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N4UM
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« Reply #6 on: April 29, 2012, 12:12:30 PM »

What AA4PB said... plus...why operate using a mode with a bandwidth of 31 Hz and setting the passband on your receiver at 3000 Hz?  That's what's often happening when another station complains about how your signal is "splattering all over the band" and messing up his QSO 2 Khz away.  Not so, if your signal is clean, what he's really telling you is that he doesn't know enough to crank in some i.f. filtering.  I understand that most stations run 20-30 watts on psk31 and that's fine, as pointed out by AA4PB, but there are times when 20-30 is more than necessary and times when it's not enough.   Switching to Olivia or MFSK is usually preferable to cranking up the power and staying on psk31.  The only problem with that is that good operating practice would suggest that you QSY out of the area where other stations are operating psk31 since the bandwidth of the more robust modes is usually greater.
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W8JX
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« Reply #7 on: April 29, 2012, 12:15:51 PM »

There will be those that will try to tell you that you "must" use low power to support their wide waterfall theory with all signals being same. This pipe dream is not reality as propagation and antenna types can cause a 10 watts signal to be 10 or 20 over 9 and 100 watts signal far less. Use the power you feel you need to use for conditions and also consider the heating of rig at that power level with extended key down modes like PSK. A 100 watt signal can be as clean as a 5 watt one. Do not let someone try to tell you that you are responsible for someones weak receiver that lacks ability to reject strong signals in band pass that can swamp AGC too.  That is not your problem. I use a 270 hz xtal filter in digi modes here and run 10 to 50 watts depending on conditions.
« Last Edit: April 29, 2012, 03:32:20 PM by W8JX » Logged

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KG6AF
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« Reply #8 on: April 29, 2012, 01:07:08 PM »

High power does NOT necessarily equate to "splattering" if things are adjusted correctly. However, on the receive end any strong signal in the waterfall will activate the receiver AGC and reduce the sensitivity causing weaker signals to disappear. For this reason it is generally best to run a reasonable 25-30W of power output and be a good neighbor. On the other hand, it is quite possible for a 5W station to be the strongest signal on the band if the propogation is right.

Using a narrow IF filter in the receiver is a great way to reduce the strength of unwanted signals on the band - if your receiver has that capability.


Yes, exactly.  PSK31 operators need to differentiate between a transmitter that's overdriven (i.e., too high a transmit audio level) and a clean signal that happens to be loud for any of a number of reasons: high power, great antenna, good propagation, etc.  The former is a problem that should be solved by the person transmitting (turn down the audio gain!), while the latter should be solved by the person on the receiving end (by using a narrow IF filter).

Confusing the two types of signal--overdriven vs. loud--just leads to a lot of pointless discussions.
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VA3KBC
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« Reply #9 on: April 29, 2012, 05:32:07 PM »

The default on my DM780 was 50 on tx mute.
When I went down to 25 the transmit would not work. I set it up over 50 and it would work.
Guess thats where I leave it.

I ran a test cq and ran 75 watts on the sound card and my qso said I wasn't splattering and had a nice signal.

VA3KBC
Don
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WA7KPK
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« Reply #10 on: May 01, 2012, 11:34:47 AM »

I just leave my signal at 30 watts regardless of conditions, with one exception: I had a QSO one time with a guy about a mile south of me who asked me to turn down the power because I was overloading his receiver. I've got things adjusted to where people can decode my signal and I don't want to mess with it any more than I have to.

I could probably get away with lower power. Someday I may spend a Saturday running 5 watts just to see what happens. I'd volunteer to go lower but that's as low as my FT-857 will go.
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G0GQK
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« Reply #11 on: May 02, 2012, 01:40:55 PM »

When PSK31 was first invented, it was said that there would be no need to run more than 10 watts to have a worldwide transmission, but that was when the bands were generally in a better condition. This was designed a narrow band DX medium for those unable to have a decent antenna, and probably an attic antenna.

I use about 25 watts as indicated on a power meter, remember the power indicated is doubled, and those using yagi antenna's are pushing quite a substantial signal. There are many distorted signals around caused by using excess power. Originally a signal trace was regarded as good when it had the shape of an anchor chain. How many of those do you see these days, like an 8 vertically ?

Mel G0GQK
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KX5JT
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« Reply #12 on: May 03, 2012, 06:58:46 PM »

There are many distorted signals around caused by using excess power.


This is a total misconception.  Excess AUDIO DRIVE causes distortion. 
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STAYVERTICAL
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« Reply #13 on: May 08, 2012, 02:38:44 PM »

Hi Don,

It's a slow day in the office, so I will put my hat in the ring.

PSK31 is a mode which requires a linear transmit chain to radiate a clean signal.
Unlike modes like RTTY and CW which can be run on non-linear class C power amplifiers, PSK31 is more like AM.

Like AM, the modulation is by two audio tones, shaped in both rise and fall times to emulate a cosine switchover to reduce transients and "key clicks".

This means you need to avoid modifying the waveform in the audio and RF sections of your transmitter.
Overdriving the audio or RF stages will lead to clipping or distortion and this always leads to extra frequencies being transmitted.

The actual power you radiate is technically irrelevant as long as it is a linear transmit chain.

The problems that appear on PSK31 sections of the band, are usually about strong signals, not dirty signals.
Dirty signals certainly exist, but they are generally handled quickly by the op, once they are informed by others.
In many cases, the dirty signal is caused by too much audio drive to the transmitter, which is cured by simply winding back the audio.
In other cases, the audio compressor is activated, or the ALC (which is a type of RF compressor) is the culprit.
Remember, anything which changes the modulated signal from the original PSK31, results in extra sidebands.

Clean, but strong, signals are another matter.
As others have said, sometimes strong signals are qrp with great propagation, other times they are guys who run 500 watts
into a 7 element yagi on PSK31.

The propagation led strong signals change in time, but the big power guys usually stay the same.
The result is like when someone puts an annoying beacon right in the middle of a DX band, they are generally a nuisance.
But, changing peoples behaviour is often a lost cause, so the solution is to adapt.

Many PSK31 ops use the "browser" type of waterfall displays common on digimode programs.
They enable ops to see around 2.5Khz of spectrum, and will decode signals within this range, directly on the screen.
While most signals are around the same strength, the display and decoding goes smoothly.
But, when one signal is extremely strong - the problems begin.

First, because your receiver is viewing 2.5Khz of bandwidth, its AGC (Automatic Gain Control) will reduce the gain of
its I.F. stages to keep the signal constant.
But the weaker signals will now fade away, or disappear completely.
If you were working a weak signal, this is very disconcerting, hence the vehemence directed at the strong signal.

However, this is not justified, since if you were using CW, it would be natural to use narrow I.F. filters in normal operation.
The solution, like in CW, is to use narrow I.F. filters in PSK31, and you will not be affected by the supersize signal,
unless he is right on top of your contact.

In summary:

- ALC is a means of keeping the transmitter within a power limit.
- If you see ALC movement on PSK31, it means the transceiver is modifying itself to restrict power.
- Depending on the design of the ALC, this may mean non linearity entering the TX chain, distorting the signal.
- If you have a menu which sets power, frequently ALC is used to limit output to this power.
  This is why many ops recommend setting the power to 100W, to stop ALC being invoked by default.
- High output power by itself does not mean a dirty signal.

- The oft quoted "IMD" (InterModulation Distortion), is frequently measured incorrectly by receiving stations.
- To measure IMD on a received signal, what you are measuring is the ratio of the main signal to it's 3rd order sidebands.
- These 3rd order sidebands are adjacent to the main "railroad track" signals and make the signal appear wider.
- To measure IMD, it is necesary to have an "idle" signal (no typing - just railroad tracks), enough signal to noise ratio
   so that you are not measuring noise as 3rd order sidebands, and a program which is known to be accurate.
- An IMD of -30dB is considered very good, but -25dB is acceptable as well.
- Basically, the lower the IMD figure, the stronger 3rd order (and higher) sidebands are in your transmitted signal,
  so the wider your signal is, and the more you interfere with others.

- Use the "browser" 2.5Khz wide option on digimodes programs by all means, but realize this is not going to work at all times.
- When a "big un" appears on the waterfall, switch to a narrow filter and he will disappear, leaving you in peace.

Well, back to work,

73s
« Last Edit: May 08, 2012, 02:42:11 PM by STAYVERTICAL » Logged
AA4PB
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« Reply #14 on: May 08, 2012, 05:43:59 PM »

Good explanation. I did some bench testing here and found that the receiver AGC can distort the received signal in the same way that the transmitter ALC can. With a test signal set to an S-5 meter reading I would get a significant change in the waterfall display and the IMD reading when I change the AGC between fast, medium, and slow. The faster the AGC response, the worse the IMD reading. The lesson here is to be careful about giving out poor IMD reports. If most of the signals on the band have poor IMD then it just might be your receiver AGC. Poor IMD can be cause by distortion anywhere in the signal chain from the transmit station's sound card to the receiving station's sound card.

This lead me to do some experiments with the ALC timing on my 10W K2. I found that by adding a capacitor to slow the ALC decay time I could make it so that ALC action didn't distort the PSK31 signal. The received IMD reading was the same with full ALC reading as it was with no ALC reading. With the normal SSB ALC timing I could actually see a copy of the PSK31 signal on the ALC line with a scope as the level was following the signal. Adjusting the ALC timing on the K2 for PSK31 allowed it to provide automatic output adjustment so that my power output was exactly the same regardless of the band or the position of my PSK31 signal within the IF filter. Without ALC the power output would vary as I move around the waterfall due to ripple in the IF filter or gain changes on different bands - I was constantly adjusting the audio drive level to maintain a clean signal with a decent output power.
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