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Author Topic: PSK with 30L-1?  (Read 12031 times)

Posts: 12501

« Reply #30 on: June 18, 2012, 02:18:26 PM »

While it could possibly be argued that using an amp to compensate for a poor aerial system is in some sense inelegant, it is not in fact an unreasonable strategy even for PSK or the like. Obviously the amp must be clean, but that really should be the objective anyway.

For PSK mode, I must disagree with this.  Using an amp IS an  unreasonable strategy for PSK for a variety of reasons.  The most important being that IMD in PSK mode isn't some imaginary thing that may happen or not.  Even when unmodulated, an idling PSK signal with less than excellent IMD has sidebands that will interfere with adjacent stations.  Often, we hear someone use the term "clean" as you have here applied it to amplifiers.  It might surprise many to learn that some common and expensive amplifiers have IMD ratings that aren't exactly excellent.  For example, the Acom 2000 (IMD rated at 31 dB), Icom PW1 (IMD rated at 29 dB).  Would you say those are "clean" amplifiers in terms of IMD?  Using an amplifier for PSK mode is just a bad idea, even with a poor antenna.  

73, Floyd - K8AC

I really do not think that a amp is going to cause a IMD problem unless it is improperly driven or loaded.  (BTW the IMD figures you gave for amp was at rated output, not reduced output) PSK is not that sensitive. As to whether amp is needed is debatable but there are times when even a good antenna is not enough for poor conditions. Personally I do not do much PSK myself. I prefer other digital modes.

Ham since 1969....  Old School 20wpm REAL Extra Class..

Posts: 1757

« Reply #31 on: June 18, 2012, 05:04:01 PM »

The convention to run 25-35 watts comes from not over driving any of the stages in our typical 100 watt radios into a non-linear state. Every one of these stages are Linear Amplifiers, no doubt. Having them built into the radio doesn't make them magically not Linear Amplifiers.

Over driving any amplifier will increase IMD. If you use an IMD monitor and are careful to not overdrive any stage, there is nothing about high power that by itself creates excessive IMD.

With that said, I've got over 2000 PSK-31 qso's in the log. High power is not the answer. Good antennas, receivers that don't overload easily and with IF DSP carry my load. Transmitting is easy. Hearing the other guy is the hard part and no amount of output amplification will fix that.

A dirty signal on the band ruins the enjoyment for everyone on that segment of that band and makes it impossible for anyone to copy the weak signals. Please refrain from any activity that might create that kind of mess whether it is from over driving your microphone input or any other stage.

73 de N4CR, Phil

Never believe an atom. They make up everything.

Posts: 1816

« Reply #32 on: June 19, 2012, 04:25:01 AM »

The problem here is that PSK is a special case when it comes to IMD.  The two tones are there all the time and the IMD products that result are there all the time, unlike with SSB, where the products will be fleetingly scattered around.  The fact that an amplifier is "linear" doesn't mean it doesn't have ANY IMD.  If the figures I quoted for those two amplifiers resulted only from overdriving the amps, do you really think that those are the numbers that would be published?  Those are BEST CASE numbers. 

I agree that the best approach is to use some sort of PSK meter to measure the IMD distortion and adjust for minimum IMD.  If everyone did that, they'd have a better understanding of the fact that IMD doesn't just result from overdriving an amplifier somewhere in the chain.  And, accepting the best IMD you can achieve with your setup may not be good enough.  If the best you can do is -17 dB, is that good enough?  Examining the waterfall trace of a signal with that amount of IMD says NO. 

I'm amazed that guys spend thousands of dollars to upgrade to the transceivers that top Sherwood's list (which is sorted ONLY on IMD for two close-spaced tones), and then put out a PSK signal with extremely poor IMD.  Most guys wouldn't recognize an IMD problem in the receiver if they heard it, but everyone on the PSK portion of the band is effected by a signal with high IMD coming from the same rig. 

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