Re: FW: [FlexRadioSmartSDR] Adaptive Predistortion Progress
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Easy to sacrifice gain to get negative feedback (and better linearity) when power isn't an issue and there is excess gain available (e.g. low power stages)
Much more expensive and risky to add high power gain stage after high power gain stage trying to keep them at "low power" -- I don't know for sure but I worry that the phase shift at parasitic frequencies could become an issue and cause instability (would have to go back to 50 years ago to review all that....)
The available power amplification devices we have just aren't perfectly LINEAR, and we usually don't have huge gain around to throw away in that stage.....so tough to deal with.? ? I'm guessing it is easier if we run the corrections to an ealier?stage some how that doesn't risk oscillation.? ?Certainly not an expert here.?
But if the output becomes cleaner and we didn't have to spend a bazillion $$$, that's a good thing, right?
Gordon KX4Z
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More mumbling about APP¡ operating the amp at less than 25% of its average available output gives better IMD.? DUH. ? ? Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ VP2EHZ ? Owner - Operator Big Signal Ranch ¨C K9ZC Staunton, Illinois ? Owner ¨C Operator Villa Grand Piton ¨C J68HZ Soufriere, St. Lucia W.I. Rent it: ? Moderator: North American QRO Group at Groups.IO. Moderator: Amateur Radio Builders Group at Groups.IO. ? email:? bill@... ? ? ? In my recent talks about IMD, through observations of many amplifiers, most of which were Class A units, I came to the conclusion that runnunig 6 dB below the P1dB compression point kept the 3rd order product below 30 dB. Often much lower. Just a rule of thumb. ?There is at least one mfgr of amplifiers that rates them at 3kW (max, whatever that means) and gives good IMD performance at 1kW. Many hams solid state amplifiers are run at or close to their P1dB point on voice peaks.? ?Care has to be taken at all stages of analog design because every amplifier and mixer throughout a transceiver produces IMD. I believe that most modern radios are pretty clean up to the final amplifier where cost often dictates non-linear choices. All that said, producing -50 dBc or better IMD products before the power amplification requires detailed engineering of analog circuits. This low level is easily obtained and exceeded in a good SDRs of 14 or 16 bits RF sampling. Most readers of this already know that SDRs are good radios for many reasons. Clean signals is one.? ? ?The low level (0 dBm) 3rd order IMD output from my 6700 at 28 MHz was below -80 dBc. I say below because my two tone audio source measured about that level of IMD.? ? I'm not suggesting that we all run 6db down. Military applications often require better IMD performance due to Co-Site issues. Examples: shipboard operations (especially the big decks) may have many HF stations running with antennas close together. I'm just pointing out that observing signals on the Pan could be deceiving because of varying power levels.
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