I would like to know if some kind of feedback (maybe complicated) could bring about a quantum leap in linearity, like sampling the output of the amp and feeding that back to the K3 to digitally pre-alter the audio to cancel the distortion going out.
On Thu, May 12, 2011 at 9:12 AM, Geoffrey Mackenzie-Kennedy <[email protected]> wrote: > Hello Wes, > > The practical question "how much non-linearity is acceptable" in commercial > HF systems has been debated by some Regulators and their advisers from > industry for years - certainly since the early 1960s in my experience. > > In the context of linear amplifiers (or complete transmitters) designed for > commercial single channel voice SSB, their linearity (or lack of) is usually > expressed in terms of occupied bandwidth - from which the acceptable levels > of odd order intermodulation products can be determined, which in turn leads > to the commonly used specification limit for 3rd Order Intermodulation > products. Even back in the 1960s the Type Approval specification limit for > 3rd Order products was typically 30db below test tone for HF transmitters > rated up to and including 1 kW PEP output. This spec had to be met over the > supply voltage range as specified by the manufacturer, together when driving > a RF load whose impedance was not only fixed at nominal (e.g. 50 +j0) but > also at other values which escape me. > > For some reason this magic number of 30db appears to have remained as part > of our ham folklore, worse yet 30db below PEP not test tone, and IMHO needs > to be increased now that the bands are more crowded. Driving a so-so > "linear" with a transceiver whose IMD performance is poor is a recipe of > course for wideband crud, and there are many crud generators here in Europe. > > Whereas a plot of output power vs input power will give some idea of > linearity, in particular when an amplifier enters compression, unfortunately > it will not expose any "misbehaviour" of the level of IMD products at lower > output levels even if the plot appears to be "linear" at these levels. > > To give a SSB rig a clean bill of health you really do need to perform > two-tone tests at vaious power output levels, which I suspect Elecraft has > done. > > 73, > > Geoff > GM4ESD > > > > > > > > > > > > > > > > ----- Original Message ----- > From: "Wes Stewart" <[email protected]> > To: <[email protected]>; "G4ILOJulian" <[email protected]> > Sent: Wednesday, May 11, 2011 8:02 PM > Subject: Re: [Elecraft] KPA-500 "Output for Input" schedule? > > >> Then by definition, they aren't linear. >> >> A graph of output power vs. input power should be a straight line. It it >> isn't then it's non-linear. The practical question is how much >> non-linearity is acceptable. >> >> --- On Wed, 5/11/11, Julian, G4ILO <[email protected]> wrote: >> >>> >>> If it does that then it will be the first linear amplifier >>> I have ever seen >>> that actually keeps that relationship across the whole of >>> its range. >>> >>> Most solid state amplifiers exhibit noticeable compression >>> as you increase >>> the input power, so you get a lot more than half the rated >>> power out for >>> half the specified drive power. >>> >>> Take a look at the figures for this Mirage amp (selected at >>> random as being >>> the first spec sheet I could find that proved the point.) >>> It is only truly >>> linear up to half its rated output. 10W in gives the rated >>> 160W output, but >>> 5W gives you 150W 2.5W would give you somewhere around >>> 120W. The VHF amp I >>> have here has a similar characteristic. Although it is >>> designed to be driven >>> with 5W input I am actually driving it with the K3/K144XV >>> with less than >>> half that power and still get virtually full output. >>> >>> So I think the OP actually asked a very reasonable >>> question. >>> >> ______________________________________________________________ >> Elecraft mailing list >> Home: http://mailman.qth.net/mailman/listinfo/elecraft >> Help: http://mailman.qth.net/mmfaq.htm >> Post: mailto:[email protected] >> >> This list hosted by: http://www.qsl.net >> Please help support this email list: http://www.qsl.net/donate.html > > > -------------------------------------------------------------------------------- > > > > No virus found in this incoming message. > Checked by AVG - www.avg.com > Version: 9.0.901 / Virus Database: 271.1.1/3632 - Release Date: 05/11/11 > 20:12:00 > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
