[Elecraft] KPA-500 "Output for Input" schedule?

Thu May 12 09:12:23 EDT 2011

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" <n7ws at yahoo.com>
To: <elecraft at mailman.qth.net>; "G4ILOJulian" <julian.g4ilo at gmail.com>
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 <julian.g4ilo at gmail.com> 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.
>>
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