[AMPS] IMD Question

[Steve Thompson]

-----Original Message-----
From: Steve Harrison <ko0u@os.com>
To: amps@contesting.com <amps@contesting.com>
Date: 14 February 2000 11:10
Subject: Re: [AMPS] IMD Question
>It is not commonly realized that "soft" compression is due to harmonic
>energy. Power meters cannot differentiate between the fundamental and
>harmonic power so the harmonic energy contributes to the total power
>measured. In addition, grounded-grid amplifiers feed through driver energy
>which is also measured.
>
>The only way to measure fundamental power without harmonic contribution is
>with a tuned receiver or spectrum analyzer.
>

If the spectrum analyser indicates that the output contains insignificant
harmonic levels, then a thermal power meter is fine. The characterisation is
of non-linearity in the fundamental frequency, whatever the cause might be
within the amplifier circuit.

On one amp I did a comparison between long hand measurements and the network
analyser on amplitude sweep. I got good agreement.

>>Many power mosfets have gain which drops
>>continuously with power (Po/Pin is banana shaped)
>
>If this is being observed with an untuned power meter, it is likely because
>MOSFETs and FETs readily generate copius amounts of odd-order harmonic
>energy, most notably 3rd and 5th harmonics.

I've not noticed that FETs generate more odd order harmonic than other
devices.

>
>>.. latest LDMOS has an 'S' shaped Po/Pi characteristic at most bias
levels.
>
>Likely the same problem: untuned power meter and harmonics being measured.


Or maybe not.

>
>It's also not commonly realized that spectrum analyzer measurements are
>notoriously inaccurate at amplitude measurement; even the latest Hewlett
>Packard instruments with digital readout of the cursor's location are
>specified to no better than about +/- 1 dB accuracy. This inaccuracy can be
>reduced considerably by comparing the power measured by the analyzer to a
>known, clean signal source level established by a power meter.


Agreed - although for the measurement in question, absolute amplitude
accuracy isn't essential; amplitude linearity at any given frequency is
needed, and typically a spectrum analyser isn't up to that job either.

>
>>Most bipolar amps are
>>dreadful because the particular transistor isn't suited to linear
operation
>>and/or the biassing arrangements are rubbish.
>
>Bipolar RF power transistors are built with and without emitter resistance
>which introduces negative feedback. The emitter resistance helps raise the
>real part of the base resistance for easier matching. Motorola called these
>devices "Controlled Q". A very-low-Q, low-pass network is usually used to
>connect the base junction to the base lead of the package and raises the
>base impedance further (in V/UHF transistors, this network is often the
>reason why the high frequency response drops drastically beyond the
>intended operational frequency range).


When I was making them, all rf power transistors had emitter resistance
built in to ensure current sharing and prevent hot spots and thermal
runaway, not for any matching function. As I understood it, and as Motorola
EB19 describes it, 'contolled Q' (or 'J zero' or whatever) and emitter
resistance are not deliberately interlinked.

Steve


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