[Amps] Fwd: Re: Fwd: Linear Amplifier Tuning---PROPERLY!

[jeff millar]

Thanks, Gary...  I'm trying to be helpful.

On the subject of harmonics vs 2xF1.  When I mentioned harmonics before, it 
wasn't as
clear as could be.  I'm not referring to harmonics at the amplifier output, I'm 
referring to
harmonics at the _device_, before the output network.  The plate, collector or 
drain
waveform is _not_ a sine wave for any amplifier other than class A.  The clipped
plate/collector/drain waveform generates a lot of harmonics at the device that the
output matching network filters out to make the clean RF sine wave sent to the
antenna.

The device is switching on and off (if not class A) and generally acting 
non-linear, so in
addition to generating harmonics, it also mixes all the harmonic products and 
generates
all those inter-modulation frequencies.

The output matching network puts the correct impedance on the device for best
performance, and whether that makes a conjugate match, load line match, or whatever
  is not at issue here.  What matters is, What does the output matching network 
present
as an impedance at the 2nd harmonic, 3rd harmonic, etc?  Tube Pi-Networks have a
capacitor across the plate terminals and present a low impedance for the higher
harmonics.  This has some effect on the phase of the 2nd harmonic currents running
around the tube's output network.  At the high harmonics, the parasitic 
inductor, stray
wiring may presents a higher impedance to the plate which again changes the phase
of the harmonics currents.

The output 3rd order IMD appears at 2xF1 +/- 1 x F1.  But that IMD also includes
components from 3 x F1 +/- 2 x F1, 4 x F1 +/- 3 x F1, etc and all those components
land on the exact same frequency as the 3rd order components.  When two RF signals
land on the same frequency, they can add _in phase_ and make a larger signal or
add _out of phase_ and make a small signal.  Since the output network doesn't 
control
  the phases of the high harmonics reflected back to the device, it's impossible 
to predict
how all those components will add up.

VHF output matching networks tend to use transmission lines and those have 
resonances
at or near the higher harmonics...which tends to present a wide range of 
impedances to
the devices at those higher frequencies...resulting in even less predictability 
and control.

All the IMD mixing takes place at the non-linear device...namely the 
tube/transistor/FET.
The mixing depends on the phases and amplitudes of the all the currents running 
around
at the various harmonic frequencies of all the components that make up the modulated
signal.  The resulting mixing products that land near the fundamental are properly
matched by the output network and make it to the antenna.  But mixing products that
fall far from the fundamental, get filtered out.

As an aside, Class F amplifiers try to raise and control the impedance at higher 
harmonics
to make the output transistor switch more completely and improve efficiency.  
But in
general the impedance and phases of harmonic voltages and currents running around
the output network is uncontrolled.

jeff, wa1hco


On 11-12-27 01:16 PM, Gary Schafer wrote:
> Nice article Jeff! I got through part of it but it takes some digesting!
>
> I think that the kind of IM products that we are most concerned about in our
> amps is based on 2xF1 +- 1xF2 for 3rd order IM rather than the harmonics of
> the fundamental. This assumes two signals being amplified by the amplifier
> as in a two tone test. Of course things become a little more complicated as
> the number of tones increase.
> Of course we are also concerned with the higher orders of IM also such as
> 5th, 7th etc. as they all can mix with one another once the amp is operating
> in the non-linear mode caused by any order of IM products.
>
> I don't think that the phase of the output of the amp has much to do with IM
> with narrow band signals such as we use for SSB. When the bandwidth is much
> wider such as that used by cellular transmitter amplifiers maintaining a
> near constant phase match over the bandwidth seems to be more important.
>
> Our output matching network is not providing an impedance match to our
> amplifiers as the actual device (tube or transistor) output impedance is
> typically much lower than what our network is set for to obtain maximum
> output. We really do not provide an impedance match we just provide an
> optimum load for the tube to work into.
>
> <...>
>
> 73
> Gary  K4FMX
>
>