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

[Carl]

Very excellent Jeff.

Having been deeply involved with both cell phone IC design as well as 
microwave to millimeter wave RF module design before retiring I got an 
eyefull and brain full of what was both required and possible with the multo 
channel data and voice world.

And then have to come home and climb the wall listening to garbage 
signals......Often from well known world class contesters and DXers who are 
regulars on many forums.

And then see a completely assinine review of the TE 2M amp in the latest 
QST. Its just another example of the ARRL kissing butt for the advertising 
dollar.
Garbage like that should be banned from QST as well as 2M or any other band 
such trash is sold on.

Carl
KM1H



----- Original Message ----- 
From: "jeff millar" <wa1hco at wa1hco.net>
To: <amps at contesting.com>
Sent: Saturday, December 24, 2011 12:21 PM
Subject: Re: [Amps] Fwd: Linear Amplifier Tuning---PROPERLY!


> Here's more about IMD than you may want to know...
>
> I used to work with very low IMD amplifiers for the cellular industry and 
> spent
> a lot of
> time studying IMD sources and means of control.  I learned the most about 
> the causes
> of IMD from a PhD thesis, and later book, by Joel Vuolevi...
>
>     http://herkules.oulu.fi/isbn9514265149/
>
> Most literature describes Intermodulation distortion as coming from 
> harmonics of
> the
> fundamental mixing with the main signal.  3rd order IMD means
>
>     2 x fundamental +/- 1 x fundamental.  (2+1 = 3rd order)
>
> But, the total contribution to the IMD also comes from 3 F +/- 2F, 4F +/- 
> 3F,
> etc up to
> the 9th harmonic and beyond.  All those products stack up on the same 
> frequency
> at the output.
>
> Each of these products has a phase and the resulting IMD depends on how 
> all the
> phases line up.  This can make the IMD level very sensitive to the test 
> signal,
> power level,
> temperature, bias, etc.  It also can make the high and low side IMD have
> different levels.
>
> The FCC recognizes this effect and mandates that RF transmitter tests uses 
> a
> random modulation to measure IMD, so the testers don't tweak the test to 
> find a
> sweet spot in IMD performance where some of the components cancel out.
>
> The level of harmonics and their phases in the output matching network 
> affects
> the total resulting IMD.  But it's nearly impossible to design for or 
> control
> all those
> effects.  Every matching and bias network presents a different impedance 
> at all
> those
> harmonics.
>
> Most UHF power transistors designed for very low IMD (-50 dB) have an 
> output match
> specification that is _NOT_ the same at the optimum power transfer match.
>
> So, the optimum IMD setting of the tune capacitor is _probably_ not peak 
> power
> output
> but there's not enough information available to set it...and it would 
> drift
> around anyway.
>
> The biggest contributor to IMD is the amplifier going into compression on
> peaks.  This
> produces a nonlinear input vs output response and those non-linearities 
> produce the
> harmonic mixing effects that create IMD.
>
> The shape of the compression curve affects the level of IMD.  A curve with 
> a
> sharp break
> has more IMD than a more gradual curve.  As a guess, a more heavily load 
> amplifier
> will have a softer compression curve and produce less IMD when driven into
> compression.
> A heavily loaded amplifier limits on current and a lightly loaded amp 
> limits on
> voltage swing.
> Intuitively, voltage clipping seems "sharper" than current limiting.
>
> The cellular industry implements predistortion of the input signal to 
> compensate for
> amplifier compression.  The amplifier compression curve bends down, so the
> predistortion
> circuit bends the the gain curve up to compensate.
>
> The future of amplifiers for _should_ include an IMD monitoring circuit 
> and
> predistortion.
> This would produce significantly more power with a smaller and more 
> efficient
> amplifier.
> IMD levels should be set to about -40 to -50 dB. This would have to apply 
> to both
> transceivers and amplifiers, because much of the IMD these days comes from 
> the
> crummy amplifiers in the transceiver.
>
> Crowded bands would seem completely different if everybody's IMD dropped 
> 10-20 dB.
>
> jeff, wa1hco
>
> ________________________________________
> From: amps-bounces at contesting.com [amps-bounces at contesting.com] On Behalf 
> Of TexasRF at aol.com [TexasRF at aol.com]
> Sent: Wednesday, December 21, 2011 8:12 PM
> To: deswynar at xplornet.ca; donroden at hiwaay.net
> Cc: amps at contesting.com
> Subject: Re: [Amps] Fwd: Linear Amplifier Tuning---PROPERLY!
>
> Hi All, it makes no sense to not peak the tune C as the last step in a
> matching procedure.
>
> The tuning network is fully adjustable, allowing a match from your nominal
> 50 ohm load to what ever plate load impedance you want to use. If the tube
> needs  2000 ohms for best efficiency and power output, it is a simple 
> matter
> to provide  that. But, the transformed impedance is resistive only when 
> the
> complete network  is resonant. The only way that can happen is when the
> plate tune C is adjusted  last, for maximum power transfer. The complete
> network includes all sources of  reactance, including tube output C and 
> stray C as
> well as the effect of the rf  choke and tank inductor and tune C.
>
> When you tweak the tune C for maximum output, you are making all of the
> reactance contributors parallel resonant, leaving only a resistive load 
> for
> the  tube.
>
> Sure the plate load impedance changes when you tweak the tune C, that is
> what is supposed to happen. Over coupling slightly is just another way to 
> say
>  that the plate load impedance is reduced slightly. If you don't end up
> with the  desired over coupled value, then the plate load C is not set to 
> the
> required  capacity and needs further adjustment.
>
> Operating a tube into a reactive load just doesn't make any sense in my
> mind. It may not be reactive enough to hurt anything but how can it help 
> in
> any  way?
>
> Standing by for other view points and arguments.
>
> 73,
> Gerald K5GW
>
>
>
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