Re: 1/4 vs 1/2 wavelength antenna

From: Thomas Magma (somewhere_at_overtherainbow.com)
Date: 03/04/05 

Date: Fri, 04 Mar 2005 01:13:39 GMT

Wow this is a long thread. Don't really know where I should put my two bits
in, but here it goes.

I have designed several RF PA sections in the past. 500MHz at about 50W.
Pretty easy stuff if you have the right tools and know how to use them. The
tools I like using for matching the power output FET is two triple stub
tuners. One on the input of the FET and one on the output. So it
goes...pre-amp (50 ohm output) -> stub tuner -> FET -> stub tuner -> 50 ohm
dummy pad -> spectrum analyzer. Then just tune the stubs for the performance
you desire, these include: efficiency (thermal issues), harmonic content,
spurious emissions, load VSWR considerations, cold start, ect. Then remove
the FET and look into the triple stub tuners with the network analyzer.
Model and duplicate the network out of discrete components that can handle
the voltage/power, send the design off to the enviro test lab, and head home
early for the day.

Cheers,
Thomas

"gwhite" <gwhite@deadend.com> wrote in message
news:422772C9.A4A9508E@deadend.com...
> Richard Clark wrote:
> >
>
> > As I've noted in the past, you can fill a library with negative
> > assertions...
>
> The troublesome assertion is not the negative one. It is that RF PA's are
> conjugate matched. Neither you nor Ken has provided a single example of
such a
> design that also extracts the maximum amount of "linear" power from a
device and
> essentially its power supply (after all, that is what it is: a _power_
amp).
> Your example said nothing about output-Z, which suggests you have no clue,
since
> you didn't even remotely address the issue.
>
> For Ken's part, he recently obfuscated by dismissing an example that was
> primarily intended to be illustrative, but yet holding the salient points.
He
> completely ignored (or didn't understand) the clipping issue. Further
> obfuscation was provided by talking about "protection circuitry," which
may or
> may not exist in a circuit, but adds zero to a discussion regarding how
the PA
> is to be loaded. "Protection" is a non-stater because the PA is either
off or
> impaired.
>
> Ken's argument is circular. He say's that if a design is done for
conjugate
> match,
> then it will behave as if it is conjugately matched. Well of course (or
at
> least sort of under specific test conditions and circuits)! It is
> self-fullfilling prophecy but it unfortunately makes no statement
regarding
> obtaining the maximum power out of the circuit in the sense of turning DC
power
> into RF power (yes, *extracting* power from the DC supply and transformed
to
> RF). This is paramount to PA design. To use the device to maximum
efficacy, as
> Cripps puts it, a load-line match is needed. Ken's "conjugate match"
design
> won't do that, and that's why PA's aren't designed that way.
>
> The bottom line is that if I design an amp via load line techniques using
the
> same device and power supply as Ken (him using conj-match), my amp will
deliver
> higher unclipped PEP than his. That is the factual result you resist.
Now if
> you want to pay for extra power and big devices, that's your business--go
ahead
> and attempt to conj-match your amp--but engineers who design PA's don't do
that.
>
> Another idealized and hypothetical example to elucidate the load-line
principle
> is offered.
>
> Let's say we have a 10 W FET we'll build into a class A circuit. An RF
choke is
> used to supply drain current. We DC bias it to Vd = 10 V and Id = 1 A.
Just
> for argument sake, let's say it has a constant internal resistance of 110
ohms
> and the device will break down at 25 V. According to the most idealized
and
> standard load-line theory, we should load it to rL = Vd/Id = 10 Ohms.
This
> idealization includes the definition of positive and negative clipping --
> whichever comes "first" -- of being the operational limit for output
voltage
> swing. Clipping is associated with severe distortion.
>
> Since we need rL to be 10 ohms, and Ri = 110 ohms, we need to make the
actual
> load resistor equal to: RL = 11 Ohms. Let's check that result and see if
it
> meets the clipping constraint for maximum available power.
>
> positive swing = Id*rL = 1*10 = 10 V
> negative swing = Vd = 10 V
> Power delivered to RL: Pload = 10^2/(2*11) = 4.55 W
> The efficiency is a little under 50% because of the internal resistance.
Note
> the Load resistance is decidely not the conjugate of the internal
resistance.
>
> Let's spot check the load to see if it at least appears to be the peak
available
> power, by testing two loads "immediately" on either side of our optimum 11
ohms.
>
> Let RL = 10 ohms
> positive swing = Id*rL = 1*9.17 = 9.17 V
> negative swing = Vd = 10 V
> Since we positive clip at 9.17 V, we are limited by our design clipping
> constraint to only driving the PA such that 2*9.17 V is the maximum
available
> voltage swing.
> Power delivered to RL: Pload = 9.17^2/(2*10) = 4.20 W
>
> Let RL = 12 ohms
> positive swing = Id*rL = 1*10.82 = 10.82 V
> negative swing = Vd = 10 V
> Since we negative clip at 10 V, we are limited by our design clipping
constraint
> to driving the PA such that 2*10 V is the maximum available voltage swing.
> Power delivered to RL: Pload = 10^2/(2*12) = 4.17 W
>
> Sure enough, the power peaked at a load of 11 ohms, just like load-line
theory
> says it will. Now let's see what the available power hit of conjugate
matching
> is.
>
> By definition, conj-match insists RL = Ri = 110 ohms. Again we are
limited in
> our clipping constraint by static drain current, and supply voltage,
> specifically 10 V.
>
> Our negative swing limit is, as ever, 10 V (the drain voltage).
>
> positive swing = Id*rL = 1*55 = 55 V
>
> This would breakdown the device, but the lower negative swing will force
us to
> back down the drive to meet the design defined clipping constraint.
>
> Pload = 10^2/(2*110) = 0.455 W
>
> Conjugate matching resulted in a 10*log(0.455/4.55) = 10 dB available
power
> hit. Power amplifiers are not designed with conjugate matching in mind.
You
> don't need to re-invent the wheel. Just follow well established
principles when
> doing cookie cutter PA design.
>
> > The list could go on,...
>
> LOL. Given your pattern, I am sure it will.
>
> > You sighed with content at being offered a "relevent
> > question/statement" Your re-iterative response contains the same (how
> > could it be otherwise?) slack of precision that started this. Want to
> > try again?
>
> Not really. The problem isn't precision, it is you can't, or refuse, to
> comprehend what is being said, which I presume is why you instead write
with the
> most bizarre terms and phrasology that has nothing of import to the topic
at
> hand.
>
> > You could have as easily expressed what sense they ARE matched,
>
> For what seems like the billionth time now: they are load-line matched.
>
> > ...but instead this time offer what Basis
> > of Matching you are attempting to
> > describe.
>
> I've given a didactic example (actually a couple), you just don't--or more
> likely won't--get it. If you don't like my example, you can refer to
Cripps,
> who is considered one of the preeminant RF PA experts in the world.
>
> Even more simplistic is Malvino's discussion on pp177-185 of the first
edition
> ((c) 1968) of "Transistor Circuit Approximations." It is basically a
technician
> level description, so perhaps it is well-suited to you. In academics,
load-line
> theory is presented down to tech level courses and up across to
engineering.
> That some engineers and techs aren't clear on the load-line concept for
PA's (or
> *any* circuit needing a wide symmetrical swing) is notwithstanding.
>
> > This is the more rigorous approach that eliminates vague
> > descriptions and uses standard terms. If you have to query about what
> > "Basis" means (used by professionals - namely metrologists who can
> > quantify Output Z of all sources) - then we can skip it as a topic out
> > of the reach of amateur discussion.
>
> I see you still don't know what impedance is. In any case, it doesn't
mean that
> looking into a properly designed PA output with a network analyzer
confirms the
> conj-match precept, it doesn't.
>
> Impedance is a *linear* conception, a portion of linear theory, and again
by
> definition:
>
> Z = V/I
>
> V and I are sinusoids (phasors). But with power amps, substantial
non-linearity
> exists (destroying the linearity assumption of impedance), thus applying a
> linearly defined concept to a non-linear milieu is a misapplication. You
are
> attempting, as is Ken, to stuff a square peg down a round hole. Why?
>
> The concept is even questionable for the most linear of the power amps:
class
> A. In any case, given real devices with real supplies, the conj-match
ideal is
> next to worthless. While I could agree that the borderline may be fuzzy
> regarding where and when to drop the impedance notion, it still stands
that the
> concept is not useful in determining how to optimally load an RF PA.
>
> At this point you own the conj-match assertion as much as Ken. Prove it!
You
> can't because it is fundamentally incorrect.
>
> > Note:
> > >Again, RF PA's should be load-line matched.
> > Does not qualify as a Basis.
>
> Load-line matching is such a basic electronic concept it is unbelievable
how
> oblivious you are to the concept. Read a basic book. Don't rely on me:
look it
> up and do your own design!
>
> > It is suggestive of one, but because you indiscriminately
> > mix several Basis within your discussions, it is your
> > responsibility to be precise.
>
> You just like to hear yourself talk. I've been explicit and precise. You
just
> don't know anything about the elementary electronics principle of load
line
> matching. I presume this is why your comments have zero substantive
> responsiveness.
>
> > If you can accomplish this, then we can
> > proceed to review how little it all matters.
>
> If you keep ignoring what I've written, and that which is written in
elementary
> electronics texts, you can remain happily ignorant of understanding the
> simple-basic-fundamental concept presented. Your choice.
>
> > Barring resolving any of these issues of precise language,...]
>
> The guy ignorant of the definition of impedance and that s-domain theory
*is*
> linear circuit theory (and more goodies) is talking about "precise
language."
> Amusing.
>
> > I notice
> > that you rather enjoy...
>
> No, I don't enjoy it at all. Your lack of electronic understanding is
dismal,
> especially given your tone. It would have been a lot easier for me if Ken
> hadn't made the erroneous
> statement in the first place and made a correct one instead. That would
have
> been my preferance.
>
> > ..fruitless jousting with them than challenging my
> > support of Ken's (supposed) statement that you say is your focus:
> > >> However, responding to the bald statement, I find nothing
> > >> objectionable about it.
> > >
> > >That's because you don't understand the difference between impedance
matching
> > >and ac load line matching.
> > We will leave that as another dead-end.
>
> I suspect you will. I already understand it -- you're the one who
doesn't.
>
>
> "One of the principal differences between linear RF amplifier design and
PA
> design is that, for optimum power, the output of the device is not
presented
> with the impedance required for a linear conjugate match. That causes much
> consternation and has been the subject of extensive controversy about the
> meaning and nature of conjugate matching. It is necessary, therefore, to
swallow
> that apparently unpalatable result as early as possible (Section 1.5),
before
> going on to give it more extended interpretation and analysis (Chapter
2)." --
> Cripps, p1
>
>
> The quote is on Page 1. Swallow it now. Learn something for a change.

Relevant Pages

  • Re: 1/4 vs 1/2 wavelength antenna
    ... essentially its power supply ... He say's that if a design is done for conjugate ... The bottom line is that if I design an amp via load line techniques using the ...
    (sci.electronics.design)
  • Re: Work Proceeds- Experimental Status
    ... A basic power as slow quanta appears the cause to the anomaly. ... 1kW laser can equal a 1kW magnetron. ... And so I decided to follow a basic design method of ducted microwave. ... A single magnetron to power a rotating set of waveguides. ...
    (sci.physics)
  • Re: battery charger topology
    ... substantial powers used to charge a 300Ah 24V lead acid battery bank ... normally have to build into the design. ... sets the voltage you must boost to. ... 100KHz may be a bit fast for the power level. ...
    (sci.electronics.design)
  • Re: Async Processors
    ... Coal-Fired Power Plant ... "With the Forecast Icing Potential tool, ... design has significant advantages over standard sync clocked logic ... using sync designs if your goal is to reduce power consumption. ...
    (comp.arch.fpga)
  • Re: battery charger topology
    ... substantial powers used to charge a 300Ah 24V lead acid battery bank ... normally have to build into the design. ... sets the voltage you must boost to. ... 100KHz may be a bit fast for the power level. ...
    (sci.electronics.design)
Loading