Re: LF: CLASS D E.... Instrumet induced errors ? Misunderstanding ?

["Graham" <g8fzk@g8fzk.fsnet.co.uk>]

Hi Alan,

.. Thats the point ,

> > > > (((((Note that it is not a 50% squarewave)))). Conversely if the design
> criteria are not followed the efficiency will be compromised. These PAs 
> are
> really just switching power supplies and those achieve very high
> efficiencies at similar frequencies.<<<<
Its a good system ,



I was noting a lot of 'posts' referring to  , P=I*V , , even if it was a 1:1 
square wave, you would still need either a form factor , or better still , a 
'true rms' meter . to make the final measurement , assuming it has 
sufficient bandwidth ,


A scope with a floating differential input , measuring across a very low 
'passive' resistance would do  the trick .. ..


Its a bit like measuring the peak power out of a ssb set , do you  measure 
the peak envalope power , by peak votatage and calculate the power and then 
mult by 0.707 to  get the rms level .or just take the peak ... or just dont 
ask ?


Graham ..

















----- Original Message ----- 
From: "Alan Melia" <alan.melia@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Tuesday, November 06, 2007 12:34 AM
Subject: Re: LF: CLASS D E.... Instrumet induced errors ? Misunderstanding ?


> Well the thermo RF current meters measure RMS current, but have you looked
> at the current from the PSU on a scope?? It shouldn't vary much if the 
> choke
> is doing its job properly. The FET switches the current to ground and when
> it opens the current is diverted to flow into the shunt capacitor. The 
> choke
> tries to maintain the current flow by generating a "back emf"....this is
> what chokes do. If the design is done properly (the choke and the shunt 
> cap
> are the right size) there should ve very little "ripple" in the current, 
> so
> the load seen by the psu should be fairly constant and it should not show
> "sag". Note that it is not a 50% squarewave. Conversely if the design
> criteria are not followed the efficiency will be compromised. These PAs 
> are
> really just switching power supplies and those achieve very high
> efficiencies at similar frequencies.
>
> Alan G3NYK
>
> ----- Original Message -----
> From: "Graham" <g8fzk@g8fzk.fsnet.co.uk>
> To: <rsgb_lf_group@blacksheep.org>
> Sent: Tuesday, November 06, 2007 12:15 AM
> Subject: Re: LF: CLASS D E.... Instrumet induced errors ?
>
>
> Thats the point, the out put is 'pure' no problem to  measure , but the dc
> feed looks to be a 'complex' function ...any allowance made in the calc's 
> ,
> past  P = I*V ?
>
> Same problem, pulsed radars are very sensitive to psu ripple causing
> 'spurious side bands'
>
>
> ----- Original Message -----
> From: "hamilton mal" <g3kev.ham@virgin.net>
> To: <rsgb_lf_group@blacksheep.org>
> Sent: Monday, November 05, 2007 4:49 PM
> Subject: Re: LF: CLASS D E.... Instrumet induced errors ?
>
>
> It looks like you have a serious problem, sounds more like a Radar tx. 
> The
> output from my class E is a beautiful sine wave, this is followed by a 5
> pole LPF.
> For testing it is preferable to use a spectrum analyzer, and a good
> calibrated scope and a quality DL
>
> G3KEV
>
> ----- Original Message -----
> From: "Graham" <g8fzk@g8fzk.fsnet.co.uk>
> To: <rsgb_lf_group@blacksheep.org>
> Sent: Monday, November 05, 2007 12:53 PM
> Subject: Re: LF: CLASS D E.... Instrumet induced errors ?
>
>
> ..... The 'I' supply to  the stage dosent look like a sine wave , more 
> like
> a pulse train ? , thats the case will need a little more than a avo-8 to
> get the  correct value,  vlaues may be actually higher than conventionally
> measured, giving lower conversion factors ? supply bandwith may also be a
> issure, caused by switching -rise times- is there a voltage  collapse on 
> the
> leading edge, there must be some , but how much  , reducing  the 'area 
> under
> the curve' ?
>
> G ..
>
>
>
>
>
> ----- Original Message -----
> From: "John GM4SLV" <gm4slv@sighthound.demon.co.uk>
> To: <rsgb_lf_group@blacksheep.org>
> Sent: Monday, November 05, 2007 12:13 PM
> Subject: Re: LF: CLASS D E
>
>
> On Mon, 5 Nov 2007 11:39:54 -0000
> "hamilton mal" <g3kev.ham@virgin.net> wrote:
>
> > Hi all
> > Tnx to all for the information, but some of the figures do not work
> > out. How are the % figures measured.
> > One simple method is to calculate the DC input PWR in watts (I x V)
> > then using an RF current meter calculate the PWR out (Isq x R) let R
> > be the norm 50 ohm D/load. Compare the two figures and calculate the
> > Eff % 73 de Mal/G3KEV
> Again...with my TX
>
> At 50W output :-
>
> Dc supply = 18.7v at 3.4A
>
> Efficiency = 78%
>
>
> The math's is trivial.....
>
> 18.7V x 3.4A = 63.58W DC input
>
> 50W RF output (as measured on a homebrew power meter calibrated against
> a £20,000 R&S Spectrum analyser and precision 40dB power attenuator)
>
> Efficiency = Power Out / Power In
>
>
> 50 / 63.58 = 0.786
>
> Hence 78.6% efficient.
>
> Of course if you can't accurately measure any of the necessary
> quantities then the end result will be meaningless.
>
> Measuring RF output using an RF ammeter and 50 ohm load is all well and
> good...but...how accurate is your RF ammeter? How accurate is the 50
> ohms. How is the 50 ohms measured... at DC? Is it the same at RF?
>
> With a good Class-E amp and 90% efficiency it only takes one of your
> measurements to be out by a few percent to make the efficiency
> calculated to be really way out.
>
> Say (plucked out of thin air example):-
>
> DC input (most accurately measured parameters?)
>
> 13.8V at 5.3A = 73.14W
>
>
>
> RF output as measured by your RF ammeter/50 ohm load
>
> 1.17A in 50 ohms = 68.4W
>
> Efficiency = 93.6%
>
>
>
> If your RF ammeter reads 5% high...
>
> I = 1.23A in 50 ohms = 75W Out
>
> Efficiency = 103%
>
> Contact the Nobel Prize Committee... you've invented the prepetual
> motion machine.....
>
>
>
> If your 50 ohm load is really 49 ohms however...
>
> 1.17A in 49 ohms = 67W
>
> Efficiency = 91.7%
>
>
>
> Don't assume there's some black magic mumbo-jumbo Emperor's New Clothes
> syndrome with MOSFET amps just because it appears that you can't tie
> down a reliable efficiency figure. Little errors make big differences.
> The important thing is that they work, don't get hot and can produce
> serious power for little engineering. Try getting a 2N3055 to produce
> 150W on 500kHz.
>
> Cheers,
>
> John
>
>
>
>
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