Re: LF: RE: PA matching oddity

[Andy Talbot <andy.g4jnt@googlemail.com>]

A follow up to this sage  

 

After much puzzling over the design, and comment here about how it must be 3:1 teh only solution was to measure it.  So I connected up and measured voltages. Indeed, it does turn out to be 3:1 -   So   how could it deliver 1kW plus...

 

Next, fitted the module to a big heatsink,  got out the 'telephone exchange' PSU (50V 25 A) and wound up the amp to the max it would give.   Unfortunately the PSU current limited when supplying only 800 or so watts, but by winding teh PSU down to 44V, it current limited at a higher value and I got 950 Watts before the PSU gave up (off 44V).    So it really was giving more power out than it 'ought to'

 

BUT, as Clemens mentioned I'd ignored the saturation element and in fact, if used as a switching amp can give well over 1kW.   I'd used this aspect in the 700W 137kHz Tx, http://www.g4jnt.com/137tx.pdf  and forgot it.   Enough said.

 

Anyway, tested the PA, and found it would go down to about 5MHz where the transformers began to give up, and up to about 20MHz where the FETs lost gain / efficiency.   So as a 13.56MHz industrial unit its fine, but of little use as a general HF amp.   BUT, with suitable change of output transformer, could probably be persuaded to give 1.5kW on 137 or 500kHz - if a suitable PSU were to hand.  That's where it sits now.   Experimentation done, and any further work can wait for other projects to get pushed of fthe stack.

Andy
www.g4jnt.com

On 16 February 2010 19:07, Clemens Paul <cpaul@gmx.net> wrote:

Andy,Rik,

 

after thinking a while over the transformer of Andy's PA and referring to

http://www.g4jnt.com/pamatch.gif  

I'm sorry to say that

I'm still quite (say 99,9%) sure that it is a conventional 1:9 (Z) transfomer.

 

Here are some crucial points which exclude *absolutely* the above design

to be an autotransformer,(1:16 or whatever).

1.) Autotransformers provide no dc isolation, i.e. you have a dc path from

the primary to secondary side.

There is no such dc path here.

2.)In a bootstrap design the braid of one coax end is connected to the center

conductor at the start of the same  coax (1:4) or of the next  coax section (1:9 or higher).

This is not the case in this design.The center of the secondary is nowhere connected

with a braid.

 

BTW there is no reason to worry about the theoretical 900W limit in linear mode.

If the PA is slightly overdriven you get easily one 1dB more power on the fundamental

frequency which is about 1133W so you have enough margin for losses of the low pass filter

losses etc.

It's very unlikely that this PA which was used as industrial RF heater

was designed to be superlinear.

 

73
Clemens
DL4RAJ

 

 

 

 

----- Original Message -----

From: Andy Talbot

To: rsgb_lf_group@blacksheep.org

Cc: ukmicrowaves@yahoogroups.com

Sent: Tuesday, February 09, 2010 10:43 PM

Subject: Re: LF: RE: PA matching oddity

BINGO -  Tnx Rik, thats it, convincing.  Sorted

 

So my thing  IS 1:4 turns ratio, (1:16 Z ratio)  just as the calculations said it needed to be, and is indeed, effectively, an auto transformer.  Otherwise known as a bootstrapped coaxial transformer, but just configured with different reference points to confuse everyone. Your second reference, the Semelab paper, (UKuW see below)  gives the answer.  

 

Look at page 10,   the 1:9 Ruthroff   UnUn .   With just two bits of coax, which are then bootstrapped up on top of the input to give a 3:1 ratio (for 1:9 impedance).   Add another turn, so three windings are  bootstrapped on top of the primary, rearrange the grounding points and there, in all its glory, is my mystery.   

 

The reference / grounding points can be shifted as the mosfet PA  has  two separate, non magnetically linked cores.  This is is just normal push-pull HF PA practice, and being so normal in its concept, completely hides hides a big real advantage - it enables the designer to float just whatever ternainals need to be floated for balanced / unbalanced operation, and even to provide DC isolation,   Any conductor passing through the core allows one end to be completely Rf decoupled from the other end, so that's how balanced operation is permitted with the basic Ruthrof design shown in the paper.

 

On the same page, a single length of coax is shown giving 1:2 transformation by the same bootstrapping arrangement, and when the transmision line is shown as a twisted pair rather than coax, then wrapped on a torroid, it becomes the 1:4 balun beloved of HF antenna constructors.   A natural progression that helps illustrate the concept  of  a loop of coax giving one turn more than it appears to.

 

It all falls out. 

... life is worth living again ...

... can retire happy ...

 

Andy
www.g4jnt.com

... now, next job, how to sort out the world financial crisis ...

On 9 February 2010 19:53, Rik Strobbe <Rik.Strobbe@fys.kuleuven.be> wrote:

Andy,

 

have a look here :

 

http://www.polyfet.com/HFE0503_Leong.pdf

http://www.semelab.com/rf/documents/Push-Pull%20Circuits%20and%20Wideband%20Transformers.pdf

 

73, Rik  ON7YD - OR7T

 

---

Van: owner-rsgb_lf_group@blacksheep.org [owner-rsgb_lf_group@blacksheep.org] namens Andy Talbot [andy.g4jnt@googlemail.com]
Verzonden: dinsdag 9 februari 2010 18:10
Aan: rsgb_lf_group@blacksheep.org; ukmicrowaves@yahoogroups.com
Onderwerp: LF: PA matching oddity

 

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