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Date: Fri, 01 Jun 2012 00:53:58 +0200
From: =?windows-1252?Q?Stefan_Sch=E4fer?=
 <Stefan.Schaefer@iup.uni-heidelberg.de>
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Subject: Re: LF: Re: Current mode class D PA for 475 kHz - success
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Hi Jay, LF,

Thanks very much again!

Well, if the PA efficiency is 90% with standard parts, what can you=20
achieve with the speial core material? I can obtain the 77 ferrite=20
material here but never heard about the 79 material. I can hardly=20
imagine that there is much more room for significant improvements. More=20
infos apprechiated.

For now, as a result of this day i want to show some photos i've just=20
taken. BTW the PA is now driven from the Elecraft K2 which i built in=20
2003. Just 1W RF is needed (at 1900 kHz =3D> 475 kHz) to drive the PA inp=
ut!

Gate voltage waveform from the ICL7667:=20
http://dl.dropbox.com/u/19882028/LF/Gate%20voltage%20475%20kHz%205VperDIV=
%20ICL7667%2BIRFZ48N.JPG
Drain, unloaded:=20
http://dl.dropbox.com/u/19882028/LF/Drain%20Source%20voltage%20without%20=
a%20load.JPG
Drain at 40W RF:=20
http://dl.dropbox.com/u/19882028/LF/Drain%20Source%20voltage%20%40%2040W%=
20RF%20out.JPG

Comments?

No, i don't have a Hello Kitty scope, the pink colour comes from the=20
auto adjustment of the image...


73, Stefan/DK7FC

Am 31.05.2012 20:27, schrieb jrusgrove@comcast.net:
> Stefan
>
> Well ... guess you have already answered your own question ;~)
>
> I've stuck with transformer coupled designs ... VMCD for 137 / 500=20
> kHz  and CMCD for 160 / 75 / 40 meters. I don't have much experience=20
> with the bridge circuits which I believe is what you normally use.
>
> Without a load at the transmitter output both types behave well - they=20
> act as though they aren't even keyed. I've mistakenly keyed them into=20
> incorrect loads (antennas for other bands) with no ill effects.=20
> Operating into impedances other than 50 ohms cause a shift in the=20
> drain voltage/current ratio as expected ... so there are limits. From=20
> experience a 1.5:1 VSWR during normal operation isn't a concern.
>
> The ringing on the off FET drain is an issue with VMCD and is mainly=20
> due to leakage reactance of the transformer. In my experience, this is=20
> best minimized by choosing a core material that provides enough=20
> winding reactance for the frequency in use  without going overboard.=20
> For my 137 kHz KW transmitter (yet to be written up) I used an=20
> FT-240-78 core. 78 material was specifically developed for the=20
> frequency range between 100 and 200 kHz and works excellent. These=20
> cores are standard parts. For my 500 kHz KW transmitter (yet to be=20
> written up) I used an FT-240-79 core. 79 material is even better than=20
> 78 material at 500 kHz. Unfortunately, these are not readily available=20
> - 79 is a newer material and available in only a few sizes that aren't=20
> conducive to what we're doing. I parted with some NRE money last year=20
> and got Fair Rite to make some FT-240-79 cores for me. Last I checked=20
> they hadn't added it to their catalog ;~(
>
> Even with the correct core material and winding technique (interleaved=20
> primary and secondary windings) some form of snubber from each drain=20
> to source is required. Choosing the correct R and C is an interesting=20
> exercise. Rather than go over it here check out this paper by Rudy=20
> Severn:
>
> http://www.ee.bgu.ac.il/~pel/links/DESIGN_OF_SNUBBERS_FOR_POWER_CIRCUIT=
S.pdf=20
>
>
> BTW, Rudy is one of the WD2XSH 500 kHz group. He also has a You Tube=20
> videos on the subject ... and a book as well.
>
> In my experience the ringing is not a problem as long as it is=20
> controlled and well under the voltage limits of the FET. In my KW VMCD=20
> the drain square wave is 100 V (2 X supply voltage (50VDC)) and the=20
> first peak of the ringing voltage brings this up to about 140 V. The=20
> FETs in use here are 240 volt parts.
>
> Jay
>
>
>
>
>
>
> ----- Original Message ----- From: "Stefan Sch=E4fer"=20
> <Stefan.Schaefer@iup.uni-heidelberg.de>
> To: <rsgb_lf_group@blacksheep.org>
> Sent: Thursday, May 31, 2012 12:37 PM
> Subject: Re: LF: Re: Current mode class D PA for 475 kHz - success
>
>
>> Jay, MF,
>>
>> I just tried the CMCD and immediately achieved 89% efficiency at=20
>> 12VDC and 36W RF at 475 kHz (dummy load). Just a first result. I can=20
>> answer my own question now: Without a load the PA behaves not=20
>> critical! The current consumption is 0.2A only (including the current=20
>> of the VFO and exciter) !! :-) I'm going to improve the design now=20
>> and will publish some photos soon...
>>
>> 73, Stefan/DK7FC
>>
>> Am 31.05.2012 15:59, schrieb jrusgrove@comcast.net:
>>> Stefan
>>>
>>> Thanks for the positive comments.
>>>
>>> Have to admit I haven't been following what you're up to ... curious=20
>>> what problems you're running into with VMCD at 500 kHz?
>>>
>>> Jay
>>>
>>>
>>
>