R: Re: VLF: Class E PA

["marcocadeddu@tin.it" <marcocadeddu@tin.it>]

Hi Chris & LF fellows,

I have also a similar problem to your Chris, but I haven't still found the origin:
at the end of WSPR transmission, in the 2 seconds pause before the ptt switches to rx, I see a high peak in the return loss (very high SWR if you prefere) with lower associated froward power.
This should mean a short transmission outside the matching of the antenna I guess.. maybe an audio tone few kHz away or a sort of regeneration through the driver..
It happens both in the LFTX which is an half bridge Decca style and in the MFTX which is a class E amplifier.
No worries of course for the LFTX which is very sturdy ;-) but I guess this could be one of the problems I encountered testing the MFTX.

73, Marco IK1HSS

----Messaggio originale----
Da: dead.fets@gmail.com
Data: 14-ott-2018 11.00
A: "DK7FC"<rsgb_lf_group@blacksheep.org>
Ogg: Re: VLF: Class E PA



Hello DK7FC,

The biggest voltage and current spikes in my W1VD Class d amp are when
a WSPR or OPERA transmission ceases, from back EMF I presume.

I  am  curious  as tvo whether a "soft voltage start" circuit could be devised
and  added  to  other  power  supplies  similar  to  the one discussed
earlier?

Sunday, October 14, 2018, 9:30:20 AM, you wrote:

>    
>  Hi Tom, 
>  
>  Good luck with your PA.
>  Tell us the component values when you're finished, as well as the
> actual efficiency. From my LF PA i remember the coil, which was
> quite large already ('air-cored').
>  
>  Regarding current peaks at the first switching: I would simply
> ignore the problem and see if the FET will survive (and how old it
> becomes). If it's lifetime is actually to short, then i would start
> thinking about a solution for the problem. One of them just costs 2
> EUR or so, so it is worth to try...
>  
>  73, Stefan
>  
>  
>  
>  Am 14.10.2018 00:16, schrieb DK1IS: 
>   
> Hello group,
> Just to close this thread:

> Concerning the problem of the first drive pulse with DC power
> already on: Bob, W1XP, made an important contribution in a private
> mail. He pointed out that the current spike I was worried about
> would be limited by the FETs output characteristic anyway. Looking
> to the data sheet of my intended IRFB 4227 this seems to be true
> although there could persist a certain limitation by the save
> operation area. Perhaps I was a little bit too naive estimating the
> peak current as Vdc / Rdson ... Be that as it may, using the newly
> discovered DC soft start feature of my power supply should overcome
> this challenge. Controlled by a logic level input it runs up the
> output voltage with a time constant of about 100 ms. This will
> protect the FET, avoid any key clicks and won´t affect the
> readability of the long lasting vlf signals. To keep the signal
> phase constant (essential on vlf) oscillator and gate drive of the FET won´t be keyed at all.

> I´ve just modified the antenna / loading coil system to resonate at
> 8270 Hz without transformers, - 3dB band width now being 168 Hz.
> Including 25 m RG213U from the arbour to the shack (being later on
> part of the matching network) the feeding impedance is now 480 Ohms
> pure resistive which will be matched to the rest of the class E PA
> by its output network. Thus I`m able to omit the two matching
> transformers being used up to now, see www.qrz.com/db/dk1is. 

> Now I´m waiting for the driver ICs coming from China - in Germany
> up to now without additional tax ;-) !

> Thanks for all the feedback, 73,
> Tom, DK1IS 
>    


-- 
Best regards,
 Chris                            mailto:dead.fets@gmail.com