R: Re: LF: On air again in LF WSPR-15

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

Hi Chris,

if I may confuse you a bit more..... :-)))
Stefan worries fron too large cores but I suggest you to avoid too small cores ;-)
I'm using the same IC driver in an half bridge PA I had a 20/20/20 turns xfmormer wound on a binocular core size BN43-202. it was fine at low level power but.. increasing the supply voltage suddendly the output became erratic with tendency to decrease ... after searching for sometime what was wrong, just for fate I touched such xfmr: it was veeeeeeery hot!!! the problem was soved stacking 2 cores thus increasing the cross area of the core. If you are planning to use a toroid will suggest a diameter between 15 and 20 mm any core of type from X830 or higher AL should work fine

Marco, IK1HSS

----Messaggio originale----
Da: selberdenken@posteo.de
Data: 22-nov-2018 12.09
A: <rsgb_lf_group@blacksheep.org>
Ogg: Re: LF: On air again in LF WSPR-15

Hi Chris,

Am 22.11.2018 11:36, schrieb Chris Wilson:
> Thanks  for the schematics Stefan! Do you know exactly what toroid you
> used (assuming it is a toroid) for the gate driver transformer please?
>    
I tried to describe it as good as possible.
If it helps, here is the link: 
https://www.reichelt.de/ringkern-aus-ferroxcube-n30-rik-20-p15247.html?&trstct=pos_0
It is not so critical. As long as the AL value is reasonably, it should 
work. Avoid to use a too large diameter because of stray inductances and 
so on. This would affect the gate-source voltage.

> Would  the  output level of the Si chip in a QRP Labs U3S be enough to
> drive  this  amp  direct?
I don't know about the output of the U3S but i assume it is certainly 
enough to drive the optocoupler.
The optocouplers are not a must but they can help to avoid problems in 
the shack, ground loops and so on, you know. Another transformer may be 
an alternative instead. There are many ways to build a system running 
without troubles during the operation.

>    I  do not fully understand what the BF862 /
> 6N136   and   10K  pot  do,  would mind just explaining that bit about
> duty cycle please?
>    
Well the 6N136 is an optocoupler providing a very good galvanic 
decoupling between the signal generator and the PA. Also it is part of 
the safety considerations of the system operating at rectified mains level.
Depending on the signal source in front of the optocoupler and the 
optocoupler itselfe, the input voltage of the IXDD will most likely not 
be a perfect rectangle with 50 % duty cycle. Using the pot it was 
possible to adjust for a nearly perfect 50% duty cycle of the 
rectangular (+-320 V @ 137 kHz) output voltage of the H bridge.
In this design it is not super-essential to have a 50% duty cycle. With 
53 % for example there will be a residual DC component but this would be 
taken by the C in front of the output transformer. The PA would not be 
destroyed but the output power would be a little bit lower. Anyway, 50 % 
is desired.

BTW you know that this design is not intended for linear operation of 
course, unless you add another stage modulating the bridge supply 
voltage. But this is not necessary for QRSS/DFCW/OP32/WSPR/JT9-X/EbNaut, 
i.e. for almost any mode.
I admit i even transmitted slow hell with that PA. The result didn't 
look as pretty as with a linear PA but it worked as well :-) In fact it 
shows that the design seems to be quite robust. Anyway i heared the FETs 
complaining (krrzzkrrrktz), you know.
See the attachment from the old days :-)

73, Stefan