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From: N1BUG <paul@n1bug.com>
Message-ID: <f5673545-5bc1-c140-e373-9c8714261e39@n1bug.com>
Date: Fri, 29 Dec 2017 08:18:38 -0500
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Subject: Re: R: Re: LF: TXing 2200m WSPR
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Hi Clemens,

OK, I understand about capacitive coupling in transformers. So I 
tried a simple common mode choke. At first 22 turns using RG-178 
coax on a FT-114-JC (yes, made a typo last time). No change for 
better or worse with the choke.

Maybe 22 turns is too much so I made it 18 turns. No change. 16... 
14... 12... 10 turns. No change. Is the core too small? I could try 
stacking a few of these.

I still have to try building a shielded box for the PA and LPF. This 
will take some days. Maybe it also needs shielding between input and 
output circuits of the PA.

BTW I forgot to mention during the night I also tried a CM choke on 
the DC power feed (16 turns, FT-114-JC). No change at all.

If I knew for sure that a different (properly built, proven design 
class D) PA would work here, I would not put any more effort into 
this PA. The trouble is I can't know that another PA would be OK.

Note: RG-178 on a coated core is very slippery! But I was able to 
keep the turns tight with some tape!

Paul N1BUG


On 12/29/2017 07:08 AM, Clemens Paul wrote:
> Hi Paul,
> 
>> I can try common mode chokes instead of isolation transformers, but
>> will this really be better? Seems if the problem is common mode
>> current the iso xfmrs should kill it...??
> 
> Basically yes.
> But we don't know how much capacitively coupling there is between primary and secondary
> which might partially spoil the RF isolation.
> I would just give it a try:
> One chocke at the PA output is no big deal.
> 
>> I still have a couple of ideas to try: Replace 50uH choke in PA with
>> different type core.
> 
> In Spice there's only a ca. +/- 0.5Vp voltage swing across the choke.
> Hence the flux is less than 10% (3.7mT) of the max. flux for this core.
> So no problem with that choke.
> 
>> Use completely separate bias supply.
> 
> In Spice it makes no difference.
> 
> 
> 73
> Clemens
> DL4RAJ
> 
> 
> 
>> -----Original Message-----
>> From: owner-rsgb_lf_group@blacksheep.org
>> [mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of N1BUG
>> Sent: Friday, December 29, 2017 11:40 AM
>> To: rsgb_lf_group@blacksheep.org
>> Subject: Re: R: Re: LF: TXing 2200m WSPR
>>
>> During the night I made a few more tests...
>>
>> Iso xfmr at PA output. 14T:14T on FT-114-JC core. Made the problem
>> slightly WORSE.
>>
>> Iso xfmr at amp input. 4T:4T on 3 x BN73-202. Made the problem
>> slightly WORSE.
>>
>> Removed iso xfmr from amp output, left iso on PA input. Slightly
>> better than without iso anywhere, but still bad.
>>
>> Tried again all configurations of iso xfmrs (at antenna, at PA
>> output, at PA input) using HP 3325B to drive the PA. No difference
>>from using the Ultimate 3S.
>>
>> When I use an iso xfmr at the antenna the problem is WORSE than when
>> I have the coax connected to the antenna ground. It's not a big
>> change, but measurable and repeatable.
>>
>> Considered this might be an instrumentation problem with RF
>> affecting the HP 1980B oscilloscope, but I can see the signal
>> quality degrade in a receiver when the problem occurs so I don't
>> think so.
>>
>> I can try common mode chokes instead of isolation transformers, but
>> will this really be better? Seems if the problem is common mode
>> current the iso xfmrs should kill it...??
>>
>> I still have a couple of ideas to try: Replace 50uH choke in PA with
>> different type core. Use completely separate bias supply. However
>> since the PA works perfect into a dummy load I am not sure the PA is
>> the problem.
>>
>> I will try to make a shielded box for the PA. That will take some time.
>>
>> The Ultimate 3S is in the metal case sold for it. This is not great
>> because of the paint (or powder coat) at mating surfaces. I will try
>> to work on that but since using the HP 3325B as a PA driver does not
>> make any change I have doubts this will help.
>>
>> Paul N1BUG