Re: Extra capacitors in your BITX...?

Rahul Srivastava

Dear Arv,
From what I could make out of mechanical translations, the concept among RU friends was popularised with Ural 84 design. Desna was the more updated version probably it means dew.
the RU page for Desna design is here:
The main board ckt I am posting under files section for thers to have a look at another way of implimenting Bi-directional concept. 
Rahul VU3WJM

Arv Evans wrote:

I did a search (using Google) for :Ural 84" and for "Desna" and did find some textual mention of these units, but I did not find any web sites that showed equipment or schematics.  Do you have a URL that we might use to see schematics for this equipment?


On Wed, 2004-09-29 at 22:17, Rahul Srivastava wrote:
On mention of PSK mini I also suggest looking at RU designs Ural 84 and Desna. Similar concepts are used there while using bipolar transistors.
I am sure the coupling caps for filters can be removed.  Had I been and audiophile  I would  like not to have even one of them in signal path.
Rahul VU3WJM

Arvid Evans wrote:

As I sit here looking at the BITX20 schematic it 0ccured to me that
some of the capacitors may not be necessary:

  1)  There is a 0.1 mf on the base of Q1, that connects to a 0.1 on
the collector of Q13.  These then connect via a 10 pf to the front-end
bandpass filter.  Would it not be acceptable to use only one 0.1 mf
between the collector of Q13 and the Base of Q1, and connect the base
of Q1 to the bandpass filter via the 10 pf cap?

  2)  There is a 0.1 mf on the collector of Q2 that connects to the
crystal filter, and another 0.1 that connects the base of Q12 to the
same point on the crystal filter.  Would it be sufficient to use just
the 0.1 mf from the collector of Q2 to the crystal filter, and then
connect the base of Q12 directly to the same point on that filter?
Since crystals do not pass DC, the same amount of DC isolation would
be maintained.

  3)  There is a 0.1 mf between the end of the crystal filter and the
base of Q3, and another 0.1 mf between that same point on the crystal
filter and the collector of Q11.  It seems that it would suffice to
have the 0.1 mf between the collector of Q11 and the base of Q3, with
the crystal filter simply connected to the base of Q3.  Again, the
crystal does not conduct DC, so there would still be the same level of
DC isolation.

OK, for all you purists out there, I know that it looks more
symetrical with the "pairs of 0.1 mf capacitors", but are they really

What spawned this evaluation is my comparison of RV3GM's PSK-20
schematic with the BITX20 design by Farhan.  I was wondering if Oleg's
method of using one transister in bi-directional mode could also be
applied to the BITX20 where we are using pairs of transisters to
obtain bi-directionality. 


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