Trip - KT4WO
The infamous BITX-Click has been well documented on this forum, but it was mostly
some time ago.?
There is a capacitor that couples the receive audio from the BFO mixer and another
capacitor that couples the microphone amplifier to the BFO mixer.? In Rx mode and in
Tx mode these capacitors get charged to different voltages.? When you change modes
these capacitors take on different voltages.? This causes a click when going from Tx
to Rx mode, and a 20 ms carrier burst when going from Rx to Tx mode.?
There have been several methods suggested to alleviate this situation, but basically
the situation is something that most will live with as a trade-off for simplicity and low
cost.? Once you have an antenna connected the receive-mode click should be less
noticeable.? 0.1 mfd across your speaker terminals might help minimize the click.?
Some of the early BITX20 and BITX17 builders resorted to using a relay to switch
microphone and speaker circuits as a way to eliminate this situation.? That too is
documented in the forum.
It is possible to make an LTSpice simulation that documents this situation, and that
might lead to simulator experimentation to come up with a solution to the problem.
In fact, any of the BITX circuit sub-sections can be ran in LTSpice as a means for
providing better understanding how they work and to trial proposed modifications
without having to melt solder.
Arv? K7HKL
_._
