38Sexperimenters (whoops, need a space in there methinks.....)
Whilst helping to develop Ori's design into a full fledged kit, the one
"feature" that we could not cure was the key thumps. I spent many hours
searching for the source of the thumps and never got to a final
conclusion!
The timing of the -T and -R signals into the 4066 switch seemed to
matter. After some experimentation, I found that delaying these signals
by slightly different amounts helped reduce the thump to a level that
was almost acceptable but not perfect (you should have heard it before
we added the delay components!!!). BTW the delay is acheived with D4/5,
R7/8 and C12/13 - you can fiddle with the values of R7 and R8 if you'd
like to "fine tune" your rig for minimum thump (but it won't go to
zero).
Like I said, I never got to the bottom of the problem. I did rule out
the RF leakage from the T/R switch getting into U1 and causing problems.
It's definately not that!
There were negative excursions on just about all the signals entering
and exiting the 4066 switch - it was not possible to isolate which was
the source and which were effects. However, since there appeared to be a
significant thump pulse even on pin 1 of the 4066 (from the xtal
filter), I concluded the following: the T/R changeover pulse that causes
the thump is being generated INSIDE the 4066 device, and as such, there
is not a whole lot that can easily be done about it.
Perhaps someone with some spare experimentation time (Mike?) could try
to add another 4066 device in parallel with the existing one, but split
out the switches into different devices. This might just tell us
something. It's possible that the pulse is being propogated from one
switch to another inside the device. Remember, this device was not
really designed for the exceedingly small signals were using it for; it
was really meant to switch large signals (of the same order of magnitude
as the power supply voltage at least).
Keep hunting, we'll crack the problem yet!
72, Dave Fifield, KQ6FR
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