I am partially done building my 38 special. I built it with the TiCK
Keyer
and 5 watt mod. I used K8IQY's optimizations for the front end, transmit
preamp and final filter and W5VBO's technique for variable output power.
The electronics worked right away. Its a great kit and an EXCELLENT
quality circuit board. My thanks and congratulations to the NorCal team
and the QRP-L.
As soon as I keyed the rig, I noticed the audio thump. It was very
pronounced
in my unit, so I resolved to reduce it. I saw on the scope that the
thump
occurred after the transmitter was unkeyed and consisted of a rather
large,
exponentially decreasing envelope of 500 Hz sine waves. I also noticed
there was quite a bit of transmitter RF on the audio line coming out of
U2-11.
The 500 Hz component is due to the audio bandpass filter ringing at its
center frequency. High Q bandpass filters tend to ring when they are
excited by a large pulse or a step because those signals contain energy
in
a wide range of frequencies, including 500 Hz.
I first assumed that the problem was feed through, to the audio line at
U2-11, from one of the pins carrying RF. I decided to construct a simple
RC filter in the audio line between U2-11 and U5-6 to reduce the amount
of RF on the line. When I started looking at where to cut the trace from
U2-11, I discovered the primary cause of the thump.
The way the board is layed out, the ground connection for C14 goes to
the
ground plane that serves U4. This chip conducts a lot of current. That
causes the voltage on that ground plane to rise on transmit and drop on
receive relative to the ground point for C38, the audio amplifier
reference
voltage. This causes the thump when the transmitter is unkeyed.
I went ahead with the filter. I removed C14 and also cut the trace on
the
bottom of the board close to U2-11. I soldered a 2.7K Ohm resistor to
the
bottom of the board, across the newly cut trace from U2-11 to the pad
where
the ungrounded side C14 used to be. I then added a new .1 uF capacitor
on
the bottom of the board from the C14 pad to a ground point at one of the
two through holes in the ground plane beneath U2. Last, I removed R19
and replaced it with a 2.2K Ohm resistor.
There is good news and bad news. The good news is the thump is gone and
the loudness of the "sidetone" produced by the transmitter RF in the
audio
filter is slightly reduced.
The bad news is that now the sidetone from the TiCK keyer produces an
audible
beat note with the filter ringing caused by the transmitter RF that is
still
getting into the audio filter. This sounds ugly.
My engineering side wants to eliminate the beat note by reducing the
amount
of RF hitting the audio filter so it doesn't ring at all. However, this
will be
difficult and it will remove the audible TC2 "tuning indicator." My
practical
side wants to eliminate it by using a two pole push button for the TiCK
keyer
programming line. The first pole grounds the programming line and the
second
pole connects the sidetone to the audio filter during programming.
I guess I'll ponder all this while I mount the rig in its case. I hope
this helps
the rest of you get rid of the "thump". As always, your milage may vary.
See you on the short waves.
73, Ron KA1KCU
[
QRP-L Archive |
]
[
1993 |
1994 |
1995 |
1996 |
1997 |
1998 |
1999 |
2000
]