A CW Sidetone
Bypass for the NESCAF Audio Filter
By: Bertrand Zauhar, VE2ZAZ
audio filter is a small and low cost circuit that works great. It is a
good addition to any amateur radio setup. It is especially useful with
radio receivers or transceivers that lack an optional CW filter. I have
made a product
of the NESCAF
audio filter on
my Amateur Radio Blog.
One feature this filter lacks though is a bypass path for the audio
transmitting in CW. This would allow to hear the CW sidetone geneated
by the radio through its speaker. So I have
come up with a simple solution to this issue.
Referring to the altered NESCAF
audio filter schematic
below, all added connections and components are shown in
wider/bolder black color. The bypass path is provided by a reed relay.
simply bridges the audio input signal to the output audio
amplifier. The relay coil is energized via the ground return path
provided by the Morse code key. The silicon diode and the zener diode
insure that interaction with the radio is kept to a minimum. On the
audio path, a 10K resistor is inserted to reduce the gain and match the
audio level seen when the filter is inserted. A coupling capacitor is
also inserted to block any DC current from flowing. Grounding the "Morse key"
terminal will operate the bypass
figure to enlarge it
Nothing is really
critical in the implementation, and you may want to experiment with the
component values to suit your needs. The following are things you
should consider when adding this feature to the filter.
- Use a
reed relay with a 5V coil. Try to select one with the least amount of
current drawn from the coil. Remember that the U2, the 9V regulator, is
limited to 100mA of current. The relay I use is a DIP package reed
relay (Clare PRMA 1A05) I had on hand. You should be able to find an
equivalent relay very easily.
5.1V Zener diode is important, as it insures that the relay does not
get much more than 5V on its coil, keeping the coil currrent low.
Additionally, it limits the floating voltage at the morse key to less
than 5V, which is normally the floating voltage on modern rigs. You may
verify this on your radio and select a different Zener voltage diode to
suit your radio.
blocks any voltage from flowing back into the relay coil,
which would have a loading effect on the radio's CW key signal.
discrete components are soldered right onto the relay pins, in other
words the relay pins supports all discrete components.
the relay pins; just keeping the wide part of the pin is
sufficient to solder onto.
relay is simply glued to the top of the filter chip with flexible goop.
soldered to the PCB on the bottom side. I used 30-gauge
wire-wrapping wire to make the connections.
surface mount resistor and capacitor. The surface mount
version is optional but it keeps the overall solution small.
following picture shows the added circuitry, with the reed relay (in
blue) sitting on top of the filter chip. All connections that are part
of the add-on circuit are made with the thin red wires.
Click on picture to enlarge it
Operating the add-on bypass circuit
is a matter of connecting the "Morse key" terminal of this circuit in
with the radio's key input (hot wire and ground) and on the the morse
code key. Please observe