In the course of my ham
radio experiments I tried to work packet radio with my PC. I wanted to do
this on the road, i.e. connect to the PR service with a laptop. I use the
Flexnet packet radio software with my Kenwood TH-F7E.
However, I found the setup very difficult;
adjusting the soundcard settings is a very tedious task, but the main problem
was that the HF generated in the laptop traveled over the wires to the rig,
and I could not get a clear reception any more since the PR signals were
drowned in S5 HF spam.
Finally I made up this circuit which solved my
problem. It relies on the frequency characteristics of the optocouplers. The
type I use has its 3dB point at about 100kHz, which means that any harmful HF
on 2m or 70cm is completely prohibited from entering the transceiver.
There is one important drawback, though: I have
only been able to get it to work with 1k2 baud PR, not 9k6. I'm not quite sure
why; maybe this circuit's frequency transmission curve is not linear and it
needs some adjustments. Or maybe it's a problem with another component. If you
want to do 9k6 baud PR, don't use this circuit.
schematic (Eagle 4.11 format, ca. 200 kB)
As voltage supplies I suggest that you use two
9V block batteries. It is important that the voltage supplies are not
connected to each other. The exact optocoupler type is not critical, I
believe. It's only important that the pins match.
Once you have built the circuit you need to
adjust level settings using an oscilloscope. It is best if you use a
two-channel oscilloscope, however, you can get along with a single-channel osc
Connect a suitable sound source to the PC Sound
Out pin. You can use your PC and play a wav file repeatedly, or you may use a
wave generator with a frequency of about 1200 Hz and an amplitude of about 1V.
If you use a PC, set its output level to maximum, that should give you about
1V. Connect one oscilloscope probe to the TRX Audio In pin (ground to TRX GND)
and adjust R11 so that you can see a signal. Now you must adjust R8 so that
the signal you see is exactly the same signal that comes from the PC sound
card output. This is much easier if you are using a sine wave generator as
sound source, as you can easily spot the point from where the signal gets
distorted. If you are using a two-channel oscilloscope, connect the second
channel to the PC Sound Out pin and overlay the two graphs.
After adjusting the signal to appear
undistorted, adjust R11 so that you get the same output level that you put
into the circuit.
Now repeat the same procedure for the reverse
side, using R16 and R3 for the distortion and level controls.
This circuit assumes that your rig requires a
connection to GND on the PTT pin to go into transmission mode. I believe that
this is the case with most modern rigs, at least the TH-F7E does so.
Please note that building this circuit
occurs on your own risk! If you are not sure what you do here, leave it. I
do not assume responsibility for any damage you or any equipment may suffer
from building or using this device. Please respect the common security
guidelines when working with electric equipment. A description of these can be
found at the sci.electronics.repair FAQ.