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Packet Radio with the PC soundcard


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. 


Download the 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 also.

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 FAQ.