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Operating RTTY, PSK, SSTV, etc. may require interfacing the sound card on your computer to your Amateur Radio Transceiver. 1. A computer sound card outputs about 1 volt peak-to-peak on the line-out connector. (Mine outputs a maximum of 2.5 volts pk-pk). 2. Amateur transceivers with microphone and phone patch inputs are designed to amplify the signal that a microphone puts out, about 10 millivolts peak-to-peak. Use of the audio processor (compression) will make the transmitter even more sensitive. The low-level audio amplifier stages in the transmitter are in front of the microphone gain control. Once those stages are overloaded, the audio has been destroyed regardless of the MIC GAIN setting. 3. If you directly connect a sound card lineout to the microphone input you will likely have problems. · In the case of voice (such as a contest voice keyer provided by software such as Writelog) the transmitter will be badly overdriven resulting in severe distortion, and pickup of a lot of noise (such as hum). · In the case of data modulation several different effects may occur. On RTTY the most common one is that second, third, even fourth and fifth harmonics of the mark and space tones may be generated in the low level audio amplifiers. These then generate image RTTY signals with weird tone spacing that badly and unnecessarily QRMs other stations. · In the case of PSK transmission, the filtered audio signal is clipped resulting in the generation of modulation sidebands that unnecessarily QRM other stations. This is reported as “Poor IMD” (Intermodulation Distortion). The worst possible IMD for a PSK signal is –13 dB (it’s a square wave). Fair IMD is –20 db, and good IMD is –30 db. A LARGE PERCENTAGE OF PSK31 SIGNALS HAVE AN IMD OF –13 DB (i.e. they are as bad as they possibly can be). · For all modes 60 and 120 Hz hum pickup can overdrive the audio stages resulting in 3 KHz of wideband trash in the transmitter output. This ought to get you an instant pink ticket from the FCC.
The solution is to ATTENUATE the SOUNDCARD LEVEL going to the transmitter. Use of the MIC GAIN is not enough—the first two stages of audio gain in most rigs is BEFORE the MIC GAIN control. An attenuation of about 25-30 dB has worked well for me. Two resistors are all that is needed. 10K in series with the soundcard lineout, and 600 ohms to ground. Alternatively, you can buy an Attenuating Audio Cable at Radio Shack, part number 42-2461, which incorporates a 60 dB attenuator. It’s not expensive.
Finally, sometimes it’s possible to have common ground return currents shared between digital and analog interfaces to the transmitter. These can be isolated with a 600 ohm 1:1 audio transformer. You’ll know that you need one if the RS-232 interface on your rig causes beeps or squawks in your transmitted audio. This is due to the sharp edges of the RS-232 signal causing ground offset voltages between the computer and the transmitter. Also ground loops may cause hum, this is very noticeable if the speech processor is turned on.
Additionally millivolts of ground voltage differential can exist between the computer chassis and the HF transmitter chassis. This can be exacerbated if the computer and transmitter are plugged into different AC power outlets.
· Plug both pieces of equipment into the same AC outlet strip. · Run a low-resistance electrical connection (heavy ground wire) between the chassis of the computer and the chassis of the transmitter to help reduce this voltage difference. |
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10000 |
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600 W
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600 ohm 1:1 transformer
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From soundcard line out
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To MIC input or
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patch input on radio
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(optional)
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