Basically the Signalink and it's soundcard are like a modem. The information encoded into audio tones is fed to the transmitter by this device and ultimately radiated out into the ether. The shielded USB connection cable is not visible but it's there plugged into the back of the unit and drops down behind the table top. Here is where things get interesting and more confusing. The TX knob on the Signalink controls the amout of audio "drive" to the radio. To provide the most efficient and "clean" signal this drive has to be carfully controlled. The TX knob on the Signalink works in conjuction with the radio's ALC or Automatic Limiting Control. ALC limits the RF drive level to the power amplifier during transmit to prevent distortion. To adjust for proper operation I switch my rigs output coax to a dummy load. This is just a giant resistor that absorbs the RF signal and bleeds it off as heat instead of sending it on to an antenna. While transmitting a PSK signal I watch the ALC meter on the rig. If the TX knob on the Signalink is turned up too high one or more bars will show on the meter. Backing off the TX knob until no indication of ALC is evident on the meter lands you in the sweet spot and the ideal balance of drive power is met. This is why I like the Signalink USB. My laptop's operating system is quirky enough and I think it would be a real pain to minimize fldigi (digital mode software) and go to the control panel window and jump through those hoops just to make a small adjustment to the soundcard drive.
That's it! Once the ALC conditions are met and the output is switched back to an antenna all systems are go for digital radio fun.
Click on the picture for a close up view and look at the yellow signal on the waterfall display. You will notice two fine red lines that straddle the yellow strip. This is the tuning indicator and is movable by using the mouse. If you see a signal and click on it the two red lines will jump to that spot and the software will begin decoding. The same applies when transmitting. Find a clear space on the waterfall, click on it and where the red indicator rests shows the exact frequency of the transmitted PSK signal.
Last night I spotted a bright PSK signal on 30 meters. I copied the callsign that belonged to the trace and when the QSO was over I gave Jean VE2GHI in St Georges, Quebec a call. He responded and we had a pleasant textbook PSK-31 QSO. I am very proud of the unsolicited comment that Jean made concerning my signal. The tricky part with the digital modes is that you cannot see your own signal on the waterfall so you can't tell if it is overdriven. This is why monitoring the ALC is so important. With the system properly adjusted 15 watts is more than adequate to send a good clean signal just about anywhere.