Radio Modem Circuit
This radio modem is popular for amateur radio packet application. This radio modem is powered by the data and control lines, so it need no additional power supply. Just plug to your standard RS232 port and this modem will work. As seen in our previous modem circuit, this radio modem circuit also uses TCM3105 modem chip from Texas Instruments.
To align the modem, using an oscilloscope, connect it to the receiver and open the squelch on an unused channel and watch the waveform on pin 6 of the 74HC04. Adjust the 47K pot so that the noise is centered on the screen and not all at the top or bottom of the screen – i.e. the noise should be just as much positive 5v as it is 0v. If you do not have an oscilloscope then adjust the 47K for a DC voltage of 2.5v on pin 6 of the 74HC04.
[Schematic diagram source: Harry Lythall]