Dr. Scott M. Baker's 1970s Ham Radio Gets a Big Upgrade, Courtesy of a Raspberry Pi DDS-VFO
Engineer and radio ham Dr. Scott M. Baker has created a significant upgrade for a classic 1970s amateur radio, turning a Raspberry Pi single-board computer into a high-accuracy Direct Digital Synthesizer Variable Frequency Oscillator (DDS-VFO).
"My Drake R-4C/T-4XC Amateur (Ham) Radio has been a lot of fun," Baker explains, "but it can also be a lot of work keeping it on frequency. The mechanical nature of the VFO/PTO assembly I think lends itself to frequent drift as temperatures shift, or as the voltage inside the receiver changes."
The result of these shifts: a radio that has a tendency to drift away from a target frequency and which is harder than it should be to dial-in in the first place. The solution: adding a little modernity, in the form of a Raspberry Pi Zero W.
The resulting Direct Digital Synthesizer Variable Frequency Oscillator (DDS-VFO), connected to the Drake radio through its injection line, is built from a Raspberry Pi, or a lower-cost Microchip ATmega328 as an alternative, an Analog AD9850 DDS chip, and a handful of extras — including a 3D-printed case with control buttons and a rotary encoder dial driven by a high-accuracy Bourns optical encoder.
"The DDS-VFO is driven by four lines: clk, load, data, and reset," Baker explains. "It works like a shift register. You clock in your data and then hit the load line to load the data and make it active. The DDS-VFO outputs a waveform that we couple with a capacitor and then send to an optional Butterworth filter. After the Butterworth filter there are a pair of output drivers. Each one uses a 10k pot for level control and then a two-transistor driver using 2N2222 transistors.
"I power the AD9850 module with 3.3V. The 7812 regulator is option[al]. If you plan to power this with 12V, then just wire the 12V regulator across. The mute input can be used to implement separate transmit and receive frequencies. When the T-4XC goes to transmit, it’ll ground the mute line. We could detect this, and switch frequencies. I don’t implement that, yet."
Full details of the project are available on Baker's blog; the Python script running on the Raspberry Pi, providing control from both the physical keypad and a web server if built using the Raspberry Pi option, is available on GitHub under an unspecified open source license.