Low-band AM Tx: Digital VFO/Controller

Lockdown has made 2020 a year unlike any other. Melburnians were dealt a long and painful period of isolation with a CoronaVirus second wave, from around July to September, still in force, and looking like continuing to (at least partially) keep us housebound for some months yet. Days merge together, work and leisure time is largely indistinguishable. People are rediscovering reading, knitting, and endlessly bingeing TV. Makers are melting solder and stringing wires in the air.

Some winter nights were passed dreaming up an AM transmitter, and this VFO module is the first piece of it. It is designed for use in a two-band AM transmitter capable of around 200 watts carrier power, but could easily be used for other projects, such as a transceiver, or a transmitter for CW or digital modes. This module provides a 5v square wave clock at 1.8MHz or 3.5MHz (or any frequency you desire up to 144MHz), and transmit control lines needed to sequence a transmitter. It also includes a few ‘nice to haves’ including a transmit timer, CW ident, over-beep (or any CW character such as di-da-dit or da-di-dah), and a sleep mode which dims the displays when idle.

Schematic

160-80-transmitter_Page1-VFODownload

Code

The code for Arduino is here.

Notes

The video narration covers the functional features of this VFO and controller unit. It will become the heart of a 200 watt AM transmitter for 160 and 80 meters. This is a slower-paced Corona-lockdown project, so there’s no big rush to finish this project! I’d picked up some of the 0.56 inch 7-segment LED displays, very cheaply, and was looking for an excuse to try them out. They are large and bright, and it is an interesting challenge to use the 7-segment character segments and tyhe brightness and blink controls to convey the kinds of information that a transmitter needs to communicate.

If you want to reproduce something like this, you will no doubt want to use your own variations, and you should have no problems tweaking the code to do what you want. Drop a comment in below if you want to discuss any aspects of this unit. The gallery below presents some angles on the module.

The module is built on an aluminium angle and sheet half-chassis.

A collection of components and breakout boards — Arduino Nano, si5351 breakout, RTC, MCP9809 precision temp sensor, control logic, VFO clock buffer/squarer.

In transmit mode, right display alternates between over timer and temperature.

Map of the display segments to allow new characters to be coded in ‘raw format.

Nano and si5351 breakout.

Clock buffer MMBT3904 and components.

12v DC switching circuits for band change relays, T/R relay, PWM enable lines.

Displays have a clear front face so they can be mounted flush with the aluminum angle front panel.