19 October, 2020

Longwave AM transmitter based on the XR-2206

Some years ago I described some AM transmitter experiments for the longwave band from 150 to 300 kHz. A transmitter that worked over a few meters was based on an XR-2206 function generator IC. Instead of building it from scratch as that first article indicated, I ended up purchasing and modifying the "Geekcreit® XR2206 Function Signal Generator DIY Kit Sine Triangle Square Output 1Hz-1MHz" from Banggood. 

That kit is not intended for AM modulation as the modulation input, pin 1, is grounded. It therefore needed these modifications:

  • Cut the connection from pin 1 of the XR-2206 to ground on the reverse side of the PCB.
  • Add an audio input circuit for pin 1 that mixes the left and right inputs from a minijack. It consists of pin 1 connected to a coupling capacitor of 100 nF in series with one 1K resistor to the left input connection of a minijack and one 1 K resistor to the right connection.  The minijack input plug can be seen on the upper right in the image. This input circuit is taken directly from a circuit by PA2RMX from 2012.
  • Add DC bias for pin 1 with 22 k to ground and 47 k to Vcc (pin 4) on the reverse side of the PCB.
Run the function generator from 9-12 VDC with sinusoidal output and frequency in the 65kHz - 1 MHz range, connect a mobile phone to the minijack plug, and a suitable wire as an antenna to the sinusoidal output. I used the square output as an input to a frequency counter in order to set the frequency.

Hopefully this simple circuit can inspire others!

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