01 May, 2010

Pixie sidetone oscillator and reduced broadcast interference

This circuit provides a sinusoidal sidetone oscillator for the Pixie. As an added benefit, it will also cause reduced broadcast interference, since the muting circuit of the Pixie is a major source of broadcast interference as the LM386 goes into some sort of nonlinear mode and acts as an old-fashioned crystal detector.

The sidetone is generated by a standard phase-shift sidetone oscillator copied from G4GVM's FOXX-3 circuit in SPRAT 98. The transistor can be any general purpose NPN transistor with a fairly high current gain like BC109, 2N2222 or 2N3904. The variable resistor sets the sidetone level, start with it in the mid position or with the wiper even closer to the top (lower output) and adjust for a comfortable level. My circuit produces a sidetone of about 1100 Hz.

The modifications required for the Pixie are:
  • Connect the key line and the Vcc to the sidetone circuit. 
  • Connect the output of the sidetone circuit to pin 7. The original Pixie has pin 7 free, but some variants of the Pixie have pin 7 bypassed with a capacitor to ground. If it is only 0.1 uF as in the Knightlite SMiTe circuit, the capacitor may be kept there, but the oscillator load may become too large so the oscillator may stop working as the pot's arm is moved towards higher amplitude. Therefore it is better to remove the bypass capacitor.
  • Remove the muting circuit of the LM386 by removing the diode from pin 6 to the key, and by reducing the 1K series resistor from Vcc to pin 6 to a smaller value, say 10-47 ohms. In some cases it can just be shorted out, but at the risk of oscillation in the LM386.
  • Optional click reduction can be achieved by reducing the input coupling capacitor for pin 2 of the LM386 from 0.1uF to 0.022 or even 0.01uF.
This works successfully in my 80 m Pixie.

More on this subject as well as a simple way to mute the LM386 was published in SPRAT No 113 (Winter 2002) pages 18-20: “Using pin 7 of the LM386 to reduce BCI and add side tone to Pixie 2”.

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