Whisper Sidetone


Here is a easy way to monitor your CW transmission on those simple rigs like the homebrew 49er and Pixie that don't have sidetone. Just three parts. The emitter-base junction of an NPN transistor is reverse biased into avalanche condition by exceeding its breakdown voltage and limiting the current with a high value resistor. The resulting noise signal is then fed to the audio amplifier through a blocking capacitor.

Practical use

The schematic shows the circuitry necessary to add Whisper Sidetone. Change the value of the resistor to achieve the best sound from your particular transistor and voltage source. At high impedance levels the capacitor value is not critical. 0.1uF should suffice.

Breakdown voltage varies between transistors, often within the same production run, so pick through available transistors for the lowest avalanche voltage. Mine vary between 7.5 volts and 8.8 volts so be sure to pick a supply voltage that exceeds what your device requires.

In the Forty-9er [1] I inserted the new sidetone into the amplifier input at the junction of C9 and RFC2. In that rig I used a 100K ohm resistor and a .01 uF capacitor. Instead of ground, I connected the key line to the base. Remember to leave the collector open since you're using the transistor as a diode.

The best transistors seem to be silicon NPN planar passivated switching types like the 2N2222, PN2222, 2N3904, or 2N4401. Experiment with what you have.


If you reverse bias a diode past its breakdown voltage it will start to conduct. Valence electrons will be ripped from their atoms. In their headlong flight through the semiconductor structure they knock more of their fellows from their positions in the lattice to create an avalanche of current bursts. If you limit the current sufficiently then the junction will not be destroyed by heat and can avalanche indefinitely.

Avalanching diodes produce predominately shot noise. Depending upon the design of the diode structure the shot noise amplitude can be essentially the same from below audio frequencies to well into the microwave spectrum. Shot noise is much stronger than thermal noise (which is produced by anything warmer than absolute zero).

A third form of noise is called “flicker noise”. This phenomenon occurs at low frequencies, mostly below 1 Megahertz. The amplitude drops as the frequency increases so it is sometimes referred to as “inverse frequency (1/f) noise”. By choosing the proper limiting resistance you can increase the ratio of flicker noise over shot noise by as much as 5 dB at frequencies below 3 KHz. I found that with very low avalanche currents the noise was a low rushing sound, a larger low frequency component prevailing.

A sampling oscilloscope shows varying amplitude especially around 4 microamps current through a PN2222a emitter-base junction. As current is increased more of the samples return as higher voltage levels until, at currents above 25 microamps, the gap between the zero-voltage baseline and the peak voltage excursions nearly clears. Almost as if it were saturating. Measurements with voltmeters and a Tektronix 5L4N low frequency spectrum analyzer confirmed that as current levels increased the ratio of low frequency noise signal decreased until it was equal to the shot noise.

"Random” noise refers to the randomness of timing between events but this is frequency domain. Shot noise is nearly random over a wide spectrum, bound by the chemistry and geometry of the generating device junction and parasitic reactances. These noise bursts are nearly all the same amplitude, varying mostly due to the additive function of multiple concurrent events.

Flicker noise includes more of an amplitude component. At low current levels singular avalanches result in noise pulses of random amplitude in addition to random timing. They appear to have a Gaussian distribution with a maximum amplitude nearest direct current. As current increases the shot noise becomes stronger and predominates.

The sound can be quite pleasant, more like a whisper than a hiss

Sounds nice. Three parts and just a few microamps. Real minimalism.

de ND6T


[1] Link to N6KR's NorCal Forty-9er documentation: http://www.norcalqrp.org/files/49erOrigMnl.pdf

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