Single-Mode Inversion Voice Scrambler/Descrambler

Originally printed in Circuit Circus, by Charles D. Rakes.

This month we are going to take a very popular IC and use it in several circuits in the hope that at least one of them will tweak your interest enough for you to heat up your soldering iron and join in.  We're going to spotlight on Signetics' NE602 low-power VHF double-balanced mixer.  That 8-pin chip features a built-in local oscillator, a differential input amplifier, and a voltage regulator.  The internal oscillator circuit will operate up to 200 MHz with either an external crystal or tuned-tank circuit.  Its input and output resistance is about 1.5 k with an input capacitance of only 3 pF.  The IC requires less than 3 mA of current with a supply voltage from 4.5 to 8 volts.

After looking over the NE602's specification sheet, one of the applications that came to mind was how well the IC would perform as a balanced modulator/demodulator in a single-mode inversion voice scrambler circuit.  Excellent results were obtained with the voice scrambler/descrambler circuit shown.

A LM567 tone decoder operates as a carrier oscillator supplying a 2.5 kHz to 3.5 kHz audio square-wave to the NE602's oscillator input at pin 6.  A voltage divider reduces the square-wave signal to about 600 mV peak-to-peak.

The two mixer outputs of the NE602 (at pins 4 and 5) are coupled through a 1 k to 8-ohm audio-output transformer, for a balanced output that provies maximum attenuation to the input and local oscillator signals.  A LM386 low-power audio amplifier is used to increase the descrambled output sufficiently to drive a small 4- or 8-ohm speaker.

When a single-mode inversion signal is fed to the input of the circuit and the carrier oscillator is tuned to the original encoded frequency, the audio is re-assembled to its normal condition and sounds like a properly tuned single-sideband radio signal.  Actually, the scrambler will operate either as a scrambler or a descrambler.

It just depends on what type of audio signal (scrambled or unscrambled) is fed to the circuit.  When normal voice is applied to the input of the circuit, the circuit outputs an encoded signal.  If the scrambled output is recorded and fed back into the circuit, the circuit outputs a decoded signal.  You could use a cassete recorder with the scrambler/descrambler to give your personal notes a degree of security without spending a bundle for a commercial system.

Schematics

Notes & Datasheets

Can also be used to scramble radio communications.  Connect between the microphone input and speaker output of your radios or the audio input of your surveillance bug.  Will only provide some degree of security to casual eavesdropping, as this method is fairly easy to defeat.

The scrambler/descrambler can also be used with your surveillance bug to defeat certain audio-correlation bug detectors/locators.  Since the audio output of the bug doesn't match the correlator's speaker output, there is no audio match.