Introduction, SS70A Circuit Description

INTRODUCTION

The Ramsey SS70A is a complete two-way speech scrambler/descrambler with suitable inputs and outputs for scanner, two-way radio, cordless telephone, or tape recorder operation. It is designed to help the user secure total privacy for communications or recorded memos. Just as its scrambling, technically called “speech inversion”, can be decoded by knowledgeable users of similar equipment, the SS70A in turn can be used experimentally to descramble some public service communications, cordless phones and other systems where low cost scrambling is employed.

Similar to the regulations affecting the use of mobile VHF scanners, it is the responsibility of the user of this equipment to know and observe any applicable laws regarding the interception and descrambling of transmissions employing speech inversion.

“Speech inversion” is a process that interchanges high and low speech frequencies by removing the carrier frequency and transmission of only one sideband in a communications link. This renders the speech unintelligible unless received by a device capable of replacing the carrier frequency exactly. The single inversion scrambling technique used in the SS70A is adequate to minimize casual eavesdropping or ambulance chasing. Greater security is possible, using precision audio filters, and by inverting more than one audio range simultaneously.

SS70A CIRCUIT DESCRIPTION

Radio amateurs will notice some similarity between the SS70A and a basic sideband transmitter. However, in the speech scrambler’s case, the audio input is mixed with a local oscillator running right in the audio range (3.3 KHz) rather than an RF oscillator, and it is precisely that modulation of such a low frequency that results in speech inversion. Let’s look more closely at what happens in the circuit. Have a glance at your schematic and follow along.

At the heart of the speech scrambler circuitry is the MX128 full-duplex frequency inversion scrambler IC. This chip contains a balanced modulator and several digital audio filters for both transmit and receive audio paths. In addition, the 3.3 KHz injection frequency is generated within the MX128. J3 provides selection of either a 10.24 MHz or 3.58 MHz crystal.

Notice the similarity between the transmit (TX) and receive (RX) circuit paths. Both inputs are "limited" by the overload protection diodes D1,2,3, and 4. An additional "voltage divider" (R1,R3) is used to reduce the RX input level when speaker level input path is selected. The TX path is designed for microphone level inputs/outputs.

SS70A • 4