Circuit Description; Introduction To The Ss70A - Ramsey Electronics SS70A User Manual

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