Spread spectrum chip rate tracking system

Abstract

In a new receiver design, the specific transmitter unit is unchanged from a prior patent application but may also be substantially as described in the prior art. The transmitter is initially activated by the input of the pseudo-random generator code. This code may be any alpha-numeric series, but must be identical to that input to the receiver. When the code is input through the keypad, the transmitter uses internal algorithms to generate a unique shift-register feedback combination. This creates a pseudo-random (PN) code which repeats after a programmable number of bits, depending upon desired maximal signal correlation time. That is the longer the code the longer the correlation time. This PN code sequence is clocked at the desired clock (CHIP) rate inside the transmitter unit. The PN code sequence which is thus generated is then added in phase to the data which is a substantially lower rate than the CHIP rate. After mixing with the PN code and data, the RF frequency CW signal power is spread across a frequency band equivalent to twice the PN code clock rate. The signal power density per hertz is now well below an identical non-spread signal at the communications data rate and ideally below the systems noise level. At the receiver the same pseudo-random code which was used to transmit the information is entered into the receiver unit for activation. This is then used to configure three programmable PN sequence generators which output three identical PN code sequences which vary only in a one-half phase shift of the CHIP rate from each other. The signals received by the antenna are converted to an intermediate frequency and mixed with the locally generated PN code sequences. When code alignment occurs, threshold detector circuitry senses the presence of the correlated data signals. The relative level of the signals are then sensed by the microprocessor which varies the PN clock oscillator to maintain lock with the incoming signal. The data is now available as useable information at the data port.