Harris Corporation

Luther L Crabtree

Guy M Pelchat

David K Campbell

William F Hartman

William P Osborne

To enhance the detection of a desired signal in the presence of interference, the degree of modulation of a received input signal is amplified. In the preferred embodiments, this is accomplished by amplifying the degree of amplitude modulation. This works especially well in situations where constant or substantially constant envelope jamming is employed. However, the arrangement is also effective in some cases where the jamming signal has a rapidly fluctuating envelope. The success of the technique in cases of rapidly fluctuating envelopes depends upon the probability distribution of the envelope.

Receiver having a modulation amplifier for enhancing reception of a

An automatic alignment system for use in a digital modem, having a performance indicator circuit associated therewith, adjusts the control voltages applied to selected alignment points therein in order to minimize the error indication output of the performance indicator circuit. The alignment system interfaces with a microprocessor and provides the necessary timing mechanism to generate write pulses which control the operation of respective processing circuits associated with the various alignment points in the modem analog unit. Each of the processing circuits in the modem analog unit includes a digital-to-analog converter, a data latch, and an amplifier which processes the output of the microprocessor interface to generate an analog control voltage.

Automatic alignment system for a digital modem

In a spread-spectrum receiver, a predetermined noise floor voltage, supplied together with the output of the correlation detector as a control voltage for an AGC amplifier, is subjected to a pre-signal reception adjustment or calibration, in order to compensate for drift in the noise floor or reference voltage and to correct for gain errors in the correlation detector channel thereby insuring proper operation of the detector during the sequential detection process. For this purpose, a prescribed random signal sequence, totally different from that contained in any useful transmission to be acquired, is initially applied to the correlation detector for a predetermined period of time or calibration interval. The output of the correlation detector is sampled and stored, successive samples are compared with the predetermined or fixed reference noise floor voltage, and successive adjusted noise floor reference values are produced, representative of differences between the noise level in successive correlation samples and the predetermined noise reference. These adjusted reference values are accumulated and this accumulated noise floor is fed back to the AGC amplifier as the noise floor control reference. At the end of the calibration interval, the receiver is switched to a normal operating mode and control of the AGC amplifier is determined by the noise floor reference level which has been adjusted or calibrated to the voltage represented by the accumulated noise floor level at the end of the calibration interval. Preferably, the calibration circuitry is implemented in digital form, with the duration of the calibration interval being controlled by a digital counter which counts clock pulses up to a prescribed number and then prevents further accumulations of noise reference values. The output of the counter may also be coupled as a control signal to cause a spread-spectrum signal acquisition PN sequence to be substituted for the random noise sequence which had been applied to the correlation detector during the calibration mode.

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Melbourne, FL, United States of America

Luther L Crabtree