Abstract: A direct digital sampling and synthesis general purpose radio system is disclosed which employs single or multiple receiver and/or transmitter sub-systems that require no analog frequency conversion or translation circuitry. Receiver signal processing is disclosed that describes methods of conditioning and digitizing an entire received RF signal band in which the down conversion, channelization and demodulation are performed digitally. In addition, a method for direct synthesis of transmitter signals is also disclosed where up conversion and carrier modulation is performed digitally. Several mitigation techniques are described which aid in overcoming device limitations as well as overcoming problems created by combining multiple digital transmitters and receivers into a single integrated system. One embodiment of the invention describes an integrated VHF/UHF aircraft NAV/COMM radio system which combines a VHF transmitter with four VHF/UHF receivers all of which require no IF circuitry.

Abstract: A step attenuator for use in attenuating an electromagnetic signal. The step attenuator includes a first path having a plurality of attenuator structures provided therein, each attenuator structure being selectively actuated to permit the signal to pass therethrough. A second path is disposed in parallel with the first path, the second path permitting the signal to selectively bypass the first path. A third path is disposed in series with the first and second paths and includes at least one attenuator structure that is selectively actuated to permit the signal to pass therethrough.

Abstract: A TCAS receiver/transmitter which uses an improved calibration scheme for improving the ability of the device to transmit in a narrow directional field where the device uses the transmitter frequency source to generate signals for calibration purposes. The calibration uses a technique of simultaneously transmitting from two antennas and manipulating the phase of the signal through successive settings. The signals are monitored by the remaining two antennas and calibration adjustments are made in response thereto.

Abstract: A TCAS receiver, including a relative bearing measurement radio receiving apparatus for use with an antenna array having four antennas where each antenna has associated with it, its own receiver capable of demodulating both I and Q components of transmissions from an intruding aircraft.

Abstract: A digital video quantizer for use in TCAS and ATCRBS/SIF systems. A digital delay receives a digitized video signal and produces a plurality of delayed digital signals which are used throughout the quantizer. A threshold generator then produces a predetermined threshold and a dynamic threshold, and a quantized video generator compares one of the delayed digitized signals to a threshold to produce a quantized video signal. A slope quantizer detects a slope of the digitized signal which exceeds either a positive or negative threshold. In addition, a rise time detector detects an excessive rise time in the digitized video signal, and a chip amplitude comparator provides a signal which is used in detecting Mode S data bit values. The digital video quantizer's use of digital references rather than analog references, eliminates the inaccuracies caused by a reference voltage varying over time or temperature. The digital references also facilitate modification of the reference values at a later time.

Abstract: A receiver system for measuring the bearing of a target having ambiguity correction. The receiver system includes a four element antenna array, three receivers and two phase detectors. The system further includes switching means for selectively connecting the antenna elements to the receivers and the receivers to the phase detectors to obtain two pairs of bearing components, from which the target bearing is computed.

Abstract: A transmit system for collision avoidance systems having autocalibration. A four element antenna array is provided. Each element is selectively connectable to a transmitter through a power divider and a isolator. Three of the elements further have a programmable phase shifter in line to the transmitter. A phase detector having a first input selectably connectable to one of the elements and a second output connected to the transmitter is provided. By selectively connecting one element to the transmitter and one element to the phase detector and then selectively programming the appropriate phase shifters, the differentiated phase delay in each channel of components associated with an antenna element can be calibrated.

Abstract: A direction finding receiving system for use on a protected aircraft in which a signal from an ATCRABS transponder on an intruder aircraft is received has been described incorporating a four element interferometer type antenna array, reversing switches, individual receivers, a phase detector and a processor for processing the outputs of the phase detector. The invention overcomes the problem of correcting for the difference in phase delay of signals passing through the receivers.

Abstract: A d.c. power supply for computer logic circuits which is sensitive to brief interruptions in a.c. source power. The power supply includes an input transformer providing first and second a.c. voltages which are rectified to first and second d.c. voltage, the second d.c. voltage being of greater magnitude than the first d.c. voltage. A linear voltage regulator receives the first d.c. voltage as normal input and provides a third d.c. voltage of lesser magnitude than the first d.c. voltage. A storage capacitor is charged by the second d.c. voltage. A differential amplifier detects an interruption in source power and causes the storage capacitor to be connected to the input of the regulator whereby the regulator output is maintained during a brief interruption in source power by the discharge current of the storage capacitor.

Abstract: A wideband phase locked loop oscillator (PLLO) for tracking the frequency of an altitude signal in an FM/CW type radio altimeter. The phase locked loop oscillator conventionally includes a voltage controlled oscillator (VCO), a phase comparator for comparing the phase of the altitude signal to be tracked with the phase of the VCO output and an error signal amplifier for applying the output of the phase comparator as control voltage to the VCO. The frequency range of the PLLO is extended by changing the natural frequency of the VCO in steps spanning the useful frequency range of the altitude signal. Selection of the proper value of VCO natural frequency is effected through microprocessor directed digital control.

Abstract: A linear sweep frequency modulator for a varactor tuned transistor microwave oscillator used in an FM/CW radio altimeter where the oscillator-frequency v. varactor bias characteristic is non-linear. A linear triangular wave is converted by means of a function generator to the non-linear wave required for varactor bias to cause linear frequency modulation of the oscillator. Linearity of modulation is tested by measuring variations in the period of a calibration signal derived from a delay line. Variations in the period of the calibration signal are measured by determining the difference between the average and the instantaneous calibration period and sampling the difference during each cycle of the triangular wave, once during the upswing thereof when the oscillator is at low frequency and once during the down swing thereof when the oscillator is at high frequency and separately integrating the up swing and down swing samples.

Abstract: A radio altimeter of the FM/CW type which is linearly frequency modulated by a triangular wave exhibits step error in the output indication at times near the peaks of the modulating wave. Step error is eliminated by gating the data which is processed to produce the altimeter output indication to inhibit processing during times the modulation wave is near peak values and to enable processing at other times. Accuracy is further improved by adjusting the duration of the time during which data processing is enabled so as to equal an integral number of cycles of processed data.

Abstract: A filter for tracking the ground signal return in an FM/CW radio altimeter. The filter has a variable band pass frequency response which is adjusted as a function of the altitude measured by the altimeter. The control means of the filter respond to altitude measurements in digital format.