Northrop Grumman Systems Corporation

Motorola Corporation

Trw Limited

Northrop Grumman Corporation

Michael H Myers

Sam M Daniel

An apparatus and method for generating an envelope predistorted radio frequency signal which avoids undesirable spurious emissions. A complex baseband signal, having an in-phase component I and a quadrature component Q, is sampled and filtered in a sampling circuit and filter circuit to obtain samples Iof the in-phase component and samples Q, the quadrature component. The magnitude xof each sample pair is determined in a first calculation circuit. An amplitude and phase distortion factor D, based on scaled values of the archyperbolic tangent and the hyperbolic tangent of the baseband sample magnitude is determined in further calculation circuit and a multiplier. Each sample Iof the in-phase component and Qof the quadrature component is multiplied by the corresponding distortion factor D, and the resulting predistorted components combined and upconverted to provide a predistorted baseband signal which is amplified in a power amplifier having hyperbolic tangent distortion.

Bandpass predistortion method and apparatus for radio transmission

A radio receiver () adapted to receive an AM-MSK modulated carrier signal includes an envelope detector () and a transformer () which restores the real signal to complex MSK format. A Doppler remover () removes any Doppler shift of the MSK due to relative motion of the transmitter and receiver (). A sample rate reducer () may reduce the sample rate used in the Doppler remover () to the transmitted symbol rate used during the modulation process.

Radio receiver AM-MSK processing techniques

A compensation circuit for phase modulation systems, such as QPSK and QAM systems, which compensates for phase errors in the I and Q components of the QPSK or QAM signals to minimize carry over of such phase errors in analog up and down conversions of such signals. In particular, the invention relates to a relatively simple circuit, which compensates for channel phase errors by providing a direct correction of one of the channels based on the measured correlation between the I and Q components, which should ideally be 0. As such, cross talk between I-Q channels is minimized, which improves the signal-to-noise ratio of transmitted and received QPSK or QAM signals.

Quadrature error reduction for QAM modulated up or down conversion

A steerable antenna array ( ) includes a plurality of antenna elements ( ) which receive broad band signals and narrow band interfering signals which together form antenna signals. The antenna signals are combined with weighting signals by combining functions ( ) to form a combined signal, which is filtered by a filter function ( ) to filter the narrow band signals. The filtered signals are sent to a weight processor ( ) which also receives the antenna signals as an input. The processor ( ) generates the weighting signals so that interference from the narrow band signals is reduced.

Steerable antenna interferer rejection techniques

An apparatus for and a method of determining the envelope of a complex baseband signal having an in-phase component I and a quadrature component Q by determining (I &plus;Q ) for sampled values of the baseband signal. The maximum and minimum values of I and Q are detected by detecting the larger of I and Q in a first detection circuit ( ) and the smaller of I and Q in a second detection circuit ( ). A value x&equals;&frac12;&lcub;(the detected minimum value)&divide;(the detected maximum value)&rcub; is calculated in a first calculation circuit ( ). The value of (I &plus;Q ) is then calculated based on the value of x. In a first embodiment the value of (I &plus;Q ) is calculated in a second calculation circuit ( ) as the (the detected maximum value)&times;(1&plus;x)/2&plus;&frac12;(1&plus;x&minus;x &plus;x &minus;x &plus;x &minus;x ). In a second embodiment, the value of (I &plus;Q ) is calculated in a second calculation circuit ( ) as (the detected maximum value)&times;(1&plus;2x&plus;x /2)&divide;(1&plus;x). Preferably, the apparatus is implemented in a gate array, such as a field programmable gate array, providing high speed operation.

Complex baseband envelope computation

Partial frequency offset digital upconversion

An apparatus for and a method of generating an envelope predistorted radio frequency signal. A complex baseband signal, having an in-phase component I and a quadrature component Q, is sampled and filtered. The magnitude u of each complex baseband sample pair is determined by the square root of the sum of the squares of the in-phase component sample and the quadrature component sample. A distortion factor D , is determined based on a scaled value of the inverse hyperbolic tangent or archyperbolic tangent (&ldquo;atanh&rdquo;) of the baseband sample magnitude divided by that scaled sample magnitude. Each sample of the in-phase component I and of the quadrature component Q is multiplied by the corresponding distortion factor D so as to provide predistorted components. The predistorted components are then combined to provide a predistorted baseband signal.

Bandpass predistorting expansion method and apparatus for digital radio transmission

Demodulating a DPSK encoded data signal. The received signal has a symbol tracking value determined in a synchronization circuit. Doppler error in the signal is estimated. The signal, the symbol tracking value, and the estimated Doppler error are applied to a demodulator within which the signal is multiplied by a signal at a frequency which is the negative of the estimated Doppler error to remove that Doppler error. The resulting signal is filtered and decimated and normalized to generate the arctangent of the in-phase and quadrature components of the baseband signal. This arctangent signal is stripped of phase angle modulation and applied through loops to remove derivatives of the input phase signal. The arctangent and the resulting angles are mapped in a nearest neighbor map circuit, and the difference between successive angles is applied to a gray code to binary mapping circuit which detects the encoded data.

Differential phase demodulator incorporating 4th order coherent phase tracking

A radio carrier signal is detected ( ) after being converted to digital form and is filtered ( ). A noise estimator ( ) generates a noise estimate signal by estimating the noise in the detected signal due to atmospheric conditions and the noise due to the gain in noise figure properties of the circuitry. The output of the noise estimator is used to calculate a threshold signal ( ) and another detection operation ( ) determines whether the power of a signal derived from the detected signal exceeds the threshold. If the threshold is exceeded, the detected signal is passed through a signal conditioner, such as a switch ( ) to an output path ( ).

Radio squelch techniques

An automatic frequency control loop implemented by a digital signal processor ( ) down converts an IF signal carrier to a residual-carrier within an allowed range of frequencies by a down converter operation ( ). A frequency detector ( ) detects any error frequency in the residual carrier with reference to a reference frequency, such as 0 Hz, in order to shift the frequency of the residual carrier by a shifter operation ( ) which reduces the frequency error in the shifted signal. An envelope detector operation ( ) retrieves information from the residual carrier, and the resulting signals are processed by a low pass filter operation ( ) to reduce cross talk.

Radio receiver automatic frequency control techniques

An aviation radio receiver provides automatic gain control in three zones including an RF zone employing an RF attenuator ( ), an intermediate frequency zone employing an intermediate frequency logarithmic attenuator ( ) and a baseband zone implemented in a digital processor ( ).

Radio receiver automatic gain control techniques

A radio receiver ( ) processes a carrier signal having a Doppler shifted frequency and carrying a coded signal having variable phase and amplitude. The coded signal is divided into a preamble portion and a message portion. A memory ( ) stores samples of the coded signal representing at least some of the predetermined symbols of the preamble. A logic unit ( ) divides the stored samples into a plurality of sample groups and identifies one sample group having the minimum variation of envelope amplitude. That sample group is used to provide additional samples by which the Doppler shift of the carrier frequency is estimated ( ). The coded signal is delayed by a delay operation ( ) and has its Doppler shift reduced. The Doppler reduced coded signal is processed by a filter operation ( ) and a demodulator ( ).

Radio symbol synchronization and doppler estimation techniques

The non-linearity of a power amplifier (,) and a radio frequency variable attenuator (,) are reduced by providing multiplying factors generated by a signal processor (,) and applied to the multiplying function of the attenuator (,). As a result, an input signal can be transmitted with increased accuracy which meets FAA standards for aviation radio transmission.

Radio transmitter distortion reducing techniques

A method for generating minimum shift keying (MSK) modulation signals with relatively reduced energy sidelobes relative to known methods. In one embodiment, a second derivative of the phase waveform is taken and convolved with a selectable smoothing function and integrated twice to generate a smooth phase waveform S2D.sub.-- MSK (i)=where .o slashed. (i)=.pi. i d(i)/2T. In the alternate embodiment, the sidelobe energy can be varied by simply changing the smoothing function, which can be stored in a lookup table. In the embodiment, the MSK modulation signal has a constant amplitude envelope allowing it to be transmitted using class C amplifiers. Since the embodiment provides continuous phase derivatives, the frequency and thus the sidelobe energy is sufficiently reduced to minimize interference with cosite equipment and nearby communication channels bands.

Direct 2nd derivative synthesis method for controlling sidelobe energy

An input signal is multiplied by a reference and quadrature signal and the two output signals are integrated and multiplied by a ramp signal, having a period equal to the maximum period of the input signal, and integrated to provide four output signals. The integrated product of the input, reference and ramp signals is multiplied by the integrated product of the input and quadrature signals and the product is subtracted from the product signal of the integrated product of the input and reference signals multiplied by the integrated product of the input, quadrature and ramp signals. The difference output signal is the gradient of the envelope of the maximum posterior-probability estimate of the frequency of the input signal.

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Poway, CA, United States of America

Michael H Myers