A received, suppressed carrier, quadraphase shift key modulated (QPSK) signal is demodulated with a phase locked loop including a variable frequency, coherent reference that drives first and second channels also responsive to the QPSK signal. The channels respectively derive first and second replicas of binary signals that modulated the suppressed carrier. The replicas are combined to derive a variable amplitude error signal for controlling the coherent reference frequency. The frequency of the coherent reference is dithered at a low rate so that there is derived a relatively low level tracking error phase from the locked loop. The frequency of the coherent reference is swept when the phase of the error signal differs from the dithering phase by a predetermined value that is appreciably less than 90.degree..
A delay detection circuit and a low-noise oscillation circuit using such a delay detection circuit. When the oscillation output of a VCO is to be supplied to two high frequency mixers, a .pi./2 phase shifter 24 in the front stage of one of the high frequency mixers gives a quadrature form. High frequency components are removed from the output voltage of the high frequency mixers by LPF. DC components are also removed from the output voltage of the high frequency mixers by HPF. The output voltages of low frequency mixers contain secondary phase noise components proportional to squared phase noise components and primary phase noise components proportional to the phase noise components and also depending on the delay time .tau. of a delay unit. When the output voltage of one of the low frequency mixers is subtracted from the output voltage of the other low frequency mixer, the secondary phase noise components are offset and the dependence on the delay time .tau. is removed from the primary phase noise components. The resulting voltage is used as a control voltage for the VCO (10). No voltage control delay unit or feedback loop for controlling it are required.
The present invention provides a high frequency lock detecting circuit for generating a signal indicative of a locked or a not locked phase tracking condition in a phase locked loop circuit. The lock detector comprises a plurality of high speed function generators two of which are coupled to the modulated data streams for indicating the phase data streams and a third high speed function generator is coupled to the voltage error signal of the phase locked loop for indicating the absence or presence of a voltage error signal. The analog outputs of the function generators are summed together in a summing circuit and applied to a differential amplifier which removes the complex modulated data products from the output of the function generators and provides a signal which is equal to the absolute value of the data signals applied to the first function generators minus the absolute value of the error signal applied by the third function generator. The lock detection signal is applied to the sweep controls of the phase locked loop for controlling the sweep circuits associated with the voltage controlled oscillator.
Apparatus is provided for locking onto a severe doppler shifted data modulated carrier signal. A phase lock loop of the type having a data detection branch, a carrier tracking branch and a voltage controlled oscillator branch is modified to provide a summing circuit at the input of the voltage control oscillator in the voltage controlled oscillator branch. A sweep control circuit is connected to the input of the summing circuit for sweeping the voltage controlled oscillator through a range of frequencies which encompass the doppler shifted carrier frequency. An automatic frequency control circuit is connected to the input of the summing circuit for automatically disconnecting the sweep control circuit from the summing circuit when the frequency of the voltage controlled oscillator reaches a predetermined value defining a window which encompasses only the center frequency of the doppler shifted carrier frequency. The automatic frequency control circuit is adapted to drive the output of the voltage controlled oscillator to the center frequency of the doppler shifted carrier frequency without overshoot and then enable the phase lock loop to phase lock onto the phase of the doppler shifted carrier frequency to provide a locked on condition.
A carrier wave recovery apparatus of the decision feedback type having analog multipliers of lower dynamic range. The modulated signal is phase detected by comparison with a VCO signal. The analog multipliers receive a signal derived by subtraction of the phase detected signal from the same phase detected signal compared with a reference. That signal is multiplied in an analog multiplier by the phase detected signal or by the signal produced from comparing the phase detected signal with a reference.
An FM transmitter is disclosed wherein a solid state microwave source at a first frequency is frequency modulated with a baseband signal, and a second such source at a different frequency is frequency modulated with the baseband signal inverted. Each of the modulated carriers is frequency doubled and input to a mixer. The output of the mixer is a carrier, having a frequency which is twice the difference of the two microwave frequencies, and frequency modulated by the baseband signal. An automatic frequency control circuit inputs equal and opposite control voltages to the two microwave sources.
