The oscillation frequency of a variable frequency oscillator is swept by a sweep signal from a sweep signal generator and the oscillation frequency is compared with a preset sweep start frequency for detecting coincidence therebetween. Upon detecting the coincidence, the sweep operation of the sweep signal generator is stopped and, at the same time, the oscillation frequency of the variable frequency oscillator is made by phase lock loop means to be phase-synchronized with a reference frequency. When the phase synchronization is established, an error signal in the phase synchronization is held and the loop of the phase lock loop means is cut off and then the error signal is applied to the sweep signal generator to re-start the frequency sweep of the variable freqency oscillator. When coincidence is detected between the oscillation frequency of the variable frequency oscillator and a preset sweep stop frequency, the sweep operation is stopped.
Improved phase synchronization circuits shorten the length of the synchronization pull-in time. The phase synchronization circuit according to the first invention changes the free-running frequency of the voltage control oscillation means starting from one limit frequency of the sweep range when detecting a state of non-synchronization and performing a synchronization pull-in operation. The phase synchronization circuit according to the second invention changes the free-running frequency of the voltage control oscillation means starting to one limit frequency of the sweep range faster than a prescribed speed and then changes the free-running frequency of the voltage control oscillation means to the other limit frequency of the sweep range at the prescribed speed when detecting a state of non-synchronization and performing a synchronization pull-in operation.
A sweep voltage from a sweep voltage generator is provided to sweep start and stop adjusting means, the outputs of which are summed by a summing circuit and, by the summed output, the oscillation frequency of a voltage controller oscillator is controlled. The oscillation frequency of the voltage controlled oscillator and the output frequency of a reference signal generator set to a sweep start frequency are compared by comparing means. The output of the sweep voltage generator is put in a sweep start state and sweep start adjusting means is adjusted in accordance with the output of the comparing means, thereby to cause the oscillation frequency of the voltage controlled oscillator to agree with a set sweep start frequency. The output of the sweep voltage generator is put in a sweep stop state and sweep stop adjusting means is adjusted in accordance with the output of the comparing means, thereby to cause the oscillation frequency of the voltage controlled oscillator to agree with a set sweep stop frequency.
Non-linearities in the sweep output of a voltage controlled oscillator are automatically linearized by a scheme that compensates for changes in slope of the variation in frequency with time during a high accuracy calibration mode, prior to placing the VCO into real time operation. During the calibration mode the sweep rate of the VCO is slowed down as the operational frequency range of interest is divided into a prescribed number of subfrequency segments or windows. During each window, an input control code, obtained from a random access memory (RAM) and representative of the intended slope of the VCO frequency sweep (for that window of interest), is applied to a digital-to-analog converter (DAC) whose output is integrated and applied to the voltage control input of the VCO. Since the output of the DAC is integrated, the frequency variation control signal to be applied to the VCO is a slope change control signal, which is incrementally adjusted, as required, during successive sweeps of the VCO, so as to produce a precompensated control voltage that will cause the rate of change of the VCO frequency output to track an intended ramp (linear with time) or other chosen frequency variation output.
Digital apparatus receives digital data and outputs a ramp-like analog waveform. The digital apparatus comprises a reference digital word source, a variable digital word source, a testing circuit, and a D/A converter. The reference word and the variable word are tested for a predetermined relationship and the variable word changes until the predetermined relationship or condition is satisfied. The D/A converter receives the variable word and converts changes in same into either positive or negative slope ramps. The D/A converter output is constant during intervals where the variable word is not changing.
A method and apparatus for generating a sweep signal, the frequency of which is accurately known at all times during the sweep cycle. The beginning and ending frequencies are fixed by a phase-locked loop and the resulting error correction voltages are stored. During the sweep, these error voltages determine the offset and slope of the D.C. voltage ramp signal which governs the frequency of the sweep oscillator.
