Two-port synthesizer modulation system employing an improved reference phase modulator

A method of two-port frequency modulating the output of a phase-locked loop frequency synthesizer providing an essentially flat modulating frequency response down to less than 1 Hertz with very low distortion and independent of the loop bandwidth, said method comprising the steps of:

• producing a high stability unmodulated reference signal at predetermined frequency;
• generating an output signal in a voltage controlled oscillator which has its output frequency proportional to an applied DC input voltage;
• feeding back a portion of the generated output signal, detecting a phase difference between said reference signal and said output feedback signal, and producing an output corrected signal for maintaining the voltage controlled oscillator at a desired frequency;
• and
• introducing modulation balanced between a first and second port of the phase-locked loop frequency synthesizer, said first port for frequency modulating the output of the voltage controlled oscillator, and said second port for frequency modulating the predetermined frequency of said high stability unmodulated reference signal down to less than 1 Hertz so as to effect equal magnitude and polarity frequency deviation with respect to said voltage controlled oscillator whereby the resultant modulation effected by each of said ports is cancelled during the step of detecting the phase difference between said reference signal and said output feedback signal.

An improved two-port frequency modulating arrangement for a phase-locked loop frequency synthesizer, comprising in combination:

• unmodulated reference source means;
• means for phase detecting having a first input coupled to said unmodulated reference source means and an output coupled to an included loop filtering means;
• direct-FM VCO means having an input port for coupling a modulating signal and the loop filter thereto and having an output which couples to an included selectable-integer frequency dividing means in a feedback path to a second input of said phase detecting means;
• and
• a modified frequency modulator having an input port for the modulating signal and interposed between said unmodulated reference source means and said phase detecting means, said modified frequency modulator exhibiting a flat, wide, modulation bandwidth, down to at least 1 Hertz, which effectively cancels modulation components induced by the direct-FM VCO, thereby providing a modulation bandwidth for the frequency synthesizer which is nearly equal to the theoretical maximum available bandwidth.

A low-noise phase modulator apparatus, comprising in combination:

• comparator means having a first input coupled to a modulating input signal;
• a capacitive storage element with a first terminal coupled to a second input of said comparator and its second terminal connected to common ground;
• an electronic switch with first and second terminals coupled to the respective terminals of said capacitive storage element and with a third terminal coupled to a reference frequency for providing control thereof;
• and
• a passive current source, including a filter section and a current-limiting section, coupled between a supply voltage and said second terminal of the comparator,
• said a passive current source providing reduced low frequency noise and cooperating with said electronic switch to provide a nearly linear ramp voltage on said capacitive storage element.

An improved two-port frequency modulating apparatus for a phase-locked loop frequency synthesizer which has a relatively low frequency RF carrier with relatively high percentage deviation frequency modulation at its output, comprising in combination:

• unmodulated reference source means;
• phase detecting means having a first input coupled to said reference source means and an output coupled to included loop filtering means;
• direct-FM VCO means having an input port for coupling a modulating signal and the loop filtering means thereto and having an output which coupled to an included selectable-integer frequency dividing means in a feedback path to a second input of said phase detecting means;
• offset mixing means having a first input coupled to said direct-FM VCO means and a second input coupled through multiplying means to said unmodulated reference source;
• and
• a modified frequency modulator having an input port for the modulating signal and including an integrator and a low-noise phase modulator,
• said modified frequency modulator interposed between said unmodulated reference source means and said phase detecting means and exhibiting a flat, wide, modulation bandwidth, down to at least 1 Hertz, which effectively cancels modulation components induced by said direct-FM VCO means, and said offset mixing means for down mixing the output of said direct-FM VCO means, thereby providing a relatively low frequency RF output carrier with relatively large percentage deviation and providing a modulation bandwidth for the frequency synthesizer which is nearly equal to the theoretical maximum available bandwidth.