Phase-locking circuit for swept synthesized source preferably having stability enhancement circuit

In a low-frequency synthesizer comprising a voltage controlled oscillator (VCO) having a tune input and an output;

• frequency divider means having a control input, an input connected to the output of the VCO, and an output;
• reference oscillator means having an output;
• phase detector means having a first input connected to the output of the reference oscillator means, a second input connected to the output of the frequency divider means, and an output;
• first integrator means having an input connected to the output of the phase detector means and an output;
• second integrator means having an input connected to the output of the first integrator means and an output;
• a passive lag-lead network having an input connected to the output of the second integrator means and an output connected to the tune input of the VCO;
• and controller means connected to the control input of the frequency divider means;
• the improvement comprising:
• a phase-lock loop stability enhancement circuit connected between the input and the output of the lag-lead network.

A swept synthesized source comprising:

• tunable oscillator means having a tuning input and an output;
• low-frequency synthesizer means having a control input and an output, the low-frequency synthesizer means comprising:
• a voltage controlled oscillator (VCO) having a tune input and an output;
• frequency divider means having a control input, an input connected to the output of the VCO, and an output;
• first reference oscillator means having an output;
• first phase detector means having a first input connected to the output of the reference oscillator means, a second input connected to the output of the frequency divider means, and an output;
• first integrator means having an input connected to the output of the first phase detector means and an output;
• second integrator means having an input connected to the output of the first integrator means and an output;
• a passive lag-lead network having an input connected to the output of the second integrator means and an output connected to the tune input of the VCO, and a phase-lock loop stability enhancement circuit connected between the input and the output of the lag-lead network;
• frequency sampler means having a first input connected to the output of the tunable oscillator means, a second input connected to the output of the VCO of the low-frequency synthesizer means, and an output;
• second reference oscillator means having an output;
• second phase detector means having a first input connected to the output of the frequency sampler means, a second input connected to the output of the second reference oscillator means, and an output;
• signal conditioning means having an input connected to the output of the second phase detector means and an output;
• electronic pretuning means having a control input and an output;
• switching means having a control input, an output connected to the tuning input of the tunable oscillator means, and at least three positions, including a first position in which the tuning input of the tunable oscillator means is connected to the output of the signal conditioning means, a second position in which the tuning input of the tunable oscillator means is connected to the output of the electronic pretuning means, and a third position in which there is no connection to the tuning input of the tunable oscillator means;
• and
• controller means connected to the control inputs of the frequency divider means of the low-frequency synthesizer means, electronic pretuning means, and switching means for generating control signals;
• the controller means initially providing a control signal to operate the switching means to the third position so that energy stored in the tunable oscillator means is dissipated;
• whereby hysteresis effects in the operation of the tunable oscillator means are eliminated.

A method of operating a swept synthesized source, comprising the steps of:

• initially zeroing the tuning current fed to a tunable oscillator means included in the swept synthesized source to eliminate hysteresis effects by initially disconnecting a tuning input of the tunable oscillator means from means for driving the tunable oscillator means;
• initially setting electronic pretuning means to produce a voltage to drive the tunable oscillator means so that the tunable oscillator means will operate near its minimum operating frequency;
• resetting a divide number N so that a low-frequency synthesizer means is initialized near its minimum frequency of operation;
• connecting the electronic pretuning means to the tuning input of the tunable oscillator means;
• disconnecting the electronic pretuning means from the tuning input of the tunable oscillator means;
• closing a main phase-lock loop so that the tunable oscillator means phase locks to its minimum operating frequency;
• and
• sweeping the low-frequency synthesizer means in the main phase-lock loop to drive the tunable oscillator means over a desired frequency span.

A pretune calibration method for determining a pretuning signal so that a tunable oscillator means incorporated into a swept synthesized source will initially phase-lock to its minimum operating frequency as the swept synthesized source is initialized to begin its sweep, comprising the steps of:

• a) setting electronic pretuning means for the tunable oscillator means to a minimum value;
• b) measuring a minimum tuning voltage in response to setting the electronic pretuning means to the minimum value;
• c) setting the electronic pretuning means for the tunable oscillator means to a maximum value;
• d) measuring a maximum tuning voltage in response to setting the electronic pretuning means to the maximum value;
• e) deriving a slope and offset from the measured minimum and maximum values to allow computing calibrated values corresponding to each pretuning voltage over a given range of available pretuning voltages;
• f) providing an initial seed value representative of a setting of the electronic pretuning means to pretune the tunable oscillator means to a frequency which is a predetermined frequency below a desired initial phase-lock frequency corresponding to the minimum operation frequency of the tunable oscillator means;
• g) setting the electronic pretuning means with the seed value so that the tunable oscillator means is initially pretuned using the seed value;
• h) initializing the frequency of a low-frequency synthesizer means incorporated into the swept synthesized source to near its minimum frequency of operation;
• i) connecting the electronic pretuning means to a tuning input of the tunable oscillator means for pretuning the tunable oscillator means;
• j) measuring an actual tuning voltage produced by the electronic pretuning means;
• k) storing the measured actual tuning voltage produced by the electronic pretuning means;
• l) disconnecting the electronic pretuning means from the tuning input of the tunable oscillator means;
• m) closing a main phase-lock loop for attempting phase-lock of the tunable oscillator means;
• n) measuring an actual tuning voltage present at the tuning input of the tunable oscillator means when the main phase-lock loop is closed;
• o) storing the measured actual tuning voltage present at the tuning input of the tunable oscillator means when the main phase-lock loop is closed;
• p) monitoring operation of the main phase-lock loop to determine if phase-lock occurs;
• q) incrementing the value used to set the electronic pretuning means so that the tunable oscillator means is subsequently pretuned using the incremented value if phase-lock does not occur at step p);
• r) repeating steps h) through p) if the value used to set the electronic pretuning means is incremented at step q);
• s) measuring an actual tuning voltage at which phase-lock occurs and comparing this actual tuning voltage with the tuning voltage at pretune using the present value used to set the electronic pretuning means if phase-lock occurs at step p);
• t) determining the relationship of the phase-lock frequency to the selected pretune frequency from the two values compared at step s);
• u) incrementing the value used to set the electronic pretuning means and repeating steps h) through t) if the phase-lock frequency is determined to be below the selected pretune frequency at step t);
• v) determining if the actual phase-lock frequency corresponds to the minimum operating frequency of the tunable oscillator means if the phase-lock frequency is above the selected pretune frequency;
• w) storing the present value used to set the electronic pretuning means corresponding to the tuning voltage which resulted in initial phase-lock if phase-lock to the correct harmonic is determined to have occurred at step v) for use in pretuning to produce phase-lock at the minimum operating frequency of the tunable oscillator means during subsequent operation;
• and
• x) incrementing the value used to set the electronic pretuning means and repeating steps h) through w) if it is determined at step v) that phase-lock to the correct harmonic has not occurred.

A method of operating a swept synthesized source, comprising the steps of:

• a) setting electronic pretuning means for the tunable oscillator means incorporated into the swept synthesized source to a minimum value;
• b) measuring a minimum tuning voltage in response to setting the electronic pretuning means to the minimum value;
• c) setting the electronic pretuning means for the tunable oscillator means to a maximum value;
• d) measuring a maximum tuning voltage in response to setting the electronic pretuning means to the maximum value;
• e) deriving a slope and offset from the measured minimum and maximum values to allow computing calibrated values corresponding to each pretuning voltage over a given range of available pretuning voltages;
• f) providing an initial seed value representative of a setting of the electronic pretuning means to pretune the tunable oscillator means to a frequency which is a predetermined frequency below a desired initial phase-lock frequency corresponding to the minimum operation frequency of the tunable oscillator means;
• g) setting the electronic pretuning means with the seed value so that the tunable oscillator means is initially pretuned using the seed value;
• h) initializing the frequency of a low-frequency synthesizer means incorporated into the swept synthesized source to near its minimum frequency of operation;
• i) connecting the electronic pretuning means to a tuning input of the tunable oscillator means for pretuning the tunable oscillator means;
• j) measuring an actual tuning voltage produced by the electronic pretuning means;
• k) storing the measured actual tuning voltage produced by the electronic pretuning means;
• l) disconnecting the electronic pretuning means from the tuning input of the tunable oscillator means;
• m) closing a main phase-lock loop for attempting phase-lock of the tunable oscillator means;
• n) measuring an actual tuning voltage present at the tuning input of the tunable oscillator means when the main phase-lock loop is closed;
• o) storing the measured actual tuning voltage present at the tuning input of the tunable oscillator means when the main phase-lock loop is closed;
• p) monitoring operation of the main phase-lock loop to determine if phase-lock occurs;
• q) incrementing the value used to set the electronic pretuning means so that the tunable oscillator means is subsequently pretuned using the incremented value if phase-lock does not occur at step p);
• r) repeating steps h) through p) if the value used to set the electronic pretuning means is incremented at step q);
• s) measuring an actual tuning voltage at which phase-lock occurs and comparing this actual tuning voltage with the tuning voltage at pretune using the present value used to set the electronic pretuning means if phase-lock occurs at step p);
• t) determining the relationship of the phase-lock frequency to the selected pretune frequency from the two values compared at step s);
• u) incrementing the value used to set the electronic pretuning means and repeating steps h) through t) if the phase-lock frequency is determined to be below the selected pretune frequency at step t);
• v) determining if the actual phase-lock frequency corresponds to the minimum operating frequency of the tunable oscillator means if the phase-lock frequency is above the selected pretune frequency;
• w) storing the present value used to set the electronic pretuning means corresponding to the tuning voltage which resulted in initial phase-lock if phase-lock to the correct harmonic is determined to have occurred at step v) for use in pretuning to produce phase-lock at the minimum operating frequency of the tunable oscillator means during subsequent operation;
• and
• x) incrementing the value used to set the electronic pretuning means and repeating steps h) through w) if it is determined at step v) that phase-lock to the correct harmonic has not occurred;
• y) initially zeroing the tuning current fed to the tunable oscillator means included in the swept synthesized source to eliminate hysteresis effects by initially disconnecting a tuning input of the tunable oscillator means from means for driving the tunable oscillator means;
• z) initially setting the electronic pretuning means to produce a voltage to drive the tunable oscillator means so that the tunable oscillator means will operate near its minimum operating frequency;
• aa) resetting a divide number N so that the low-frequency synthesizer means is initialized near its minimum frequency of operation;
• bb) connecting the electronic pretuning means to the tuning input of the tunable oscillator means;
• cc) disconnecting the electronic pretuning means form the tuning input of the tunable oscillator means;
• dd) closing the main phase-lock loop so that the tunable oscillator means phase locks to its minimum operating frequency;
• and
• ee) sweeping the low-frequency synthesizer means in the main phase-lock loop to drive the tunable oscillator means over a desired frequency span.