Frequency modulated synthesizer using low frequency offset mixed VCO

• US 5,079,526 A
• Filed: 08/29/1990
• Issued: 01/07/1992
• Est. Priority Date: 08/29/1990
• Status: Expired due to Term

• Current Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Heck, Joseph P.
• Primary Examiner(s): MIS, DAVID C
• Application Number:
  US07/574,626
• Time in Patent Office: 496 Days
• Field of Search: 332/127, 329/325, 331/2, 331/23, 375/81, 375/59, 375/120, 455/110, 455/112, 455/208, 455/209, 455/216, 455/260, 455/337
• US Class Current: 332/127
• CPC Class Codes: H03C 3/0975 applying frequency modulation in the phase locked loop at components other than the divider, the voltage controlled oscillator or the reference clock
• CPC Class Codes: H03C 3/0983 containing in the loop a mixer other than for phase detection

Abstract

An improved frequency synthesizer 100 capable of producing true DC frequency modulation without the need to DC frequency modulate the reference oscillator. The synthesizer 100 includes a phase comparator 104 compares the reference signal 102 and the feedback signal 112. The output of said phase comparator 104 filtered and coupled to the main VCO 108. A balanced image mixer 200 mixes the two signals from the main VCO 108 and an offset VCO 300. The offset VCO 300 is a low frequency (e.g. 100 to 300 KHz) integrated offset VCO 308 whose center frequency is controlled accurately, by means of matching to a VCO 306 which is controlled by a feedback loop, and by using an image balanced mixer 200 to reduce the image response to substantially reduce or eliminate the filtering required at the mixer output. The configuration of the offset mixer within the synthesizer loop, any residual spurious content due to imperfect image balance or carrier feedthrough of the mixer is attenuated by the loop response. The output signal of the mixer 200 is looped back to the phase comparator 104 after being divided by N through a divider 114.

Claims

1. An improved frequency synthesizer producing true DC Frequency Modulation, comprising:

a reference signal source for generating a reference signal;

a phase comparator, having a first input coupled to said reference signal, a second input and an output, for producing at the output a control signal representative of the phase difference of signals received at the first and second inputs;

a signal controlled oscillator producing an oscillator signal of frequency f at its output in response to the control signal of said phase comparator;

an offset signal controlled oscillator producing an oscillator signal of frequency f_OFF at its output in response to a modulating signal, the offset signal controlled oscillator including;

a reference signal source for generating a low frequency reference signal;

a phase comparator, having a first input coupled to said low frequency reference signal, a second input, and an output, for producing at the output a control signal representative of the phase difference of signals received at the first and second inputs;

a matched pair of signal controlled oscillators having a common control line producing oscillator signals in response to the control signal of said phase comparator; and

means for providing the output of the first of said matched signal controlled oscillators to the second input of said phase detector;

mixing means for mixing frequency f_OFF of said offset signal controlled oscillator and frequency f of said signal controlled oscillator and producing an output, and

divider means for dividing the output of said mixing means by a divisor "N" and producing an output coupled to the second input of said phase comparator.

2. The improved frequency synthesizer of claim 1 wherein the second of said matched pair of signal controlled oscillators includes a second input for coupling to a modulating signal.

3. An improved frequency synthesizer producing true DC Frequency Modulation, comprising:

a reference signal source for generating a reference signal;

a phase comparator, having a first input coupled to said reference signal, a second input and an output, for producing at the output a control signal representative of the phase difference of signals received at the first and second inputs;

a signal controlled oscillator producing an oscillator signal of frequency f at its output in response to the control signal of said phase comparator;

an offset signal controlled oscillator producing a modulated oscillator signal of frequency f_OFF at its output in response to a modulating signal;

an image balanced mixer for mixing frequency f_OFF of said offset signal controlled oscillator and frequency f of said signal controlled oscillator and producing an output, and

divider means for dividing the output of said mixing means by a divisor "N" and producing an output coupled to the second input of said phase comparator.

4. A receiver having a frequency synthesizer and adapted for recovering the modulating signal of a received carrier signal, the synthesizer comprising:

a reference signal source for generating a reference signal;

a phase comparator, having a first input coupled to said reference signal, a second input and an output, for producing at the output a control signal representative of the phase difference of signals received at the first and second inputs;

a signal controlled oscillator producing an oscillator signal of frequency f at its output in response to the control signal of said phase comparator;

an offset signal controlled oscillator producing an oscillator signal of frequency f_OFF at its output, the offset signal controlled oscillator including;

a reference signal source for generating a low frequency reference signal;

a phase comparator, having a first input coupled to said low frequency reference signal, a second input, and an output, for producing at the output a control signal representative of the phase difference of signals received at the first and second inputs;

a matched pair of signal controlled oscillators having a common control line producing oscillator signals in response to the control signal of said phase comparator; and

means for providing the output of the first of said matched signal controlled oscillators to the second input of said phase detector;

mixing means for mixing frequency f_OFF of said offset signal controlled oscillator and frequency f of said signal controlled oscillator and producing an output; and

divider means for dividing the output of said mixing means by a divisor "N" and producing an output coupled to the second input of said phase comparator.

5. The receiver of claim 4, wherein the second of said matched pair of signal controlled oscillators includes a second input for coupling to a modulating signal.

6. A transmitter for generating a carrier signal modulated via a modulation signal, comprising:

an improved frequency synthesizer producing true DC Frequency Modulation, comprising;

a reference signal source for generating a reference signal;

a phase comparator, having a first input coupled to said reference signal, a second input and an output, for producing at the output a control signal representative of the phase difference of signals received at the first and second inputs;

a signal controlled oscillator producing an oscillator signal of frequency f at its output in response to the control signal of said phase comparator;

an offset signal controlled oscillator producing an oscillator signal of frequency f_OFF at its output in response to a modulating signal, the offset signal controlled oscillator including;

a reference signal source for generating a low frequency reference signal;

a phase comparator, having a first input coupled to said low frequency reference signal, a second input, and an output, for producing at the output a control signal representative of the phase difference of signals received at the first and second inputs;

a matched pair of signal controlled oscillators having a common control line producing oscillator signals in response to the control signal of said phase comparator; and

means for providing the output of the first of said matched signal controlled oscillators to the second input of said phase detector;

mixing means for mixing frequency f_OFF of said offset signal controlled oscillator and frequency f of said signal controlled oscillator and producing an output, and

divider means for dividing the output of said mixing means by a divisor "N" and producing an output coupled to the second input of said phase comparator.

7. The transmitter of claim 6, wherein the second of said matched pair of signal controlled oscillators includes a second input for coupling to a modulating signal.

8. A transmitter for generating a carrier signal modulated via a modulation signal, comprising:

an improved frequency synthesizer producing true DC Frequency Modulation, comprising;

a reference signal source for generating a reference signal;

a phase comparator, having a first input coupled to said reference signal, a second input and an output, for producing at the output a control signal representative of the phase difference of signals received at the first and second inputs;

a signal controlled oscillator producing an oscillator signal of frequency f at its output in response to the control signal of said phase comparator;

an offset signal controlled oscillator producing a modulated oscillator signal of frequency f_OFF at its output in response to a modulating signal;

an image balanced mixer for mixing frequency f_OFF of said offset signal controlled oscillator and frequency f of said signal controlled oscillator and producing an output, and

divider means for dividing the output of said mixing means by a divisor "N" and producing an output coupled to the second input of said phase comparator.

9. A frequency synthesizer having a reference signal and adapted for producing a true DC modulated signal at its output, comprising:

a main signal controlled oscillator;

an offset signal controlled oscillator producing a modulated signal for offsetting the reference signal, the oscillator having a modulating signal input for accepting modulating signals down to DC; and

an image balanced offset mixer coupled to the offset signal controlled oscillator.

10. The frequency synthesizer of claim 9, wherein the offset signal controlled oscillator includes two signal controlled oscillators having matched layouts.

11. The frequency synthesizer of claim 9, wherein the offset signal controlled oscillator comprises a low frequency offset oscillator.

12. The frequency synthesizer of claim 11, wherein the low frequency offset oscillator comprises an offset oscillator operating below 300 KHz.