Method and apparatus for correcting the effect of dielectric absorption in an open-loop modulator circuit

• US 6,501,337 B1
• Filed: 03/23/2001
• Issued: 12/31/2002
• Est. Priority Date: 03/24/2000
• Status: Expired due to Fees

• Current Assignee: Cirronet, Inc. (Murata Manufacturing Co Limited)
• Original Assignee: Cirronet, Inc. (Murata Manufacturing Co Limited)
• Inventors: Tucker, Mark E.
• Primary Examiner(s): Pascal, Robert
• Assistant Examiner(s): Chang, Joseph
• Application Number:
  US09/815,664
• Time in Patent Office: 648 Days
• Field of Search: 455/260, 455/262, 455/264, 455/265, 331/17
• US Class Current: 331/17
• CPC Class Codes: H03C 3/02 Details
• CPC Class Codes: H03L 7/093 using special filtering or amplification characteristics in the loop H03L7/087 - H03L7/091 take precedence
• CPC Class Codes: H03L 7/187 using means for coarse tuning the voltage controlled oscillator of the loop H03L7/191 - H03L7/195 take precedence

Abstract

A method and apparatus for reducing the effects of dielectric absorption to a loop filter capacitor utilized in a dual-mode open-loop modulator having a transmit and receive communication frequency. The modulator incorporating a phase lock loop having means for maintaining a substantially constant voltage across the loop filter capacitor irrespective of the communication frequency implemented.

Claims

1. A method of providing an open-loop modulated carrier frequency to a local oscillator utilizing a receive mode and a transmit mode for signal communication, the method comprising the steps of:

providing a phase lock loop, the phase lock loop comprising a synthesizer, a loop filter having a voltage across a capacitor, an op-amp circuit, a voltage control oscillator having an input voltage, and a modulator being operably coupled in a continuous loop, the phase lock loop being capable of stabilizing a center frequency for cooperating with a transmit frequency, a receive frequency and an intermediate frequency;

tuning the voltage control oscillator to the receive frequency by applying a receive tune voltage to the voltage control oscillator;

tuning the voltage control oscillator to the transmit frequency by applying a transmit tune voltage to the voltage control oscillator;

disconnecting the continuous phase lock loop in response to the tuning of the voltage control oscillator to the transmit frequency;

applying a modulation onto the center frequency; and

,

maintaining the voltage across the capacitor of the loop filter substantially constant irrespective of the receive mode and the transmit mode by applying a predetermined offset voltage to the op-amp circuit in response to the signal communication mode, wherein the offset voltage applied during the transmit mode is substantially equal to the difference in tuning voltages applied to the voltage control oscillator for the transmit mode and the receive mode, respectively, and further wherein the substantially constant voltage across the capacitor reduces dielectric absorption of the capacitor, thus reducing frequency drift to the transmit frequency during open-loop modulation.

2. The method of claim 1 wherein the voltage difference is correlated to a frequency offset substantially equal to a first intermediate frequency of the phase lock loop.

3. The method of claim 1 wherein the transmit frequency ranges from 1.20-1.24 GHz.

4. The method of claim 1 wherein the transmit frequency is substantially 1.221 GHz.

5. The method of claim 1 the intermediate frequency is substantially 56.15 MHz.

6. The method of claim 1 wherein the disconnecting of the continuous phase lock loop comprises the step of operating the synthesizer in a high impedance mode.

7. A method of providing an open-loop modulated carrier frequency to a local oscillator utilizing a receive mode and a transmit mode for frequency modulated signal communication, the method comprising the steps of:

providing a phase lock loop, the phase lock loop comprising;

a synthesizer for stabilizing a reference frequency, the synthesizer having an input and an output;

a loop filter having a voltage across a capacitor and being operably coupled to the output of the synthesizer;

an op-amp circuit having a first input, a second input and an output, the first input being operably coupled to the synthesizer and the loop filter, the second input being operably responsive to an offset voltage and the output of the op-amp circuit; and

,

a voltage control oscillator having an input voltage and being operably responsive to the op-amp circuit and a modulator, wherein the voltage of the loop filter being substantially equal to one-half the sum of the offset voltage and the input voltage of the voltage control oscillator;

tuning the voltage control oscillator to a predetermined transmit frequency;

disconnecting the continuous phase lock loop in response to the tuning of the voltage control oscillator to the transmit frequency;

applying a frequency modulation to the voltage control oscillator; and

,

adjusting the offset voltage in response to the transmit frequency wherein the offset voltage applied during the transmit mode is substantially equal to the difference in tuning voltages respectively applied to the voltage control oscillator of the transmit mode and the receive mode, the voltage difference being correlated to a frequency offset substantially equal to an intermediate frequency of the phase lock loop, and further wherein the loop filter voltage is maintained substantially constant, the constant loop filter voltage reduces dielectric absorption of the capacitor, thus reducing frequency drift of the transmit frequency during open-loop modulation.

8. The method of claim 7 wherein the disconnecting of the continuous phase lock loop comprises operating the synthesizer in a high impedance mode.

9. The method of claim 7 further comprising:

tuning the voltage control oscillator to a predetermined receive frequency;

connecting the continuous phase lock loop in response to the tuning of the voltage control oscillator to the receive frequency;

discontinuing the application of the frequency modulation to the voltage control oscillator;

adjusting the offset voltage in response to the receive frequency wherein the loop filter voltage is maintained substantially constant, the substantially constant loop filter voltage reduces dielectric absorption of the capacitor, thus reducing frequency drift of the transmit frequency during open-loop modulation.

10. A dual-mode open-loop modulator for a local oscillator utilizing a transmit mode and a receive mode communication frequency, the dual-mode open-loop modulator comprising:

a synthesizer for stabilizing a reference frequency, the synthesizer being operably responsive to the communication frequency mode;

a loop filter having a voltage across a capacitor, the loop filter being operably connected to the output of the synthesizer;

an op-amp circuit having a first input, a second input and an output, the first input of the op-amp being operably responsive to the loop filter voltage, and the second input of the op-amp being operably responsive to an offset voltage and the output of the op-amp circuit;

a modulator circuit having a voltage control oscillator including an input and an output, the input of the voltage control oscillator being operably responsive to a tuning voltage, the output of the op-amp circuit and a modulator, the output of the voltage control oscillator being operably connected to the synthesizer; and

,

an offset voltage being responsive to the communication mode and being applied to the second input of the op-amp circuit, the offset voltage for maintaining the loop filter voltage substantially constant during either communication frequency mode wherein the offset voltage applied during the transmit mode is substantially equal to the difference in tuning voltages respectively applied to the voltage control oscillator during the transmit mode and the receive mode, and further wherein the substantially constant loop filter voltage minimizes dielectric absorption of the loop filter capacitor, thus reducing frequency drift of the reference frequency during open-loop modulation.

11. The dual-mode open-loop modulator of claim 10 wherein the offset voltage is correlated to a frequency offset substantially equal to a first intermediate frequency of the phase lock loop.

12. The dual-mode open-loop modulator of claim 11 wherein the reference frequency is an intermediate frequency substantially equal to 56.15 MHz.

13. The dual-mode open-loop modulator of claim 11 wherein the reference frequency is a transmit frequency ranging from 1.20-1.24 GHz.

14. The dual-mode open-loop modulator of claim 11 wherein the transmit frequency is substantially 1.221 GHz.

15. A dual-mode open-loop modulator for a local oscillator utilizing a transmit and receive communication frequency, the dual-mode modulator comprising:

a synthesizer for stabilizing a reference frequency;

a loop filter operably connected to the output of the synthesizer;

an op-amp circuit having a first input, a second input and an output, the first input of the op-amp having a voltage, VA, and being operably responsive to the loop filter and the synthesizer, the second input of the op-amp circuit being operably responsive to an offset voltage, V_C, and the output of the op-amp circuit;

the offset voltage being responsive to the communication frequency; and

,

a modulating circuit having a voltage control oscillator being operably responsive to a tuning voltage for selecting the communication frequency, the voltage control oscillator including an input and an output, the input of the voltage control oscillator having a voltage, V_B, the output of the voltage control oscillator being operably connected to the synthesizer wherein the dual-mode modulator is represented by an equation V_A(1+R_f/R_i)−

V_C(R_f/R_i)=V_B.

16. The dual-mode open-loop modulator of claim 15 wherein the offset voltage applied in response to the transmit mode is substantially equal to the voltage difference between the receive tuning voltage and the transmit tuning voltage.

17. The dual-mode open-loop modulator of claim 15 wherein the offset voltage is correlated to a frequency offset substantially equal to a first intermediate frequency of the dual-mode modulator.