Frequency Reference Compensation

• US 5,542,095 A
• Filed: 08/24/1993
• Issued: 07/30/1996
• Est. Priority Date: 08/24/1993
• Status: Expired due to Term

• Current Assignee: Cirrus Logic Incorporated
• Original Assignee: Pacific Communication Sciences , Inc. (Cirrus Logic Incorporated)
• Inventors: Petranovich, James E.
• Primary Examiner(s): Urban, Edward F.
• Application Number:
  US08/110,611
• Time in Patent Office: 1,071 Days
• Field of Search: 455/71, 455/75, 455/76, 455/164.2, 455/182.2, 455/183.2, 455/192.2, 455/197.1, 455/260, 455/264
• US Class Current: 455/76
• CPC Class Codes: H03J 1/0008 using a central processing unit, e.g. a microprocessor digital tuning in general H03J5/0245
• CPC Class Codes: H04B 1/40 Circuits

Abstract

The present invention provides a frequency tracking and compensation system and methods for minimizing the frequency offset between remote and local units of a communications system. The system of the present invention resides in the local unit and comprises a downconverter, a digital transceiver, a processor, a reference oscillator and a frequency synthesizer. The downconverter mixes a signal received from the remote unit with a local oscillator (LO) signal generated by the frequency synthesizer. The frequency of the LO signal is based on the frequency of a reference signal generated by the reference oscillator. The system of the present invention utilizes the digital transceiver to determine a frequency offset of the received signal without comparing the received signal to the reference signal. The processor then utilizes the frequency offset to adjust the frequency of the reference signal. The method of the present invention accumulates the frequency offsets and compares the accumulated offset to a predetermined threshold. The reference frequency is adjusted if the accumulated offset exceeds the threshold. In one embodiment, the reference frequency is adjusted after complete reception of each call.

Claims

1. A frequency reference compensation system, comprising:

a downconverter for mixing a received signal with a local oscillator signal to provide a downconverted signal, the downconverted signal being output from said downconverter;

a demodulator interfaced with said downconverter for determining a frequency offset between a frequency of said downconverted signal and a predetermined frequency of the downconverted signal, said demodulator providing a frequency control signal representative of said frequency offset;

an accumulator coupled to said demodulator for accepting each said frequency offset so determined and accumulating values indicative of said frequency offsets;

a comparator coupled to said accumulator for comparing said accumulated values to a predefined threshold and following a complete reception of said received signal adjusting said frequency control signal to define an adjusted frequency control signal based on said comparison;

a voltage controlled oscillator interfaced with said comparator for accepting said adjusted frequency control signal and for generating a reference signal having a reference frequency based on said frequency offset; and

a frequency synthesizer coupled to said voltage controlled oscillator and having an input of said reference signal and generating said local oscillator signal based on said reference frequency thereby adjusting said frequency of said downconverted signal to minimize the frequency offset.

2. The system of claim 1, wherein said voltage controlled oscillator is temperature compensated.

3. The system of claim 1, wherein said accumulator and said comparator are implemented by a processor.

4. The system of claim 1, further comprising:

a filter interfaced with said demodulator for filtering the frequency control signal.

5. The system of claim 1, further comprising:

a processor coupled to said demodulator having a memory for storing said frequency offset, said processor generating said adjusted frequency control signal when information is to be transmitted, said information to be transmitted having been formed into a transmit signal; and

an upconverter for mixing said transmit signal with said local oscillator signal to produce an upconverted signal.

11. The method of claim 5, wherein the frequency of said reference signal is adjusted in fixed increments.

6. In a communications system having at least one remote unit and at least one local unit which transmit signals therebetween in the form of calls, a method of adjusting a frequency of a reference signal of said at least one local unit to track a frequency of a signal being transmitted from said at least one remote unit, said signal so transmitted defining a remote signal, comprising the steps of:

generating a local oscillator signal based on said reference signal;

mixing said remote signal with said local oscillator signal to produce a downconverted signal;

determining, upon the completion of each call, a frequency offset defining a difference between a frequency of said downconverted signal and a predetermined frequency of said downconverted signal; and

adjusting the frequency of said reference signal after any said call, but not during any said call, based on said frequency offset to minimize said frequency offset thereby eliminating signal distortion of the downconverted signal when the frequency of said reference signal is so adjusted.

7. The method of claim 6, further comprising the steps of:

generating a frequency control value, based on said frequency offset, said frequency control value determining the frequency of said reference signal;

incrementing said frequency control value when said frequency offset is positive; and

decrementing said frequency control value when said frequency offset is negative.

8. The method of claim 7, wherein a number of remote units transmit information to said local unit at different times, further comprising the step of:

generating a different frequency control value based on which of said remote units is transmitting.

9. The method of claim 6, wherein said step of determining said frequency offset is repeated over a predetermined period of time, said step of adjusting said reference signal comprising the steps of:

generating a frequency control value representative of a frequency of said reference signal;

accumulating each frequency offset so determined resulting in an accumulated offset value;

comparing said accumulated offset value to a predetermined threshold;

incrementing said frequency control value if said accumulated offset value is greater than said predetermined threshold; and

decrementing said frequency control value if said accumulated offset value is less than said predetermined value.

10. The method of claim 10, wherein said predetermined threshold being based upon an expected frequency of said calls.

12. A method of minimizing frequency offset between a remote unit and a local unit wherein a number of transmissions between said remote and local units define a call, comprising the steps of:

transmitting a signal from said remote unit to said local unit such that said local unit receives said transmitted signal, said transmitted signal having a predetermined operating frequency;

determining, upon the completion of each call, a frequency offset indicative of a difference between said predetermined operating frequency and a frequency of said transmitted signal as received by said local unit; and

adjusting, upon the completion of each call, the frequency of a local reference signal generated by said local unit based on said frequency offset to minimize said frequency offset thereby eliminating signal distortion of said transmitted signal while said transmitted signal is being received by said local unit.

13. The method of claim 12, wherein said adjusted local reference frequency is utilized by said local unit to subsequently transmit a signal to said remote unit.

14. The method of claim 12, wherein said remote unit transmits to said local unit during a period of time defining a slot and said local unit receiving said transmitted signal during said slot, said step of determining said frequency offset being carried out during each slot in which a transmission is received by said local unit.

15. The method of claim 14, wherein said step of adjusting the frequency of said local reference signal is repeated upon completion of each call based on the last frequency offset so determined.

16. The method of claim 15, further comprising the steps of:

accumulating said last frequency offset following each call; and

adjusting said local reference frequency if said frequency offset exceeds a predetermined threshold.

17. The method of claim 16, wherein said predetermined threshold is based on an expected frequency of said calls.

18. The method of claim 12, wherein the frequency of said reference signal is adjusted in fixed increments.