Method and apparatus for detection of load impedance modulation in a transformer circuit

1. An apparatus for detecting load impedance in a transformer comprising:

a transformer having a primary side and a secondary side, said transformer having a total current being sourced on said primary side, wherein said total current comprises a magnetizing inductance current component and a load current component;

a first circuit component coupled to said secondary side of said transformer, said first circuit component differentially modulating load impedance on said secondary side of said transformer, wherein said differentially modulating said load impedance comprises:

obtaining a data stream for communication from said secondary side to said primary side of said transformer;

using a Manchester encoder to convert said data stream to a differential receive bitstream; and

modulating said load impedance on said secondary side using said differential receive bitstream; and

a second circuit coupled to said primary side of said transformer for detection of said modulated load impedance by differentially integrating said load current on said primary side of said transformer.

2. The apparatus of claim 1 , wherein said load current is isolated from said total current by canceling said magnetizing current.

3. The apparatus of claim 1 , wherein said differentially integrating said load current comprises:

a circuit measuring an output of a voltage driver on said primary side, wherein said output is a voltage equivalent of absolute value of said load current;

a circuit generating an error signal between said load current and an average of said load current;

a circuit generating an integrated output voltage by integrating said error signal; and

a circuit comparing said integrated output signal to a fixed value at specific time intervals to generate discrete values corresponding to said load impedance modulation.

4. The apparatus of claim 3 , wherein said generating an error signal comprises:

a first voltage to current convener converting said voltage equivalent to said load current;

a filter circuit generating an average of said equivalent voltage;

a second voltage to current converter converting said average equivalent voltage to said average load current; and

a summer circuit generating said error signal by subtracting said average of said load current from said load current.

5. A method for detecting load impedance in a transformer comprising:

sourcing a total current on a primary side of a transformer, said transformer additionally having a secondary side, wherein said total current comprises a magnetizing inductance current component and a load current component;

modulating load impedance on said secondary side of said transformer using differential coding of data through a first circuit component coupled to said secondary side of said transformer, wherein said modulating said load impedance using said differential coding of data comprises:

obtaining a data stream for communication from said secondary side to said primary side of said transformer;

using a Manchester encoder to convert said data stream to a differential receive bitstream; and

modulating said load impedance on said secondary side using said differential receive bitstream; and

detecting said load impedance by differentially integrating said load current on said primary side of said transformer using a second circuit coupled to said primary side of said transformer.

6. The method of claim 5 , wherein said load current is isolated from said total current by canceling said magnetizing current.

7. The method of claim 5 , wherein said differentially integrating said load current comprises:

measuring an output of a voltage driver on said primary side, wherein said output is a voltage equivalent of absolute value of said load current;

generating an error signal between said load current and an average of said load current;

generating an integrated output voltage by integrating said error signal; and

comparing said integrated output signal to a fixed value at specific time intervals to generate discrete values corresponding to said load impedance modulation.

8. The method of claim 7 , wherein said generating an error signal comprises:

converting said voltage equivalent to said load current;

generating an average of said equivalent voltage;

converting said average equivalent voltage to said average load current; and

generating said error signal by subtracting said average of said load current from said load current.

9. An apparatus for detecting load impedance in a transformer comprising:

a transformer having a primary side and a secondary side;

a first circuit component coupled to said secondary side of said transformer, said first circuit component differentially modulating load impedance on said secondary side of said transformer, wherein said differentially modulating said load impedance comprises:

obtaining a data stream for communication from said secondary side to said primary side of said transformer;

using a Manchester encoder to convert said data stream to a differential receive bitstream; and

modulating said load impedance on said secondary side using said differential receive bitstream; and

a second circuit coupled to said primary side of said transformer for detection of said load impedance, wherein said second circuit differentially integrates load current on said primary side of said transformer for determination of said load impedance.

10. The apparatus of claim 9 , wherein said load current is isolated by canceling a magnetizing current component from total current on said primary side.

11. The apparatus of claim 9 , wherein said differentially integrating said load current comprises:

a circuit measuring an output of a voltage driver on said primary side, wherein said output is a voltage equivalent of absolute value of said load current;

a circuit generating an error signal between said load current and an average of said load current;

a circuit generating an integrated output voltage by integrating said error signal; and

a circuit comparing said integrated output signal to a fixed value at specific time intervals to generate discrete values corresponding to said load impedance modulation.

12. The apparatus of claim 11 , wherein said generating an error signal comprises:

a first voltage to current converter converting said voltage equivalent to said load current;

a filter circuit generating an average of said equivalent voltage;

a second voltage to current converter converting said average equivalent voltage to said average load current; and

a summer circuit generating said error signal by subtracting said average of said load current from said load current.

13. An apparatus for detecting impedance modulation comprising:

a current sense circuit coupled to a voltage driver that supplies a load current to a variable impedance, said current sense circuit providing a detection input signal proportional to the magnitude of said load current;

a threshold circuit sampling said detection input signal at a first sample point to obtain a first sample value associated with a first magnitude value of said load current and at a second sample point to obtain a second sample value associated with a second magnitude value of said load current, said threshold circuit comprising an averaging circuit that receives said first sample value and said second sample value and provides a threshold value; and

a decision circuit receiving said detection input signal and said threshold value and providing an output data signal, wherein said decision circuit comprises:

a subtraction circuit receiving said threshold value and said detection input signal and providing a difference output; and

an integrator integrating said difference output, said integrator being reset at the beginning of each bit period.

14. The apparatus of claim 13 , wherein said decision circuit is configured to sample said detection input signal adjacent to an endpoint of a Manchester period associated with an output data stream of said voltage driver.

15. The apparatus of claim 13 , wherein an impedance modulated data stream in said load current is Manchester encoded, and wherein said first sample point and said second sample point lie within the same bit period.

16. The apparatus of claim 13 , wherein said integrator is configured to switch a polarity of integration at a midpoint of said bit period.

17. A method for detecting impedance modulation in a communication circuit having a voltage driver that drives a voltage signal onto a variable load impedance, wherein said voltage signal embodies a transmit data stream and said variable load impedance is modulated with a receive data stream that is Manchester encoded, said method comprising:

sensing a load current in said voltage driver to provide a detection signal proportional to a magnitude of said load current;

integrating said detection signal over a bit period of said receive data stream to provide an integration result;

switching a polarity of said integration at a midpoint of said bit period; and

determining a binary result based on a polarity of said integration result at the end of said bit period.

18. The method of claim 17 , further comprising:

sampling said detection signal during a first half of said bit period and a second half of said bit period to provide a first value and a second value;

averaging said first value and said second value to provide an average signal; and

subtracting said average signal from said detection signal prior to said integration.