Method and apparatus for transient frequency distortion compensation
View Patent ↗Transient distortion is compensated for by multiplying an exponentially-decaying phase shift onto the distorted waveform. The exponentially decaying phase shift waveform is patterned after the transient which typically takes the form of an exponential and occurs upon introduction of power to a circuit or circuit component. A digital circuit produces an appropriate exponentially-decaying waveform which is used as the input for a look up table whose output is a complex sinusoidal waveform capable of compensating for the distortion. The complex sinusoid is multiplied onto the transmitted waveform. The decaying exponential is biased so that it crosses a threshold at which point the compensating circuitry is turned off.
1. A method of compensating for transient distortions of a time-varying, information-bearing signal, the method comprising:
identifying a distortion of the time-varying, information-bearing signal;
generating a compensation waveform corresponding to the distortion; and
applying the compensation waveform to the time-varying, information-bearing signal for a limited time,
wherein said time-varying, information-bearing signal is a signal to be transmitted by a wireless communication device.
2. The method of claim 1 , wherein identifying the distortion includes:
identifying a signal transmission disturbed by start-up of a device that uses power.
3. The method of claim 2 , wherein identifying the distortion includes:
identifying a signal transmission disturbed by start-up of a device that dissipates power.
4. The method of claim 3 , wherein the device is in a broadcast chain of a wireless transmitter.
5. The method of claim 3 , wherein the device is in a broadcast chain of an 802.11a wireless transmitter.
6. The method of claim 3 , wherein the device is an amplifier.
7. The method of claim 1 , wherein identifying the distortion includes identifying a start of a packet transmission.
8. The method of claim 1 , wherein generating the compensation waveform includes generating a phase compensation waveform.
9. The method of claim 1 , wherein generating the compensation waveform includes generating a magnitude compensation waveform.
10. The method of claim 1 , wherein generating the compensation waveform includes generating a combined magnitude and phase compensation waveform.
11. The method of claim 1 , wherein:
the method is embodied in a set of computer instructions stored on a computer readable media; and
the computer instructions, when loaded into a computer, cause the computer to perform steps of the method.
12. The method of claim 11 , wherein the computer instructions are compiled computer instructions stored as an executable program on the computer readable media.
13. The method of claim 1 , wherein identifying, generating, and applying are performed via a digital signal processor.
14. The method of claim 1 , wherein identifying, generating, and applying are performed by at least one of a digital signal processor and analog circuitry.
15. A method of compensating for transient distortions of a time-varying, information-bearing signal, the method comprising:
identifying a distortion of the time-varying, information-bearing signal, wherein identifying includes identifying a signal transmission disturbed by start-up of a device that uses power;
generating a compensation waveform corresponding to the distortion; and
applying the compensation waveform to the time-varying, information-bearing signal for a limited time,
wherein said time-varying, information-bearing signal is a signal to be transmitted, wherein generating the compensation waveform includes:
identifying time increments from power up of the device; and
at each time increment,
performing a table lookup to determine a time increment compensation value; and
applying the time increment compensation value to a corresponding part of the compensation waveform.
16. The method of claim 15 , wherein performing the table lookup includes:
generating a lookup waveform having at least one characteristic identifiable with the distortion; and
applying the lookup waveform to a lookup table to produce the time increment compensation value.
17. The method of claim 16 , wherein the lookup table is an e jθ(t) based table, where j=sqrt(−1) and θ(t) is a function having a value corresponding to an entry in the lookup table at predetermined time increments.
18. The method of claim 16 , wherein the lookup waveform is an exponentially decaying angle correction function that decays at a rate approximately similar to the distortion.
19. The method according to claim 16 , wherein generating the lookup waveform includes:
generating an exponentially decaying angle correction signal by:
loading contents of an output register with a phase_ramp_init value comprising an initial value of the lookup waveform;
multiplying a phase_ramp_alpha value, comprising an amount of decay in the angle correction signal, by a value of the output register contents to produce an incremental difference value;
subtracting the incremental difference value from the value of the output register contents to produce an incremental value;
reloading the output registers with the incremental value; and
repeating multiplying, subtracting, and reloading to produce the exponentially decaying angle correction signal.
20. The method of claim 16 , wherein generating the lookup waveform includes:
generating an exponentially decaying angle correction signal; and
biasing the angle correction signal so that it crosses a reference point.
21. The method of claim 20 , further comprising:
discontinuing application of the compensation waveform to the signal to be transmitted when the angle correction signal crosses the reference point.
22. The method of claim 20 , wherein the reference point is a 0 degree angle correction.
23. An apparatus for compensating for transient distortions of a signal, the apparatus comprising:
a distortion compensating device configured to produce a compensation signal having a phase shift magnitude approximately equivalent to a magnitude of a frequency distortion occurring on a signal being transmitted, the signal being a time-varying, information-bearing signal; and
a compensation multiplier configured to multiply the compensation signal onto the signal being transmitted for a limited time during transient distortion correction and configured to multiply other signals during channel correction.