# US 5,710,521 A · Out-of-band compensation for non-linear device > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 5,710,521 A - Document ID: US5710521A - Status: Granted - Kind: A - Application number: 08/580,573 - Filed: 1995-12-29 - Granted: 1998-01-20 - Earliest claimed: 1995-12-29 - Assignees: QUALCOMM Incorporated - Inventors: Brian K. Butler - Web page: https://patentgrep.com/patent/US5710521A ## CPC - H03G 3/3042: {in modulators, frequency-changers, transmitters or power amplifiers} - H03F 1/3241: {using predistortion circuits (H03F1/3211, H03F1/3217 take precedence)} - H03G: CONTROL OF AMPLIFICATION - H03F: AMPLIFIERS ## Abstract A system for substantially eliminating nonlinear distortion, such as intermodulation products, introduced when a signal is processed by a nonlinear device. The system adds an out-of-band signal to an input signal to produce a combined signal having a power that substantially equals a nominal operating point power for a nonlinear device. Preferably, the instantaneous power of the out-of-band signal is adjusted by an automatic gain amplifier. The combined signal is input to the nonlinear device to produce an output. A filter processes the output of the nonlinear device to recover the information content of the original signal after processing by the nonlinear device. For example, if the nonlinear device is a power amplifier, the filtered output is an amplification of the input signal. ## Claims ### Claim 1 A system for reducing nonlinear distortion in a nonlinear device which has a nominal operating point above which it reaches saturation for increased levels of power in an input signal, comprising: a nonlinear device input; a signal source to produce a variable magnitude out-of-band signal; a combiner connected in series with said nonlinear device input and to said signal source to combine said out-of-band signal with said input signal to produce a combined signal for input to said nonlinear device, with the power of said combined signal being maintained at a predetermined level substantially equal to said nominal operating point for said nonlinear device; and a filter coupled to an output of said nonlinear device to remove said out-of-band signal from said output of said nonlinear device. ### Claim 2 (depends on 1) The system of claim 1, further comprising: an automatic gain control (AGC) circuit coupled to said combiner to maintain the power of said combined signal at a level substantially equal to said nominal operating point. ### Claim 3 (depends on 2) The system of claim 2, wherein said AGC circuit comprises: a detector coupled to said combiner to detect the power of said combined signal and produce a detected power signal; an adder coupled to an output of said detector to add the negative of said detected power signal with a signal having a value substantially equal to said nominal operating point to produce a power difference signal; a second filter coupled to an output of said adder to filter said power difference signal to generate a gain update signal; and a variable gain amplifier coupled to an output of said second filter to amplify said out-of-band signal according to said gain update signal to cause said combined signal to have a power substantially equal to said nominal operating point. ### Claim 4 (depends on 1) The system of claim 1, wherein said signal source is an oscillator and said out-of-band signal is a continuous wave signal. ### Claim 5 (depends on 1) The system of claim 1, wherein said nonlinear device is a nonlinear power amplifier. ### Claim 6 (depends on 3) The system of claim 3, wherein said second filter comprises an integrating filter. ### Claim 7 A method, for reducing nonlinear distortion in a nonlinear device which has a nominal operating point above which it reaches saturation for increased levels of power in an input signal, comprising the steps of: (a) combining an out-of-band signal with an input signal to produce a combined signal having a power substantially equal to a nominal linear operating point for a nonlinear device; (b) maintaining the power of said combined signal at a level substantially equal to said nominal operating point; (c) processing said combined signal in said nonlinear device to produce an output; and (d) filtering said output to remove said out-of-band signal. ### Claim 8 (depends on 7) The method of claim 7, wherein step (b) further comprises the steps of: (e) detecting the power of said combined signal to produce a detected power signal; (f) summing the negative of said detected power signal with a signal having a power substantially equal to said nominal linear operating point to produce a power difference signal; (g) filtering said power difference signal to generate a gain update signal; and (h) varying the gain of said out-of-band signal in accordance with said gain update signal such that the power of said combined signal is substantially equal to said nominal linear operating point. ### Claim 9 (depends on 8) The method of claim 8, wherein step (d) comprises band pass filtering. ### Claim 10 (depends on 9) The method of claim 9, wherein step (g) comprises an integration. ### Claim 11 Apparatus for reducing nonlinear distortion introduced when a signal is processed by a nonlinear device which has a nominal operating point above which it reaches saturation for increased levels of power in an input signal, comprising: a nonlinear device input; a signal source generating a variable magnitude out-of-band signal, with respect to said input signal; a combiner connected in series with said nonlinear device input and to said signal source to receive and combine said out-of-band signal with said input signal to produce a combined signal for input to said nonlinear device; an automatic gain control (AGC) circuit coupled to said combiner to maintain the power of said combined signal within a predetermined substantially constant power magnitude envelope which is less than or equal to said nominal operating point for said nonlinear device, comprising: a detector coupled to said combiner to detect the power of said combined signal and produce a detected power signal; an adder coupled to an output of said detector to add the negative of said detected power signal with a signal having a value substantially equal to said nominal operating point to produce a power difference signal; a second filter coupled to an output of said adder to filter said power difference signal to generate a gain update signal; and a variable gain amplifier coupled to an output of said second filter to amplify said out-of-band signal according to said gain update signal to cause said combined signal to have a power substantially equal to said nominal operating point; and a filter coupled to an output of said nonlinear device to remove said out-of-band signal from said output of said nonlinear device. ### Claim 12 (depends on 1) The system of claim 1 wherein said predetermined level of said combined signal provides a substantially constant signal envelope for said combined signal. ### Claim 13 (depends on 1) The system of claim 1 wherein said predetermined level of said combined signal comprises a substantially constant value of an instantaneous power level for the input to said nonlinear device. ### Claim 14 (depends on 7) The method of claim 7 wherein the step of maintaining the power of said combined signal comprises the step of maintaining a substantially constant signal envelope for said combined signal. ### Claim 15 (depends on 7) The method of claim 7 wherein the step of maintaining the power of said combined signal comprises the step of maintaining a substantially constant value of an instantaneous power level for the input to said nonlinear device.