Modulation system capable of accurately compensating for nonlinearities of an amplifier connected thereto

• US 4,985,688 A
• Filed: 01/05/1990
• Issued: 01/15/1991
• Est. Priority Date: 01/06/1989
• Status: Expired due to Term

• Current Assignee: NEC Corporation
• Original Assignee: NEC Corporation
• Inventors: Nagata, Yoshinori
• Primary Examiner(s): Pascal, Robert J.
• Application Number:
  US07/459,387
• Time in Patent Office: 375 Days
• Field of Search: 332/123, 329/304, 375/39, 375/60, 375/61, 455/63, 455/110, 330/149
• US Class Current: 332/123
• CPC Class Codes: H03C 1/06 Modifications of modulator to reduce distortion, e.g. by feedback, and clearly applicable to more than one type of modulator
• CPC Class Codes: H03C 1/50 by converting angle modulation to amplitude modulation H03C1/28 - H03C1/34, H03C1/46, H03C1/48 take precedence
• CPC Class Codes: H03C 2200/0058 Quadrature arrangements
• CPC Class Codes: H03C 2200/0079 Measures to linearise modulation or reduce distortion of modulation characteristics
• CPC Class Codes: H03C 3/02 Details
• CPC Class Codes: H03C 3/08 Modifications of modulator to linearise modulation, e.g. by feedback, and clearly applicable to more than one type of modulator
• CPC Class Codes: H03C 3/406 using a feedback loop containing mixers or demodulators
• CPC Class Codes: H03C 5/00 Amplitude modulation and angle modulation produced simultaneously or at will by the same modulating signal H03C7/00 takes precedence
• CPC Class Codes: H03D 1/2245 using two quadrature channels H03D1/2209 takes precedence
• CPC Class Codes: H03D 7/168 using a feedback loop containing mixers or demodulators
• CPC Class Codes: H03F 1/3247 using feedback acting on predistortion circuits H03F1/3264 takes precedence
• CPC Class Codes: H03F 1/3282 Acting on the phase and the amplitude of the input signal
• CPC Class Codes: H03F 1/3294 Acting on the real and imaginary components of the input signal
• CPC Class Codes: H03F 2201/3233 Adaptive predistortion using lookup table, e.g. memory, RAM, ROM, LUT, to generate the predistortion
• CPC Class Codes: H04L 27/368 adaptive predistortion

Abstract

In a modualtion system responsive to a first sampled signal (31) of a first sampling rate for supplying a modulated analog signal to an amplifier having nonlinearities to make the amplifier produce an amplified output signal, a selector (72) selects the first sampled signal and a second sampled signal of a second sampling rate as a selected signal when supplied with first and second control signals, respectively. The second sampling rate is equal to 1/N (N: 2, 3, . . . ) of the first sampling rate. A processing arrangement (36) processes the selected signal with reference to a difference between the selected signal and an additional digital signal (35) to compensate for the nonlinearities to produce a processed signal. The processed signal is modulated into the modulated analog signal. The amplified output signal is demodulated and converted into the additional digital signal by a converter (40). A comparator (71) compares an absolute value of the difference with a threshold value and produces the first control signal when the absolute value is less than the threshold value. Otherwise, the second control signal is produced. A controller ( 78) may control a phase of a sampling signal supplied to the converter so that a quotient of the difference by a differential of the selected signal becomes equal to zero.

Claims

1. A modulation system for processing and modulating a first sampled signal of a first sampling rate to supply a modulated analog signal to an amplifier having nonlinearities to make said amplifier produce an amplified output signal, said modulation system including:

subtracting means for producing a difference signal representative of a difference between said first sampled signal and an additional digital signal;

processing means connected to said subtracting means for processing said first sampled signal with reference to said difference signal to compensate for said nonlinearities to produce a processed signal;

modulating means connected to said processing means for modulating said processed signal into said modulated analog signal;

demodulating means connected to said amplifier for demodulating said amplified output signal into a demodulated signal; and

converting means connected to said demodulating means for converting said demodulated signal into said additional digital signal at a specific rate;

wherein the improvement comprises;

absolute value calculating means connected to said subtracting means for producing an absolute value of said difference signal;

comparing means connected to said absolute value calculating means for comparing said absolute value with a preselected threshold value to produce a first control signal when said absolute value is less than said threshold value and to produce a second control signal when said absolute value is not less than said threshold value; and

selecting means connected to said comparing means and said processing means and supplied with a second sampled signal of a second sampling rate for selecting said first and said second sampled signals as a selected signal to use said first and said second sampling rates as said specific rate when supplied with said first and said second control signals, respectively, said second sampling rate being equal to 1/N of said first sampling rate, where N represents an integer greater than one, said selected signal being substituted for said first sampled signal and processed and modulated into said modulated analog signal so long as said comparing means produces said second control signal.

2. A modulation system as claimed in claim 1, wherein said modulation system comprises:

differentiating means connected to said selecting means for producing a differential signal representative of a differential of said selected signal;

dividing means connected to said subtracting means and said differentiating means for producing a quotient signal representative of a quotient of said difference signal by said differential signal;

clock generating means connected to said converting means for generating a sampling signal having a phase and a clock frequency, said sampling signal being supplied to said converting means to decide said specific rate; and

controlling means connected to said dividing means and said clock generating means for controlling the phase of said sampling signal so that said quotient becomes equal to zero.

3. A modulation system for processing and modulating a sampled signal of a sampling rate to supply a modulated analog signal to an amplifier having nonlinearities to make said amplifier produce an amplified output signal, said modulation system including:

subtracting means for producing a difference signal representative of a difference between said sampled signal and an additional digital signal;

processing means connected to said subtracting means for processing said sampled signal with reference to said difference signal to compensate for said nonlinearities to produce a processed signal;

modulating means connected to said processing means for modulating said processed signal into said modulated analog signal;

demodulating means connected to said amplifier for demodulating said amplified output signal into a demodulated signal;

clock generating means for generating a sampling signal having a phase and a clock frequency determined by a specific rate which is equal to said sampling rate; and

converting means connected to said demodulating means and said clock generating means for converting said demodulated signal into said additional digital signal at said specific rate;

wherein the improvement comprises;

differentiating means for producing a differential signal representative of a differential of said sampled signal;

dividing means connected to said subtracting means and said differentiating means for producing a quotient signal representative of a quotient of said difference signal by said differential signal; and

controlling means connected to said dividing means and said clock generating means for controlling the phase of said sampling signal so that said quotient becomes equal to zero.