Digital signal generator with frequency-dependent corrections for DAC and analog non-idealities
A digital signal generator includes a numerically controlled oscillator (NCO) configured to generate a modulated phase signal, and to convert the modulated phase signal into a sinusoidal output signal, and a pre-distortion module configured to adjust a phase of the phase-modulated signal prior to conversion to the sinusoidal output signal, and to adjust an amplitude of the sinusoidal output signal to obtain a pre-distorted digital output signal. The phase and amplitude adjustments are frequency dependent, and the pre-distortion module is configured to determine an instantaneous frequency of the NCO by computing a derivative of the modulated phase signal, and to address a memory table correlating the determined instantaneous frequency with the phase and amplitude adjustments.
1 . A system comprising:
a digital signal generator configured to generate a digital signal x[n] of a given sampling frequency Fs; and
a digital-to-analog converter (DAC) configured to convert the digital signal x[n] to an analog signal having an effective sampling frequency Fs;
wherein the digital signal generator comprises:
a phase accumulator and first adder configured to output a modulated phase signal φ[n], where φ[n] denotes an instantaneous phase of each nth sample of the modulated phase signal φ[n];
a second adder configured to sum the modulated phase signal φ[n] and a phase correction value to obtain a phase-corrected modulated phase signal φ′[n];
a phase to amplitude converter configured to covert the phase-corrected modulated phase signal φ′[n] to a periodic output signal y[n];
a multiplier configured to multiply the periodic output signal y[n] by an amplitude correction value to obtain the digital signal x[n];
wherein the phase and amplitude correction values are frequency dependent, and wherein the system is configured to determine an instantaneous frequency of the modulated phase signal φ[n] by computing a derivative of the modulated phase signal φ[n], and to address a memory table correlating the determined instantaneous frequency with the phase and amplitude correction values, and
where the phase correction value and the amplitude correction value are set to compensate for non-idealities in the DAC.
2 . The system of claim 1 , wherein the digital signal x[n] is a continuous wave (CW) signal.
3 . The system of claim 2 , wherein the CW signal is a narrowband modulated CW signal.
4 . The system of claim 1 , wherein the DAC comprises N sub-DACs connected in parallel between an input multiplexer and an output multiplexer, where N is an integer of two or more,
wherein the input demultiplexer is configured to sequentially supply symbols of the digital signal x[n] to the respective sub-DACs at a rate of Fs/N; and
wherein the output multiplexer is configured to combine outputs of the sub-DACs to obtain the analog signal having the effective sampling rate Fs.
5 . The system of claim 1 , further comprising:
a multiplexer receiving phase correction and amplitude correction values addressed from the memory table, the multiplexer controlled by a modulus N counter to sequentially and repeatedly output the phase and amplitude correction values for the correct sub-DAC;
a selector receiving an output of the multiplexer and controlled according to the instantaneous frequency of the modulated phase signal y[n], and configured to apply the phase correction values to the second adder and the amplitude correction values to the multiplier.
6 . The system of claim 1 , wherein the phase accumulator is configured to output a frequency modulated phase signal, and the first adder is configured to phase modulate the phase signal to obtain the modulated phase signal y[n].
7 . The system of claim 6 , wherein the multiplier is further configured to amplitude modulate the periodic output signal y[n] to obtain the digital signal x[n].
8 . The system of claim 1 , further comprising analog circuitry configured to transmit the analog signal to an analog output of the system,
wherein the phase correction value and the amplitude correction value are further set to compensate for non-idealities in the analog circuitry.
9 . A digital signal generator configured to generate a pre-distorted digital signal x[n] of a given sampling frequency Fs, comprising:
a phase accumulator and first adder configured to output a modulated phase signal φ[n], where φ[n] denotes an instantaneous phase of each nth sample of the modulated phase signal φ[n];
a second adder configured to sum the modulated phase signal φ[n] and a pre-distortion phase correction value to obtain a phase-corrected modulated phase signal φ′[n];
a phase to amplitude converter configured to covert the phase-corrected modulated phase signal φ′[n] to a periodic output signal y[n];
a multiplier configured to multiply the periodic output signal y [n] by a pre-distortion amplitude correction value to obtain the digital signal x[n],
wherein the pre-distortion phase and amplitude correction values are frequency dependent, and wherein the digital signal generator is configured to determine an instantaneous frequency of the modulated phase signal φ[n] by computing a derivative of the modulated phase signal φ[n], and to address a memory table correlating the determined instantaneous frequency with the pre-distortion phase and amplitude correction values.
10 . The digital signal generator of claim 9 , wherein the digital signal x[n] is a continuous wave (CW) signal.
11 . The digital signal generator of claim 10 , wherein the CW signal is a narrowband modulated CW signal.
12 . The digital signal generator of claim 9 , further comprising:
a multiplexer receiving phase correction and amplitude correction values addressed from the memory table, the multiplexer controlled by a modulus N counter to sequentially and repeatedly output the phase and amplitude correction values;
a selector receiving an output of the multiplexer and controlled according to the instantaneous frequency of the modulated phase signal φ[n], and configured to apply the phase correction values to the adder and the amplitude correction values to the multiplier.
13 . The digital signal generator of claim 9 , wherein the phase accumulator is configured to output a frequency modulated phase signal, and the first adder is configured to phase modulate the phase signal to obtain the modulated phase signal φ[n].
14 . The digital signal generator of claim 13 , wherein the multiplier is further configured to amplitude modulate the periodic output signal y[n] to obtain the digital signal x[n].
15 . A digital signal generator, comprising
a numerically controlled oscillator (NCO) configured to generate a phase-modulated signal, and to convert the phase-modulated signal to periodic output signal; and
a pre-distortion module configured to adjust a phase of the phase-modulated signal prior to conversion to the periodic output signal, and to adjust an amplitude of the periodic output signal to obtain a pre-distorted digital output signal,
wherein the phase and amplitude adjustments are frequency dependent, and wherein the pre-distortion module is configured to determine an instantaneous frequency of the NCO by computing a derivative of the phase-modulated signal, and to address a memory table correlating the determined instantaneous frequency with the phase and amplitude adjustments.
16 . The digital signal generator of claim 15 , wherein the pre-distorted digital output signal is a continuous wave (CW) signal.
17 . The digital signal generator of claim 16 , wherein the CW signal is a narrowband modulated CW signal.