Method and system for tuning precision continuous-time filters

A method of calibrating a continuous-time filter having an input, an output, and first and second adjustable parameters, comprising the steps of:

• a) applying an approximately DC signal to the input of the filter to produce a filtered DC signal at the output thereof;
• b) measuring the filtered DC signal to determine a magnitude thereof;
• c) storing the magnitude of the filtered DC signal as a DC reference value;
• d) applying a first signal of a first frequency to the input of the filter to produce a filtered first signal at the output thereof;
• e) measuring the filtered first signal to determine a magnitude thereof;
• f) adjusting the first filter parameter until the magnitude of the filtered first signal is approximately a first comparison value;
• g) applying a second signal of a second frequency to the input of the filter to produce a filtered second signal at the output thereof;
• h) measuring the filtered second signal to determine a magnitude thereof;
• i) adjusting the second filter parameter until the magnitude of the filtered second signal is approximately a second comparison value;
• j) reapplying the first signal of the first frequency to the input of the filter;
• k) measuring the reapplied filtered first signal to determine a magnitude thereof;
• and
• l) if the magnitude of the reapplied first filtered signal as measured in step k) is not approximately equal to the first value, then repeating steps d) through i);
• wherein, the second frequency is different from the first frequency, the first comparison value of step f) is equal to the DC reference value multiplied by a first scaling factor and the second comparison value of step i) is equal to the DC reference value multiplied by a second scaling factor.

An apparatus for calibrating a continuous-time filter having at least one adjustable parameter, comprising:

• a) a generator adapted to produce square-wave signal;
• b) a continuous-time filter, adapted to filter the square-wave signal, having a cutoff frequency of less than fs and a quality factor;
• c) a quantizer adapted to sample the filtered signal at the rate fs;
• d) an N-point discrete Fourier transformer adapted to convert the sampled signal into N real output values and N imaginary output values at the rate fs;
• e) an accumulator to accumulate at least one of the real output values to form a real output signal;
• f) an accumulator to accumulate at least one of the imaginary output values to form an imaginary output signal;
• g) a adder to sum the real and imaginary output signals to form an output signal;
• h) a squarer to square the output signal;
• i) a comparator to compare the squared output signal with a comparison value;
• and
• j) a controller adapted to adjust, in response to the output of the comparator, at least one of the cutoff frequency and quality factor of the continuous-time filter;
• wherein the square-wave signal has a frequency of approximately fs/M, and M is an integer multiple of N.