Radio frequency ΣΔ-modulator
A radio-frequency ΣΔ-modulator comprises a first mixer in the forward path for down-converting the signals in this forward path with a local oscillator frequency and a second mixer in the feedback path for up-converting the feedback signal with the same local oscillator frequency. Delays between the two mixing operations cause a loss of gain in the loop of the ΣΔ-modulator. An adjustable amplifier in the feedback path compensates for this loss of gain.
1. A radio-frequency Sigma Delta (ΣΔ) modulator with a feed-forward path and a feedback path, the feed-forward path comprising a summing node for summing an analog input signal and a feedback signal of the feedback path and creating therefrom an RF error-signal, a first mixer receiving said RF error-signal and a local oscillator frequency for down-converting converting said error-signal, a noise shaping low pass filter for low pass filtering the down-converted error-signal of the first mixer and a quantizer receiving the output signal of the low pass filter and generating therefrom a digital output signal of the ΣΔ-modulator, the feedback path comprising a DA-converter and a second mixer driven by said local oscillator frequency for generating an up-converted analog feedback signal, characterized in that the feedback path comprises an amplifier with adjustable amplification factor for compensating the loss of amplification caused by delays and/or aspect ratio deviations of the local oscillator drives of the first and second mixers.
2. A radio-frequency ΣΔ-modulator as claimed in claim 1 characterized by means to insert a reference signal in the feed-forward path during a calibration mode, means to measure during the calibration mode the effect of said reference signal in the digital output signal of the ΣΔ-modulator and to calculate a gain control signal therefrom, and means to apply said gain control signal to the adjustable amplifier for adjusting its gain during normal operational mode of the ΣΔ-modulator.
3. A radio-frequency ΣΔ-modulator as claimed in claim 1 , in which the local oscillator frequency varies for tuning the ΣΔ-modulator to different channels, characterized in that the sampling frequency at which the quantizer operates varies with the oscillator frequency whereby a fixed integer relation between the oscillator frequency and the sampling frequency is maintained.