Abstract
RMS frequency error performance and spurious signals in two-point modulators due to path imbalances
Highlights
Two-point modulators are effective means of generating analogue/digitally modulated signals with wide modulation bandwidths
Since the modulation is achieved within a phase-locked loop (PLL) and no external elements are required for modulation, modulators with low power demand and high integrity can be implemented with relatively low cost
Spurious signals generated by PLL. Another distortion mechanism introduced due to imbalance between modulation paths is the spurious signals generated at the charge pump frequency and at its harmonics
Summary
Two-point modulators are effective means of generating analogue/digitally modulated signals with wide modulation bandwidths. The modulating signal is applied both to the reference signal, which exhibits a low pass response, and to the voltage controlled oscillator (VCO), which exhibits a high pass response This circumvents the bandwidth problem and provides uniform modulation sensitivity unless there exists imbalance between modulation paths. The imbalance between the modulating paths introduces two major distortions into the signal: i) due to imbalance, the modulation sensitivity is not uniform and rms frequency error is increased, ii) since the VCO and reference are not in phase at the input of the phase detector (PD), the charge pump is activated, producing unwanted spurious signals at the output of the VCO. Constant envelope M-ary modulation formats are desirable to obtain higher throughput with efficient power amplifiers Another performance parameter of two-point modulators, namely the spurious signals, cannot be analyzed using the conventional s-domain analysis since this method misses the sampling nature of PLL.
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