Matched dual-channel front-end preamplifier for differential time delay measurements

Abstract

Presented is an integrated circuit dual-channel front-end preamplifier that minimises the differential time delay (DTD) between two channels. The circuit, which is based on a chopper architecture, works in the audio range and is intended for applications in sound localisation based on bearing estimation. The circuit was fabricated in a 1.5 × 1.5mm die, in a 0.5 mm technology. Experimental results indicate a mean DTD of 0.125 ms, which is one order of magnitude smaller than the experimental results reported in the literature so far.Introduction: One of the alternatives to perform localisation based on bearing estimation is the measurement of the difference in time of arrival (TOA) of a signal source to two microphones. Several miniature systems based on integrated circuits (ICs) have been proposed in the literature [1][2][3][4][5]. As shown in [6], the parametric mismatch of the front-end filters introduces a frequency dependent differential time delay (DTD) that can mask the intrinsic DTD of the signals, introducing a frequency dependent error to the spatial localisation accuracy. For instance, the data used to evaluate three different architectures [1-3], with the physical setup described in [7], was collected using a first-order lowpass filter (f c ¼ 300Hz) preamplifier with off-the-shelf 5% tolerance components. According to [6], this produces ideally a worst-case 26 ms DTD at low frequencies. Further refinements using 1% tolerance resistors and 2% tolerance capacitors, reduced the worst-case DTD to 7.9 ms. The mean absolute error in TOA measured in [7] (for a 108 range of bearing angle) in an open field test is 65.8 ms (this includes not only the DTD from the preamplifiers, but also that of the microphones, the physical setup and the data logger). On the other hand, [2] reports 2 ms accuracy using a generator with synthetic signals (bypassing the microphones) with on-chip (switched caps) amplifier channels. To give a relative idea of these numbers, one degree of bearing angle translates into 5 to 8 ms (depending on the bearing angle) for the physical setup in [7]. Owls can orient with 18 to 28 of accuracy, similar to humans [5]. These data evidence the significant impact of DTD due to mismatch when accuracies below 18 of bearing angle are needed. This Letter presents an IC front-end preamplifier with a pair of matched filters for TOA measurements in the audio range, that provides amplification and lowpass filtering with minimum spurious DTD. The proposed circuit achieves a mean DTD of 0.125 ms, with a maximum DTD of 0.18 ms, in the frequency range of interest.