This paper summarizes experiments using a modelocked laser to produce a fast-rising, low-jitter electrical signal for clocking CMOS circuits. A simple integrating optical-to-electrical conversion scheme using two photodiodes is used to minimize jitter, area and electrical power consumption. This scheme is called receiver-less because there is no receiver to introduce unwanted skew, jitter and electrical power consumption. Receiver-less optical clock injection to a single CMOS circuit with < 6 ps rms jitter has been demonstrated. To scale the receiver-less concept low capacitance detectors are necessary. Transit-time limited rise times from low capacitance monolithic silicon CMOS photo-detectors have been simulated using 425 nm short pulses. The utility of this 'receiver-less' scheme is examined for larger clock distribution networks and varying photodiode capacitances.
We report the direct injection of precise clock signals into standard CMOS circuits using short optical pulses by a novel receiverless scheme that eliminates the delay, skew and jitter of a typical receiver. To accomplish the optical injection we designed small silicon detectors along-side standard 0.25micrometers CMOS-circuits. Due to the low intrinsic capacitance of the detectors, the photogenerated carriers can directly generate voltage swings that are comparable with CMOS voltage levels if the detectors are loaded with high-impedance circuits. As a first step to implement this scheme we characterized various detectors built in the CMOS process for their high-frequency response. In a test set-up the silicon detectors are sampled with on-chip samplers that only present a small capacitive loading to the detector node. We present the high-frequency high-impedance response measured with this scheme together with capacitance measurements and DC responsivities of various types and sizes of detectors. The characterized long tails typically observed with silicon detectors allowed us to set up a model for the power penalty we have to take into account for precise clock detection. Finally, as a proof-of-principle demonstration we present the first results of this receiverless scheme in which a totem-pole of silicon detectors directly drives an on-chip CMOS inverter.
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