Ultrafast Logic Analyser

Ultrafast Digital Electronics

Background:

The continuing trend towards the development of ever higher computer speeds is reliant on increasing the speeds of individual electronic components from which integrated circuits are constructed. Whilst recent demonstrations of single transistors with 1000GHz clock speeds have been indicative of the potential improvements in speed, there exist significant limitations imposed by the nature of the connections between components. Even in the case of superconducting electronics there are significant problems associated with the sheer speed of the circuit and it’s physical size. As the clock speed rises, the bandwidth of the signals travelling around the circuit becomes extremely large leading to several problems 

 

  • signals lose energy by radiation - bits of the circuit act like antennas and electrons in the substrates couple energy out as Cherenkov radiation.
  • signals on adjacent tracks on the circuit suffer strong interference problems that can lead to logic misfires and large bit error rates.
  • the length of signal pathways begin to become similar to the wavelength of signals resulting in resonant behaviour which can significantly disrupt the performance of a logic circuit.

 

Difficulties with layouts and the coupling of data from one track to another (when you’d prefer it didn’t) mean that it is in no way certain that the potential of these components will be realised. The only way to get around these difficulties is to construct integrated circuits in which each and every conductor is considered as a waveguide. This in itself leads to problems with layouts so new design rules need to be established in order to best make use of these extreme ultrafast electronics systems. This research project deals with the development of an ultra wideband logic analyser system which will enable the development and investigation of ultrafast logic circuit, allowing the driving of test data streams and observation of signals at bit rates from 10’s to 1000’s of GHz.

 

Objectives:

  1. Design and construct an optical system based on an ultrafast laser which is able to generate complex bit streams, clock sequences and sampling pulses from a single source with which to excite and probe test logic elements.
  2. Construct simple optoelectronic sampling systems based on the electro-optic effect and the photokinetic effect (recently demonstrated in oxford).
  3. Using simple wide bandwidth planar waveguides (of the typical dimensions one might need for a THZ clock speed microprocessor) observe the propagation of THz bandwidth electrical pulses and evaluate the radiation, Cherenkov and absorption losses in order to build realistic propagation models for simulation software packages.
  4. Observe the switching of simple ultrafast flip-flop circuits (based on superconducting RSFQ technology – made in house) to consider the effects of multiple pulse reflections on their threshold behaviour and evaluate the bit error rates.

Methodology:

Ultrafast optical techniques are the highest bandwidth measurements systems currently available. Ultrafast lasers are readily available which produce pulses <100fs in length with output powers >100mW. If one can convert these optical pulses to electrical drive signals for logic circuits, and use optoelectronic techniques to observe the electric fields on a device, one can realise a logic analyser operation with potential bandwidth in excess of 1THz. 

Several techniques exist to measure ultrafast electrical signals using light. These include the use of fast photoconductive devices made from Si and GaAs, electro-optic sampling using non-linear optical materials to observe electrically induced polarisation rotations and superconducting photomixers (developed by my group in Oxford during the last 3 years). Each of these has It’s benefits and drawbacks in terms of speed and sensitivity. Superconductors are the most sensitive but slowest (1uV sensitivity bandwidth ~ 500GHz), with EO samplers the least sensitive and fastest (0.3mV sensitivity bandwidth >2THz). 

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(Generating data words using variable thickness of glass.)
 
 
To produce data words and clock sequences for logic testing two things are required – one is a high bandwidth photoreceiver which we able to fabricate using superconductors and semiconductors in house. Typical responses produce electrical pulses as short as 1ps at present. Generation of complex pulse sequences is realised using stacks of glass plates to produce a range of delays across the optical beam which when concentrated onto a single detector produces the desired pulse stream as shown in the figure. This relatively low cost technique enables one to produce test data with bit rates as high as 750GHz for 1mm thick microscope slides..