16.6GHz oscillator made in standard CMOS process

16.6GHz oscillator made in standard CMOS processSteve Bush A US research group claims to have produced a 16.6GHz oscillator using a standard chip-making process.
“It is the first to operate above 10GHz in conventional CMOS,” said Bendik Kleveland, a member of the development team.
The main problem with making high frequency oscillators is stray capacitance.
“An inductor made from on-chip metal tracks can easily have 5pF between it and the substrate,” he said. Around and around… Input and output signals race to each transistor together, making gains add. The extra gain from using a distributed amplifier overcomes system loads. Physically the amplifier is a ring shape, so transmission lines are matched.
This loads the signal path heavily – at 1GHz, 5pF looks like 50ž and effectively throws away system gain.
The conventional way around this is to resort to substrates like GaAs, or make inductors using bond wires. Both are expensive. Instead the 16GHz oscillator uses narrow tracks, whose fields penetrate the substrate less.
“It is reverse scaling,” said Kleveland. “The current trend is for wider tracks, further apart to reduce crosstalk. We use narrower tracks, closer together.”
The circuit is also novel, being a distributed amplifier with 100 per cent feedback. Such an amplifier has only one stage, but the stage is split between several devices, three in the schematic diagram.
Each sub-stage is separated by two transmission lines, one to the gate and one to the drain. The signals run along the transmission lines, reaching both transistor terminals simultaneously. The net result of the stages is to sum the component gains at the expense of the transmission line delays.
The second diagram is a physical representation of the oscillator, with transmission lines curved smoothly to reduce reflections.
“It is the first ring oscillator to really benefit from being a ring,” quipped Kleveland.
The final oscillator has five stages, space constraints limiting it to a long oval layout.
The research was done at Stanford University, with help from HP Labs at Palo Alto and MIPS Technologies.


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