Power Dissipation Measurement in RQL Digital Logic

Abstract

Power dissipation in Reciprocal Quantum Logic (RQL) was measured by modulating the activity factor of RQL shift registers. A chip was measured that contained nine working circuits, seven 200-bit and two 1,000-bit shift registers, a total of 54,742 operating Josephson junctions on a single chip. Data was input to all shift registers for 5 microseconds followed by no input data for 5 microseconds, cycling the circuits between active, correct logical operation and idle at 100 kHz. Thus the AC clock return from the chip was modulated in power and propagation delay to produce modulation sidebands 100 kHz above and below the AC clock frequency. Power dissipation was proportional to sideband amplitude. In this experiment, for the first time, a mixer in a homodyne configuration was applied to the AC clock return to distinguish the amplitude modulation (power dissipation) from the phase modulation (non-dissipative time delay). Power dissipation measured in this way was directly proportional to the density of data ONEs across a range of data densities from 10% to 50%. The observed power dissipation increased linearly with clock frequency from 2 to 3.5 GHz. Energy per operation is expected to be some fraction of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$I_{c}\Phi_{0}$</tex-math></inline-formula> (joules/operation). The use of a mixer to distinguish AM from PM improved upon the previous measurement of power dissipation in RQL. Measurements with the new technique showed that RQL shift registers built with <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$35\, \mu A$</tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$50\, \mu A$</tex-math></inline-formula> Josephson junctions dissipated <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$0.28 \, I_{c} \Phi_{0}$</tex-math></inline-formula> joules per operation. The measured dissipation coefficient of 28% agrees with simulations done using WRspice.