Using Spectral Analysis of Output Data to Identify and Eliminate Noise on Control Lines

Abstract

A new technique was developed to measure noise and interference in a test stand for Josephson digital circuits. A spectrum analyzer measured the digital output of an RQL 10-bit shift register and found amplitude modulation sidebands due to bit errors generated by noise currents in the logic gate bias. The data pattern used was a simple 1010 … which produced a data tone at one half of the clock frequency. When the circuit was biased at the threshold between correct and incorrect operation, small noise tones modulated the bit error rate and were converted to AM sidebands of the data tone. Calibration tones were injected to measure the conversion ratio of sideband amplitude to interference amplitude, and showed a linear response over 4 decades of input tone power. The instrumentation noise floor was low enough to sense 20 nA of noise current on chip. The observation of AM sidebands was used to optimize filtering and identify defective cabling to eliminate noise and interference in the cryo-cooled test stand.