Designing and Prototyping of 2-4 GHz Bandpass SiGe Digitizers and Associated Test Equipment for the ALMA Project. I

In this first paper we describe the approach followed to design and experimentally test the high speed and broad band analog-to-digital converters (ADC's or digitizers) required for the ALMA project. Our prototypes are based on monolithic digitizers implemented in a BiCMOS 0.35 or 0.25 μm SiGe process with 3 or 8 quantization levels and 4 giga samples per second rate for an input signal from 2 to 4 GHz under full Nyquist condition. We have adopted a conventional flash converter architecture to design three different 2-bit digitizers and one 3-bit digitizer. The experimental results obtained with our first 2-bit digitizer ASIC show that sinusoidal input signals are properly sampled for clock rates up to 4.9 GHz. The design details and high dynamic range tests of our ADC's will be reported in forthcoming papers. A specific digitizer test equipment providing the auto-correlation and spectral analysis of the digitized signal has been developed to characterize our designs and provide the necessary feedback to the ASIC design team. It includes a broad band noise generator, a demultiplexing unit following the digitizer under test, a 16-lag auto-correlator, and FFT and display units. This equipment will also play a major role at the pre-production stage and for qualification/acceptance at the production stage. We suggest that the demultiplexing scheme developed for the digitizer test equipment could be used as a basis for multi-bit synchronization of the demultiplexed signal in front of the ALMA fiber optic transmitter system.