Digital Encoder for RF Transmit Waveform Synthesizer

We are designing an encoder for conversion of a Nyquist-rate N-bit digital word into a high frequency stream of single-bit data, to enable a novel architecture for a digital waveform synthesizer. The multi-bit to single-bit data conversion is performed by a new encoding scheme called the ldquostaggered thermometer code.rdquo Similar to the delta-sigma code, the staggered thermometer code also pushes most of the quantization noise to higher frequencies which can be easily filtered out with a low-pass filter. This encoder, built with elementary RSFQ cells, relies only on the merger of periodic pulse streams of different frequencies, and therefore, is much simpler to implement than a delta-sigma modulator which requires an accumulator and a multi-bit feedback loop. To avoid the permanent energy loss associated with low-pass filtering of high frequency components of the single-bit data stream, we also implement a novel digital circuit that acts as an amplifier by converting the pulse density modulated data stream into a pulse width modulated data stream. This digital amplifier changes the frequency spectrum by concentrating the energy in the fundamental while reducing higher harmonics. We have designed and fabricated a 4-bit and a 7-bit encoder circuit with integrated digital amplifier, and have demonstrated its operation up to a clock speed of 12.8 GHz.