Efficient, ROM-less DDFS using non-linear interpolation and non-linear DAC

Abstract

A non-linear interpolation based ROM-less Direct Digital Frequency Synthesisiser (DDFS) is more efficient than previous systems as each current cell in a non-linear DAC is used more effectively. This was achieved by forming an analogue voltage from a small linear DAC addressed by phase bits that are usually discarded. The analogue voltage was connected to a selected current source in a thermometer decoded non-linear DAC to allow nonlinear interpolation between the conventional, phase limited output levels. By increasing the number of phase bits the spurious free dynamic range (SFDR) was improved without increasing the size of the non-linear DAC. Modelling and simulation of the non-linear response of the differential switch based current cell revealed suitable parameters. The architecture of 64 current cells used a modified thermometer decoder and three-state switch in each current cell. Simulation and testing of 10 sample circuits demonstrated a robust DDFS with SFDR better than − 60 dBc and suitable for use in a wide range of instrumentation systems.

Compact wide-range sinusoidal signal generator for in vivo Impedance Spectroscopy

“…Memory-less or ROM-less DDFS replace the memory and the linear DAC with a nonlinear DAC (NLDAC) which directly produces the sinusoidal waveform from the variable input clocks [4]. However the NLDAC is usually of the current steering type and employs nonlinear interpolation techniques [5], which increase the power consumption and complexity. Both types of DDFS also require a very high oversampling ratio (OSR) of the updating master clock to generate low-frequency outputs in order to relax the requirements on the reconstruction filter.…”

Exact analysis of spurious signals in Direct Digital Frequency Synthesizers due to amplitude quantization

“…The LUT is commonly implemented as a Read Only Memory (ROM), and is utilized to convert the phase values to sinusoidal amplitude values. The DAC generates an analog wave in response to the digital input words from the LUT [6] . The AIF is added after the DAC to remove the unneeded components.…”