Architecture, design, and test of continuous-time tunable intermediate-frequency bandpass delta-sigma modulators

This paper examines the architecture, design, and test of continuous-time tunable intermediate-frequency (IF) fourth-order bandpass delta-sigma (BP ΔΣ) modulators. Bandpass modulators sampling at high IFs (~100 MHz) allow direct sampling of the RF signal-reducing analog hardware and make it easier to realize completely software programmable receivers. This paper presents circuit design of and test results from continuous-time fourth-order BP ΔΣ modulators fabricated in AlInAs/GaInAs heterojunction bipolar technology with a peak unity current gain cutoff frequency (f_T) of 80 GHz and a maximum frequency of oscillation (f_MAX) of about 130 GHz. Operating from ±5-V power supplies, a fabricated 180-MHz IF fourth-order ΔΣ modulator sampling at 4 GS/s demonstrates stable behavior and achieves 75.8 dB of signal-to-(noise+distortion)-ratio (SNDR) over a 1-MHz bandwidth. Narrowband performance (~1 MHz) performance of these modulators is limited by thermal/device noise while broadband performance (~60 MHz), is limited by quantization noise. The high sampling frequency (4 GS/s) in this converter is dictated by broadband (60 MHz) performance requirements