IF-sampling fourth-order bandpass ΔΣ modulator for digital receiver applications

Bandpass modulators sampling at high IFs (∼200 MHz) allow direct sampling of an IF signal, reducing analog hardware, and make it easier to realize completely software-programmable receivers. This paper presents the circuit design of and test results from a continuous-time tunable IF-sampling fourth-order bandpass ΔΣ modulator implemented in InP HBT IC technology for use in a multimode digital receiver application. The bandpass ΔΣ modulator is fabricated in AlInAs-GaInAs heterojunction bipolar technology with a peak unity current gain cutoff frequency (f_T) of 130 GHz and a maximum frequency of oscillation (f_MAX) of 130 GHz. The fourth-order bandpass ΔΣ modulator consists of two bandpass resonators that can be tuned to optimize both wide-band and narrow-band operation. The IF is tunable from 140 to 210 MHz in this ΔΣ modulator for use in multiple platform applications. Operating from ±5-V power supplies, the fabricated fourth-order ΔΣ modulator sampling at 4 GSPS demonstrates stable behavior and achieves a signal-to-(noise + distortion) ratio (SNDR) of 78 dB at 1 MHz BW and 50 dB at 60 MHz BW. The average SNDR performance measured on over 250 parts is 72.5 dB at 1 MHz BW and 47.7 dB at 60 MHz BW.