A ±1V-DC to 20-GHz Front-End Chipset with 1.5-Vpp AC and 0.5-to-1V DC Outputs for Direct Sampling Real-Time Oscilloscopes

This paper presents a 20 GHz DC-coupled RF front-end chipset for direct sampling digital real-time oscilloscopes. The chipset consists of four chips: a DC canceling amplifier (DCCA), a distributed active balun (DAB), an equalizer, and a dual differential output variable gain amplifier (DDOVGA). The DCCA cancels the input DC voltage with a range of $\pm 1 \mathrm{~V}$. The DAB achieves single-ended to differential conversion in a wideband starting from DC. The DDO-VGA has two well balanced differential outputs with output DC adjustment. All four chips are DC-coupled and the DC voltages in inter-stages are shifted to 0 V. This chipset is prototyped with a 0.10-$\mu \mathrm{m}$ GaAs pHEMT technology. The measurements of the front-end show a 4.5- to-6.5dB noise figure, $23-\mathrm{dB}$ gain tuning range, and $\lt 1-\mathrm{dB} / \mathrm{V}$ gain variation across $\pm 1 \mathrm{~V}$ input DC over DC-to- 20 GHz bandwidth.