This letter proposes a miniature CMOS stacked spiral-coupled (SSC) directional coupler for a fully integrated CMOS transceiver to isolate the large transmit power from the transmitter to the receiver. It consists of two 90 • -rotated center-aligned SSC coils. The path center of one coil is aligned to the gap center of the other coil. The grounded secondary coil plays a role of ground shielding for reducing the insertion loss. Two capacitors are utilized for tuning the center frequency and isolation level. Fabricated in a standard CMOS process, it achieves an isolation of −67 dB, an insertion loss of −0.8 dB, and a coupling of −16.1 dB and only occupies 0.3 × 0.3 mm 2 of silicon area. To the authors' knowledge, this is the first CMOS directional coupler at the UHF band, with the smallest form factor when compared with other implementations of directional couplers.

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SummaryThis paper presents an interference‐resilient receiver front end for body channel communication, which uses the human body as the signal transmission medium in the band of 40 to 60 MHz. To tackle strong out‐of‐band interference resulting from the human body antenna effect, a series N‐path filter–based balun preceding a low‐noise amplifier is proposed to improve the frequency modulation radio interference suppression and realize single‐to‐differential transformation of input body channel communication signals. Besides, an enhanced harmonic rejection with combination of a series N‐path filter and a harmonic rejection passive mixer using 8‐phase 12.5% duty‐cycle clocks as local oscillator is presented. The transconductance amplifier sharing is used to reduce the power consumption of the mixer. Fabricated in a standard 0.18‐μm complementary metal‐oxide‐semiconductor process, the proposed receiver front end occupies an area of 780 × 660 μm2 including pads. Operating at 50 MHz, the front end consumes 1.35 mA from a 1.8‐V supply and achieves a maximum gain of 50.5 dB with a noise figure of 11.2 dB, 28.5 dB frequency modulation interference suppression at the second‐order harmonic rejection, and 53.9 dB at the third‐order harmonic rejection.