Root mean square (RMS) Schottky barrier diode (SBD) detectors with detector gain of 4.6 V-1 , operating frequency range around 30-50 GHz or bandwidth greater than 20 GHz, and an area of 36 μm2 are demonstrated in 45-nm SOI CMOS. The maximum detector insertion loss up to 50 GHz is 0.1 dB relative to that for a thru structure with a maximum loss of 0.25 dB. The detectors for the first time are simultaneously compact, high impedance, and wide bandwidth. The Schottky diode and detectors are also the first demonstration in nano-scale SOI CMOS. The detectors allow measurements of internal-node voltages of millimeter wave circuits with a DC voltmeter. Using the detectors, the frequency responses of node voltages and gains in a millimeter wave mixer are measured. Adding the detectors to the mixer, while improving mixer IIP3, should not degrade the mixer's noise and gain characteristics, and does not increase the die area. With an external micro-controller, autonomous bias optimization for maximum conversion gain is also demonstrated.