Design of a broadband passive X-band double-balanced mixer in SiGe HBT technology

Abstract

In this paper, a passive double-balanced mixer in SiGe HBT technology is presented. Owing to lack of suitable passive mixing elements in the technology, the mixing elements are formed by diode-connected HBTs. The mixer uses lumped element Marchand baluns on both the local oscillator (LO) and the radio frequency (RF) port. A break out of the Marchand balun is measured. This demonstrates good phase and magnitude match of 0.7° and 0.11 dB, respectively. The Marchand baluns are broadband with a measured 3 dB bandwidth of 6.4 GHz, while still having a magnitude imbalance better than 0.4 dB and a phase imbalance better than 5°. Unfortunately with a rather high loss of 2.5 dB, mainly due to the low Q-factor of the inductors used. The mixer is optimized for use in doppler radars and is highly linear with a 1 dB compression point above 12 dBm IIP2of 66 dBm. The conversion gain at the center frequency of 8.5 GHz is −9.8 dB at an LO drive level of 15 dBm. The whole mixer is very broadband with 3 dB bandwidth from 7 to 12 GHz covering the entire X-band. The LO–IF, RF–IF, and RF–LO isolation is better than 46, 36, and 36 dB, respectively, in the entire band of operation.

A SiGe BiCMOS double-balanced mixer with active balun for X-band Doppler radar

“…It reaches a minimum of ~6.5 dB around the design frequency of 10.5 GHz. Compared to the purely passive X-band double balanced ring mixer in [3] this represents an improvement of ~3.3 dB due to the good noise performance of the active balun.…”

“…It is well-known that direct conversion mixers suffers from DC offset problems and high 1/f-noise [1]. This has led to research into passive doublebalanced mixers using either CMOS resistive mixers [2] or diode-ring mixers [3]. In a SiGe BiCMOS technology, the 1/fnoise properties are believed to be best for the bipolar transistors compared to the MOSFETs.…”

Analysis and design of complex impedance transforming marchand baluns

“…X L = 0, (13) gives X s2 = 0, a short circuit, (12) gives X m → ∞, an open circuit, and the impedance scaling factor becomes α = RL RG , which is the case for the regular Marchand balun [9].…”

“…In figure 4 the schematic is shown. The design procedure follows what is described in [9], with modification to incorporate X s2 and X m . The design equations is given as…”

Design of a double balanced mixer based on a novel planar UWB balun

“…Mixers are important components in these systems for frequency up-conversion and down-conversion. And double balanced mixers have the advantages of low LO noise, better harmonic suppression and high port-to-port isolation [2,3,4,5]. The baluns are the key component in double balanced mixers and determine their performance directly.…”