A high performance C-band subharmonic modulator exploiting branch-line couplers as input/output ports

Digital direct modulation at microwave frequencies represents the best way to develop a component requiring a simple system design, low cost and compact size. In this paper, a novel architecture of a C-band modulator, which exploits a branch line hybrid to provide an equal amplitude division of Local Oscillator signal (LO), is proposed and experimental results are reported. Using of subharmonic mixer offers the following advantages: inherent Local Oscillator/Radio Frequency (LO/RF) isolation, no bias network is required, low power of LO, rejection of LO's even harmonics. Measured conversion loss is around lOdBc with LO rejection approximately 29dBc and image frequency rejection around 26dBc. At last, the modulator exhibits Radio Frequency/Intermediate Frequency (RF/IF) and RF/LO isolation levels better than 30 dBc. A comparison is also reported to evaluate the performances of three different topologies of the same direct vector modulator.