Design Of Low Noise Modulator With Lo Divider

The up converter modulator is a critical part of a RF transmitter, and it completes the frequency transition of the desired signal from baseband to RF. As for FDD systems, the noise at the receiver band contributed by the modulator directly degrades the performance of the receiver. Traditionally an external SAW filter is used to attenuate the out of band noise from the modulator; but the introduction of the SAW filter increases the cost of the transmitter.This paper presents a low noise modulator which meets the noise requirement of eliminating a SAW filter. The core circuit is a novel two stage mixer which not only rejects noise from LO path but also contributes limited internal noise. Moreover, the modulator employs current mode logic divider and injection locked divider as LO generation circuit. The phase noise of the two dividers varies but the output noise of the modulator is nearly identical, which proves that the two stage mixer can reject noise from LO path effectively.The modulator operates at 2.5GHz and is supplied by 1.2V /2.5V . The measured results show that the output power of the modulator with different LO divider is 0.7dBm and -0.2dBm ; the noise at 40MHz offset frequency is -161.2 dBc /Hz and -161.4 dBc /Hz .The modulator achieves low noise performance to eliminate SAW filter. The chip is fabricated in TSMC 65nm process.