Cognitive radio transmitter with a broadband clean frequency spectrum

The tremendous increase in wireless communication over the last few decades has led to a congestion of the radio frequency (RF) spectrum. As suitable RF spectrum is scarce, attempts are being made to use the RF spectrum in a more intelligent efficient way. A Cognitive Radio (CR) addresses this problem by Dynamic Spectrum Access, i.e. measure which spectrum is temporarily locally free and then use it. The CR transmitter needs to be flexible to be able to transmit where ever there is free spectrum available.\ud Conventional transmitters not only produce the desired upconverted information signal but also many unwanted harmonics of the local oscillator (LO) and distortion products related to the baseband signal. These unwanted products have been usually suppressed using dedicated RF filters which are narrowband and inflexible. For CR transmitters flexibility is a key requirement, and hence other techniques are wanted to suppress unwanted products, without using the inflexible filters.\ud Previous research has shown that polyphase multipath circuits can in principle cancel a large number of harmonics and distortion products. However, the upconversion was done using a large number of paths which is not very power efficient. In this work a less complex and more power efficient implementation of this technique is proposed. The proposal is actually based on a combination of three techniques, namely: 1) 8-path polyphase upconversion, 2) Tuning of the LO duty cycle ratio to close to 7/16 and 3) Tunable first order RF filter.\ud To verify the functionality of the proposed techniques, a flexible 8-path transmitter chip was fabricated in a 160 nanometer CMOS technology. The transmitter works over 3 octaves in frequency from 100MHz to 800MHz. Measurements show that the frequency agile transmitter achieves a broadband clean output spectrum where all unwanted products are at least 40dB below the wanted transmit signal. This is the first polyphase multi-path transmitter combining the baseband multi-phase generation and RF circuit on one chip. Compared to other harmonic rejection transmitter designs with similar frequency range, it is more power efficient and has better LO leakage and image rejection. This chip suppresses ALL LO harmonics and distortion products for ALL frequencies, without any external filters