The tremendous increase in wireless communication over the last few decades has led to a congestion of the radio frequency (RF) spectrum, which is utilized for transmission and reception of information. As suitable RF spectrum is scarce, attempts are being made to use the RF spectrum in a more intelligent efficient way. A Cognitive Radio addresses this problem by Dynamic Spectrum Access, i.e. measure which spectrum is temporarily locally free and then use it. The Cognitive Radio transmitter needs to be flexible to be able to transmit where ever there is free spectrum available. 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 are not flexible. For Cognitive Radio transmitters flexibility is a key requirement, and hence other techniques are wanted to suppress unwanted products, without using the inflexible filters. Moreover, agile operation of the cognitive radio transmitter in a broad band is wanted. Previous research has shown that polyphase multipath circuits can in principle cancel a large number of harmonics and distortion products. However, a solution for wideband polyphase baseband signal generation including digital-to-analog conversion and filtering was lacking. Moreover, the upconversion was done using a large number of paths which takes quite some chip area and 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) a tunable first order RF filter. The combination of these three techniques allows to suppress all unwanted products to more than 40dB below the desired signal. It is possible to improve this further if a tunable RLC network with high quality factor is used at the RF output. The multiphase baseband signals required for an 8-path upconversion can be generated using a simplified vector modulator type of architecture. In order to allow for a Spurious Free Dynamic Range of 50dB, the use of a Digital to Analog Converter (DAC) with a resolution of at least 7 bits is proposed. It is shown that it is possible to cancel the first dominant DAC image by using a polyphase DAC architecture, which relaxes analog reconstruction filtering requirements. 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 ii 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. Note that this chip suppresses ALL LO harmonics and distortion products for ALL frequencies, without any external filters.
|Award date||2 Jul 2014|
|Place of Publication||Enschede|
|Publication status||Published - 2 Jul 2014|