By way of indication, UWB communication is regulated for frequencies that may range from 3 to 10.6 GHz and the permitted spectral templates are defined, on the one hand, by regulatory constraints (FCC in the United States, ECC in Europe, etc.) and, on the other hand, by constraints imposed by standards (IEEE 802.15.4a, IEEE802.15.6 standards, etc.) or proprietary. These constraints on spectral templates may be grouped under the term “template specification”. The bandwidth is in the range of 1 GHz and the duration of the transmitted pulses is typically of a few nanoseconds. More generally, a UWB signal is defined as a signal whose passband bandwidth at −10 dB of the maximum-power spectral density is higher than 500 MHz for frequencies that are above 2.5 GHz, or alternatively, for center frequencies that are lower than 2.5 GHz, whose passband bandwidth is greater than 20% of the center frequency.
By way of example, FIG. 1 shows the general shape of a 5 nanoseconds UWB pulse with a carrier frequency of 4 GHz. FIG. 2 shows the spectral density of this pulse. This spectrum comprises a main peak at the carrier frequency and a passband bandwidth at −10 dB of around 500 MHz, but it also comprises numerous side lobes whose frequency may be close to the carrier frequency. These peaks run the risk of making the spectrum extend outside the template imposed by the specification, and it is therefore necessary to ensure that they are reduced as much as possible.
The use of fully digital transmitter architectures that allow the spectrum of the transmitted pulse to be effectively controlled in order to reduce the side lobes thereof is theoretically known, but these architectures are not readily compatible with standard integrated-circuit technologies when the aim is to operate with frequencies that are higher than 4 GHz, and even less so at frequencies of 8 GHz. These architectures synthesize a waveform in the time domain in the manner of a digital filter operating at the Nyquist frequency. The value, as well as the precision, of the coefficients of the implemented filter have a direct impact on the level of the side lobes. However, the switching times of digital circuit transistors are too high (at least 16 GHz to operate at an 8 GHz center frequency) in the most commonly used technologies and it would be necessary to use much more expensive technologies to solve the problem.
A transmitter architecture that has been proposed in order to implement UWB pulse transmitters in the band of 3 to 8 GHz is an architecture using an oscillator to generate a carrier frequency and a high-speed switch (radiofrequency switch) that is in series and downstream of the oscillator, between the oscillator and a UWB pulse transmission antenna: the high-speed switch is normally blocked and it is turned on for a very brief duration which is the desired duration of the UWB pulse. The output of the switch provides the UWB pulse to the antenna at the carrier frequency of the oscillator. It may be thought of as a sort of time windowing, for a very brief duration T, of an oscillation generated over a longer period.
FIG. 3 shows the principle of such a UWB transmitter. The switch SW may be a switch with one or more transistors, potentially comprising inductors. The oscillator is a controlled-frequency oscillator VCO that allows the carrier frequency to be adjusted F0. A control circuit CTRL closes the switch for a duration T which is the desired duration for the pulse.
The failing of the implementations proposed so far that are based on this architecture is that they do not allow the spectrum of the transmitted pulse to be controlled correctly, and in particular they do not allow the side lobes to be reduced, if necessary, which runs the risk of making the spectrum extend outside the template imposed by the specification. Only the passband bandwidth (at the level of the main lobe) may be controlled by acting upon the duration for which the switch is closed.
The following documents describe UWB pulse generators:    Xu et al., Power-Efficient Switching-Based CMOS UWB Transmitters for UWB Communications and Radar Systems. IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, Vol. 54, NO. 8, August 2006.    Li, The Design of CMOS Impulse Generators for Ultra-Wideband Communication and Radar Systems. THESIS THE UNIVERSITY OF TEXAS AT ARLINGTON, August 2011.    Raphaeli, et al., Ultra Wideband On-Chip Pulse Generator. Patent application US 2010/0177803 A1, July 2010.    Anh Tuan Phan, et al., Energy-Efficient Low-Complexity CMOS Pulse Generator for Multiband UWB Impulse Radio. IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS, Vol. 55, NO. 11, December 2008.    Barras, et al., Low-Power Ultra-Wideband Wavelets Generator With Fast Start-Up Circuit. IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, Vol. 54, NO. 5, May 2006.    Pelissier, et al., A 112 Mb/s Full Duplex Remotely-Powered Impulse-UWB RFID Transceiver For Wireless NV-Memory Applications. IEEE JOURNAL OF SOLID-STATE CIRCUITS, Vol. 46, NO. 4, April 2011.