Chapter 2 Multi-Band/Multi-Mode Radio Systems 5
3.3 A Concurrent Dual-Band Receiver
digital converters (ADC) that can operate at multiple Gsamples/sec range with the tough dynamic-range requirements of radio standards are still not on the horizon for implementation. At the same time, the dynamic-range of these ADCs usually comes at the price of increased power consumption. As power consumption of digital circuitry increases in proportion to the frequency of operation, justifying the direct digitization for low-power portable radio receivers becomes more difficult. Upon availability of the desired ADC, the clear advantages of such implementations in the future are their performance scaling with technology improvements, ideally automated and faster design cycles, programmability, and extreme versatility in architectures and applications. In the following section we introduce an alternative approach that does not suffer from these deficiencies.
Figure 3.6: Generic architecture of a direct digitization radio
and combine it with proper passive resonant circuitry at the input and output, as discussed briefly in the previous section. This approach shapes the frequency response, ensures stability, and achieves gain and matching at the single band of interest [27].
Figure 3.7: Evolution process of two parallel receivers to a concurrent dual-band receiver
A very important observation is that the transconductance of the transistor is inherently wide-band and can be used to provide small-signal gain and matching at other frequencies without any penalty in the power dissipation. This leads to a compact and efficient front- end for a concurrent dual-band receiver which consists of a dual-band antenna [28]-[30], followed by a monolithic dual-band filter [31] and a concurrent dual-band LNA that provides simultaneous gain and matching at two bands. This is illustrated in the lower part of Figure 3.7. A detailed approach to the design and analysis of such a multi-band LNA will be described in Chapter 4. It should be noted that the concurrent dual-band receiver does not use any dual-band switch [32] or diplexer [33], since simultaneous reception at both
LO1 LO2
Band 1
LNA BPF3
ω ω ω Gain ω
ωω ωω1
BPF2 BPF1
ω ω ω ωin1
Dual-Band Filter Dual-Band
Antenna
Dual-Band LNA
ωω ωω
Gain
ωω ωω1ωωωω2
S11
ωω
ωω1ωωωω2 ωωωω
LO1' LO2'
LNA BPF3'
ω ω ω Gain ω
ωωω ω2
BPF2' BPF1'
ωωω ωin2
Dual-Band Image-Rejection Downconvertor
Band 2
Band 1
bands is desired. A dual-band down-conversion scheme is subsequently needed to translate different information-carrying signals to baseband with as few local oscillators (LO) and external filters as possible, while maintaining isolation between the two bands. This can be achieved in numerous ways (e.g., heterodyne, homodyne). Figure 3.8 shows a simplified block diagram of one such receiver that evolves from the single-band image-reject architecture proposed by Weaver [34].
Figure 3.8: Concurrent dual-band receiver architecture
The frequency of the first local oscillator (LO) that appears after the LNA and performs the first down-conversion determines the image frequency(ies) and plays an important role in the performance of the system. For a non-concurrent receiver, it has been proposed to choose the first LO frequency halfway between the two frequency bands and select the band of interest by choosing the appropriate sideband produced by an image-separation mixer [9]. Although this method is sufficient for the non-concurrent approaches, it will suffer from serious shortcomings if used for a concurrent receiver, where the LNA amplifies the signal in both of the desired bands. This is due to the fact that one band is the image of the other and there is no attenuation of the image by either the antenna or the filter. The situation is exacerbated by the LNA gain in the image band. In this scenario, one is solely relying on the image rejection of the single sideband receiver, which is limited by
Dual-Band
Filter Dual-Band LNA
A
B fLO1
I Q
Amplification & Filtering
fLO2
I (a) Q
(b) Dual-Band
Antenna
fLO3
I Q
the phase and amplitude mismatch of the quadrature LOs and the signal paths [35],[36], and is usually insufficient in a concurrent receiver.
An alternative approach, which does not suffer from the above problem and in fact significantly improves the image rejection, is to use an offset LO, as shown in Figure 3.9.
The LO frequency is offset from the midpoint of the two bands of interest (fA and fB) in such a way that the image of the first band at fA falls at the notch of the front-end transfer function at fIA. The attenuation at fIA is determined by the compounded attenuation of the dual-band antenna, filter, and LNA. Similarly, the image of the second band at fB will fall outside the pass-band of the front-end at fIB and will be attenuated accordingly. Using a quadrature first LO makes the stage fit to act as the first half of any single-sideband image- reject architecture, similar to that proposed by Weaver [34]. Since the receiver has to demodulate two bands concurrently and independently, two separate paths must be used eventually. Each path is comprised of the second half of the image reject architecture, as shown Figure 3.8, which provides further image rejection (Figure 3.9). This architecture eliminates an extra antenna, a front-end filter, an LNA, and a pair of high-frequency mixers, which in turn results in power, footprint, and area savings. In addition, large image rejection in excess of that of the single-sideband receiver is achieved through this diligent frequency planning and proper usage of stop-band attenuation. A concurrent dual-band receiver based on this proposed architecture is implemented and will be discussed in detail in Chapter 5.
Figure 3.9: Operation principle of the proposed concurrent dual-band receiver in frequency domain