Chapter 2 Fundamentals of Single-Path and Multi-Path Receiver 4
2.1.4 Single-Path Receiver Architecture
In over one hundred years of development, many receiver architectures have been proposed and demonstrated for different requirements of various wireless applications.
The choice of receiver architecture considers performance, cost, complexity, power, integrity, and flexibility. A detailed discussion and comparison about each single-path architecture is beyond the scope of this dissertation and can be found in [10][11].
Here our discussion is focused on the two most common architectures, superheterodyne receiver and direct-conversion receiver, to show the general criteria and trade-offs at the system level.
Figure 2.7 depicts the block diagram of a generic heterodyne receiver [12]. The EM power picked up by the antenna is first pre-selected by an RF filter to reject the out-of-band interference and partially suppress the image signal. The RF filter must exhibit a low loss since it is directly added to the overall receiver noise figure. The LNA amplifies the signal power and provides the necessary gain for suppressing the noise of the subsequent blocks. An image rejecting filter is inserted between the LNA and the mixer to further attenuate the image interferences. The RF signal and its image are separated by 2fif in frequency domain. If fif is large enough, the RF filter and the tuned LNA may afford sufficient attenuation to the image, eliminating the need for IR filter. However, a high IF increases the quality factor requirement for the channel selection filter. Therefore, the choice of IF is a trade-off between channel selection and image rejection.
The mixer downconverts the RF signal to IF. The LO port of the mixer is usually driven by a frequency synthesizer that generates a tunable LO frequency. The receiver
RF filterLNA
A/D A/D
DSP0o 90oLO2 LO1
IR filterIF filterMixerVGA
Baseband filter Figure 2.7: A generic superheterodyne receiver
may need to cope with RF signals at different channels in a time-division fashion. A tunable LO translates RF signal at different channels to the same IF frequency so that a fixed channel selection filter can be used. A variable gain amplifier (VGA) prevents the subsequent circuits saturated by a large input. If DR of the input power is very high, a VGA can also be employed at the RF front-end and baseband to achieve more tuning capability.
Quadrature paths are often employed to translate the signal from IF to baseband, i.e., the LO signals driving the in-phase path (I path) mixer and quadrature path (Q path) mixer differ by 90o. This is because in the bandwidth-efficient modulation scheme, the signal spectrum is asymmetric around the carrier frequency. When downconverted to baseband, the information carried in the upper-side band will be irreversibly lost in those of the lower-side band. The solution to this problem is to separate the signal into two elements differing in phase and treat the two elements together as a complex variable, whose frequency spectrum is not necessarily symmetric to dc, so that the asymmetric information can be preserved.
The downconverted signal is further amplified, filtered, and transformed to the digital domain by using an analog-to-digital converter (ADC), from where much more complex and versatile functions can be performed by digital signal processing (DSP).
Because interference rejection and gain control can be performed at various stages of the downconversion path, the superheterodyne receiver achieves superior performance to other architectures with respect to selectivity, sensitivity, and dynamic range. Since being invented by Edwin Howard Armstrong in 1918 [13], the superheterodyne receiver has served the vast majority of the commercial wireless receivers to date.
The main drawback of the superheterodyne receiver is that when implemented in integrated circuits, it requires external IR and IF filters such as the surface acoustic wave (SAW) or ceramic filter, since the quality factor of integrated filters is limited by the substrate and ohmic loss. To drive the off-chip component via package parasitics, the LNA and mixer demand more power. Most importantly, more external components are used, lowering the cost efficiency of the whole system.
Two modified superheterodyne architectures have been proposed for integrated implementation: wideband-IF receiver [14] and low-IF receiver [15]. Both architectures choose to separate the signal to I and Q path at the first downconversion instead of the second downconversion, circumventing the image problem. However, the number of IF components is doubled, as well as the power consumption.
Figure 2.8 shows the block diagram of a direct-conversion receiver [16], also known as homodyne, or zero-IF, receiver. The direct-conversion receiver employs only one frequency translation step by setting the LO frequency equal to the RF carrier frequency. This architecture minimizes the number of external components by eliminating the IF stage and using quadrature path instead of IR filter to suppress images, hence it is more amenable to monolithic implementation than the superheterodyne receiver. A reduced number of building blocks and no off-chip components can lead to a low system power consumption. Due to those advantages, direct-conversion topology has been more and more popular in modern integrated communication systems.
However, to design a direct-conversion receiver one needs to carefully address several important problems which are less serious in the heterodyne receiver. One of those problems is LO-to-RF leakage. The LO power is leaked to the RF port through parasitic components, EM coupling or substrate, and mixes with the main LO tone, creating a DC offset, which is troublesome to remove in narrow-band modulation. For wideband modulation such as WCDMA, this DC offset is removed by using a base-
RF filter LNA
A/D
A/D 0o DSP
90o LO
Figure 2.8: A generic homodyne receiver
band high-pass filter with a low cut-off frequency, which has little impact on the signal quality. Another problem caused by LO-to-RF leakage is that the leaked LO signal can intermodulate with some strong interferences (for example, in WCDMA receivers, the powerful TX signal leaks into the receiver [17]) creating in-band distortions which are difficult to eliminate. I/Q mismatch is another serious consideration in a homodyne receiver. The phase and amplitude mismatches in the I and Q paths corrupt the signal by distorting the signal constellation. Although quadrature downconversion is also employed at the last downconversion stage in superheterodyne receiver, the I/Q mismatch is a less severe issue in this case because the low frequency mixer is less sensitive to parasitic mismatches. In addition, the direct-conversion receiver is more vulnerable to second order distortion and flicker noise in the circuits.
In short, there is no receiver architecture globally advantageous to all the others.
The optimum choice is determined under certain specifications and applications.