REVIEWS ON FRONT–END RECEIVERS, LOW–NOISE
2.1 Receiver Architectures
2.1.1 Homodyne, Direct–Conversion, or Zero–IF Receiver
In a homodyne or direct-conversion receiver (DCR) which is depicted in Fig. 2.1 [14–21], the desired RF signal is directly down-converted to zero-IF in one-step frequency mixing with single LO signal. Therefore, in this type of the receiver, the LO frequency is equal to the RF frequency. The baseband signal is then filtered with a low-pass filter to select the desired channel.
For frequency- and phase-modulated signals, the down-conversion must provide quadra-ture outputs in order to avoid loss of signal information. The main advantage of DCR is that is does not possess the image problem when the incoming RF signal is directly down-converted to baseband without any IF stage. Another advantage is its simple architecture.
However, the major disadvantage is DC offsets [22]. As shown in Fig. 2.2, the severe DC offsets can be generated at the output of the mixer when the leakage from the local os-cillator is self-mixed with LO signal. The second source of DC offsets is the large nearby interferers leaking to the VCO and then self-mixing. This effect could saturate the fol-lowing stage. The DC offsets can be removed by capacitive coupling. However, the signal power near DC will be lost. Hence, the size of capacitors should be chosen quite large.
Feedback loops from the baseband or the digital part are also proposed to reduce the DC offsets. But these methods will increase the complexity of the direct-conversion receiver.
Equally critical is the flicker noise of the mixer since the mixer output is the baseband signal and can be easily corrupted by large noise. It is because that the flicker noise of active devices becomes the dominant noise source as the frequency below 1 MHz. The flicker noise should be considered in designing DCR. Active devices with large dimension can be chosen to reduce flicker noise. In addition, PMOS contributes less flicker noise than NMOS.
2.1.2 Heterodyne or IF Receivers
The most straightforward receiver architecture for implementing a cellular receiver front-end is evidently the heterodyne receiver, which is shown in Fig. 2.3 [23–28]. The
main feature is the use of an Intermediate Frequency (IF). For this reason, the heterodyne is often also called the IF receiver.
The received RF signals from the antenna are first filtered by a band select filter, BPFRF1, which suppresses interferences residing outside of the application band. By removing these out-of-band blocking signals, which could saturate the following stages, the requirement of the dynamic range of the receiver can be relaxed considerably. A low noise amplifier (LNA) amplifies the received RF signals, which are then filtered by an image-reject filter, BPFRF2, to remove the image. The image has an offset of twice the intermediate frequency by the mixer. The received RF signals after BPFRF2 are down-converted to IF by the down-conversion mixer, and then passed through the channel-select filter BPFIF to remove the interferences at the adjacent channels. Finally, the channel-selected is demodulated into baseband I/Q signals to retrieve the desired signal information.
The high-frequency noise and distortion from inter-modulation and high-order harmonics are removed by baseband low-pass filter LPFBB.
In the frequency translation, both the desired signal and image signal are mapped to the IF frequency after mixing. Although the image-reject filter BPFRF2 is used to attenuate the image signal, suitable attenuation of the image may not be practical unless the IF frequency is selected relatively high. The trade-off is that filtering at a high IF requires more complicated filters in order to maintain selectivity. It is difficult to realize an on-chip high-Q filter at the RF frequency. The required high-Q, high frequency image-reject filter is therefore placed off-chip. Consequently, the integration ability of the heterodyne or IF receiver is limited, and the cost is increased because of several off-chip filters are needed.
Additional buffers to drive off-chip filters also require high power and reduce the gain of this kind of receivers.
The path mismatch is implied not a big issue because the image rejection does not rely on any matching between two signal paths, but is mainly done by the image-reject filter.
Also LO feed-through and DC offset do not affect the signal quality since the desired signal frequency is never close to these frequencies. The same applies to self-mixing of either RF or LO signal. Another important property is that the channel selection occurs before the ADC. Hence, the ADC only requires handling minimum dynamic range. Due to the
bandpass nature of the channel, even the sub-sampling ADC can be used. Additionally, the number of bits can be kept low since both the out-of-band and in-band blocking signals have already been removed.
2.1.3 Image–Reject Receiver
The primary advantage of the image-reject receivers [29–33] is that they do not need image-reject filters. Without the image-reject filters, the IF frequency can be placed very low to reduce the design difficulty of the IF channel-select filter. Hartley [32] and Weaver [33] receivers are two famous image-reject receivers.
The architecture of Hartley receiver is shown in Fig. 2.4. The desired signal and the image signal are down-converted in both upper and lower paths. However, the desired signals at the points B and C are in-phase, while the image signals at the points B and C are out-of-phase. When the spectrum at the points B and C are combined, the image signals will be cancelled and the desired signals will be left.
The architecture of Weaver receiver is shown in Fig. 2.5. The Weaver receiver is different from the Hartley in that the quadrature mixers are used to the replace 90-degree phase shifter in the signal path. The purpose of this replacement is to perform phase shifting not on the signal path, but on the second LO which is only a single sinusoidal tone. Therefore, the phase shifting accuracy can be well controlled.
The Image-Reject Ration (IRR) of the Hartley and Weaver receivers is limited by the gain mismatches between I- and Q-path, the phase inaccuracy of quadrature LO signals, and the imperfect quadrature phase shifting. The IRR can be expected by
IRR = 1 + (1 + ε)2− 2(1 + ε) cos(θ)
1 + (1 + ε)2+ 2(1 + ε) cos(θ) (2.1.1) where ε and θ are the gain and phase mismatch, respectively. For ε = 5% and θ = 5◦, the IRR is 26 dB. Existing implementations of image rejection receivers typically achieve 30∼ 40 dB for image rejection.
2.1.4 Wideband–IF Receiver
Shown in Fig. 2.6 [34, 35] is the architecture of wideband-IF receiver. The architecture of this receiver is similar to a combined technique of heterodyne receiver and Weaver image-reject receiver. In heterodyne receiver in Fig. 2.3, the channel selecting is performed using the RF local oscillator. However, the wideband-IF receiver uses a fixed RF local oscillator at the first mixing stage, and the entire received bands are translated to the fixed IF. In the second mixing stage, a tunable IF local oscillator is used to select the desired channel from the received entire bands, and the desired channel is translated to the baseband. Simultaneous image rejection is performed in the second mixing stage which uses quadrature frequency conversion.
Since the RF local oscillator is at fixed frequency, the phase noise performance of the oscillator can be optimized. Besides, it is relatively easier to design the IF VCO with a low phase noise. Nevertheless, the disadvantage is that the blocking signals at adjacent channels are translated to the baseband without filtering. Hence, the dynamic range or linearity shall be carefully considered. Take the linearity requirement into consideration, the gain of the receiver is mostly provided from the IF section. Leaving the gain to the IF section may increase the Noise Figure of the receiver. Besides, the image signal still interferes with the desired signal if I- and Q-path at the first stage have mismatches.
Sometimes, an off-chip RF filter is required for high IRR.
2.1.5 Low–IF Receiver
The architecture of low-IF receiver is shown in Fig. 2.7 [36–38]. The low-IF receiver combines the advantage of heterodyne and direct-conversion receivers. The desired RF sig-nals are down-converted to IF in one mixing step, which is similar to the direct-conversion receiver. Since the IF is higher than DC, DC offsets and flicker noise do not affect the desired signals. In low-IF receivers, poly-phase filters are used to remove the image; hence, the high-Q image-reject filter is not required. Because the polyphase filters are operated at the low intermediate frequency and are possible to be realized on-chip, low-IF receivers are obvious to have better integration capability than heterodyne receivers.
The IRR of the low-IF receiver is limited by the gain mismatches between I-path and Q-path, phase inaccuracy of the quadrature LO. The spectra flow of the low-IF receiver before and after down-conversion is shown in Fig. 2.8. The spectrum of complex signal (I+jQ) is represented. SIGpRF and IMpRF represent the spectrum of the desired signals and image signals. LOp represents the spectrum of the quadrature local oscillator. However, SIGnRF and IMnRF represent the crosstalk image signals of SIGpRF and IMpRF, respectively. LOn represents the crosstalk image signal of LOp. After the frequency conversion, the SIGpRF, SIGnRF, IMpRF, and IMnRF are down-converted to SIGnIF, SIGpIF, IMpIF, and IMnIF, respectively. As shown in Fig. 2.8, the image IMpIF mixes with the signal SIGpIF at the positive IF frequency ωIF, and hence cannot be removed following polyphase filter. Only the signals IMnIF and SIGnIF at the negative IF frequency −ωIF can be removed by the following polyphase filter. Since mismatches in RF circuits are inevitable even in modern IC process, it is difficult to achieve high IRR without special and complicated techniques for low-IF receivers.
2.1.6 Double–Quadrature Receiver
Shown in Fig. 2.9 [39–41] is the architecture of Double-Quadrature Receiver (DQR) which is used to improve the IRR. The DQR shifts the phase of RF signal to quadrature and then the quadrature RF signals are downconverted to IF signals by mixing with quadrature LO signals. The DQR is less sensitivity to the imbalance of LO signals of I- and Q-path because the RF and LO signals are both put into quadrature phases. Since the RF signals are down-converted to IF, it is also immunity from the problem of DC offsets and flicker noises.
The spectra flow of the DQR before and after down-conversion is shown in Fig. 2.10.
The spectrum of complex signal (I+jQ) is represented. SIGpRF and IM pRF represent the spectrum of the desired signals and image signals at the output of the quadrature generator.
LOp represents the spectrum of the quadrature local oscillator. However, SIGnRF and IM nRF represent the crosstalk image signals of SIGpRF and IM pRF, respectively. LOn represents the crosstalk image signal of LOp. After the frequency conversion, the SIGpRF, SIGnRF, IM pRF, and IM nRF are down-converted to SIGnIF, SIGpIF, IM pIF, and IM nIF, respectively. The same like the spectra of low-IF receiver in Fig. 2.8, the image
IM pIF mixes with the signal SIGpIF at the positive IF frequency ωIF, and hence cannot be removed following polyphase filter. Only the signals IM nIF and SIGnIF at the negative IF frequency −ωIF can be removed by the following polyphase filter. However, the main difference between DQR and low-IF receiver can be seen from the value of IM pIF of DQR which can be represented as
IM pIF = IM nRFLOp(ISRQGISRLO+ ISRmixers) (2.1.2) where ISRQG, ISRLO, and ISRM ixers denote the Image-to-Signal Ratio (ISR) of the quadrature generator, the local oscillator, and mixers, respectively. For ISRQG, ISRLO, and ISRM ixers 1, the ISRQGISRLO term in (2.1.2) is negligible relative to ISRM ixers Therefore, IM pIF is determined by the gain/phase errors of the mixers and IM nRFLOp The DQR exhibits better image-reject performance than the conventional low-IF receiver, because IMpIF is smaller and almost unaffected by ISRLO. To achieve high IRR of the DQR, the symmetry of the layout in mixers between I/Q-paths should be regarded as reducing the amplitude of crosstalk image signals. Additionally, the polyphase filter must have a high capacity for rejecting images at the intermediate frequency.
The comparisons of the afore-mentioned receiver are listed in Table 2.1. The hetero-dyne receiver can achieve the best performance because it is immunity from the problem of I/Q-mismatches, DC offset and flicker noise. The expenses of the heterodyne receiver are high power consumption and poor integration ability. The DCR has the advantage of high integration ability and low-power consumption. However, the high performance is difficult to achieve because of the problems of DC offsets and flicker noise. Image-rejection, wideband-IF, and low-IF receivers achieve better performance than direct-conversion re-ceiver, but the imbalance of LO signals will limit the IRR. Hence, off-chip high-Q RF filters are still required for high IRR. The DQR down-converts the RF signal to IF, so it will not be affected by DC offsets and flicker noise. Besides, the DQR is less sensitive to I/Q-imbalances, and off-chip high-Q RF filters are not required.