Pick  Important: Zero IF (Zero IF) or Direct-Conversion (Direct-Conversion) receiver has the characteristics of small size, low cost and easy monolithic integration, and it is becoming a very competitive structure in radio frequency receivers. Based on the introduction of the super heterodyne (Super Heterodyne) structure and the performance and characteristics of the zero-IF structure, this article focuses on the analysis of the local oscillator leakage (LO Leakage) , even-order distortion (Even-Order DistorTIon) , and DC deviation ( DC Offset) , flicker noise (Flicker Noise) and other issues, and give the design method and related technology of zero-IF receiver.
introduction
In recent years, with the rapid development of wireless communication technology, wireless communication system products have become more and more popular and have become an important part of the development of today's human information society. The radio frequency receiver is located at the forefront of the wireless communication system, and its structure and performance directly affect the entire communication system. Optimizing the design structure and selecting the appropriate manufacturing process to improve the performance-price ratio of the system are the directions pursued by RF engineers. Because the zero-IF receiver has the characteristics of small size, low cost, and easy monolithic integration, it has become a very competitive structure in radio frequency receivers and has received extensive attention in the field of wireless communications. Based on the introduction of the performance and characteristics of the superheterodyne structure and the zero-IF structure, this paper analyzes the possible problems of the zero-IF structure, and presents the design method and related technology of the zero-IF receiver.
Superheterodyne receiver
Superheterodyne (Super Heterodyne) architecture since 1917 since the invention of Armstrong, has been widely adopted. Figure 1 is a block diagram of the superheterodyne receiver. In this structure, the radio frequency signal received by the antenna first passes through the radio frequency band pass filter (RF BPF) , low noise amplifier (LNA) and image interference suppression filter (IR Filter) , and then performs the first down conversion to produce a fixed Frequency Intermediate Frequency (IF) signal. Then, the intermediate frequency signal passes through an intermediate frequency band pass filter (IF BPF) to remove adjacent channel signals, and then performs a second down-conversion to obtain the required baseband signal. The RF bandpass filter in front of the low noise amplifier (LNA) attenuates out-of-band signals and image interference. The image interference suppression filter before the first down-conversion is used to suppress the image interference and attenuate it to an acceptable level. Using the adjustable local oscillator (LO1) , the entire spectrum is down-converted to a fixed intermediate frequency. The mid-band pass filter after down-conversion is used to select the channel and is called the channel selection filter. This filter plays a very important role in determining the selectivity and sensitivity of the receiver. The second downconversion is quadrature to generate two baseband signals in phase (I) and quadrature (Q) .
The superheterodyne architecture is considered to be the most reliable receiver topology, because excellent selectivity and sensitivity can be obtained by appropriately selecting the intermediate frequency and filter. As there are multiple frequency conversion stages, DC deviation and local oscillator leakage will not affect the performance of the receiver. However, the image interference suppression filter and the channel selection filter are both high- Q band-pass filters, and they can only be implemented off-chip, thereby increasing the cost and size of the receiver. At present, it is very difficult to integrate these two filters with other radio frequency circuits on a chip using integrated circuit manufacturing technology. Therefore, the monolithic integration of superheterodyne receivers is difficult to achieve due to technological limitations.
Zero-IF receiver
Since the zero-IF receiver does not require an off-chip high- Q band-pass filter, it can achieve monolithic integration, and has received wide attention. Figure 2 is a block diagram of the zero-IF receiver. Its structure is much simpler than that of a superheterodyne receiver. The received radio frequency signal is amplified by a filter and a low-noise amplifier, and then mixed with two orthogonal local oscillator signals to generate in-phase and quadrature baseband signals respectively. Since the frequency of the local oscillator signal is the same as the frequency of the radio frequency signal, the baseband signal is directly generated after mixing, and the channel selection and gain adjustment are performed on the baseband, and are completed by the on-chip low-pass filter and variable gain amplifier.
The most attractive feature of the zero-IF receiver is that it does not need to go through the intermediate frequency during the down-conversion process, and the image frequency is the radio frequency signal itself, and there is no image frequency interference. The image suppression filter and the intermediate frequency filter in the original superheterodyne structure are both Can be omitted. On the one hand, external components are eliminated, which is conducive to the monolithic integration of the system and reduces costs. On the other hand, the number of circuit modules and external nodes required by the system is reduced, which reduces the power consumption required by the receiver and reduces the chance of RF signals being interfered by external sources.
However, the zero-IF structure has problems such as DC deviation, local oscillator leakage and flicker noise. Therefore, effectively solving these problems is a prerequisite to ensure the correct realization of the zero-IF structure.
LO leakage (LO Leakage)
The local oscillator frequency of the zero-IF structure is the same as the signal frequency. If the isolation performance between the local oscillator port and the RF port of the mixer is not good, the local oscillator signal is easily output from the RF port of the mixer and then passes through low noise The amplifier leaks to the antenna and radiates to the space, forming interference to the adjacent channel. Figure 3 shows a schematic diagram of the local oscillator leakage. Local oscillator leakage is not easy to occur in a superheterodyne receiver, because the frequency of the local oscillator is very different from the signal frequency, and the frequency of the local oscillator generally falls outside the frequency band of the pre-filter.
Even-order distortion (Even-Order DistorTIon)
A typical RF receiver is only sensitive to the effects of odd-order intermodulation. In the zero-IF structure, even intermodulation distortion will also cause problems for the receiver. As shown in Figure 4 , assuming that there are two strong interference signals near the desired channel, the LNA has even-order distortion, and its characteristic is y(t)=a1x(t)+a2x2(t) . If x(t)=A1cosw1t+A2cosw2t , then y(t) contains the term a2A1A2cos(w1-w2)t , which indicates that two high-frequency interferences will produce a low-frequency interference signal through the LNA with even-order distortion . If the mixer is ideal, after this signal is mixed with the local oscillator signal cowLOt , it will be moved to a high frequency, which has no effect on the receiver. However, the actual mixer is not ideal, the isolation between the RF port and the IF port is limited, and the interference signal will pass through the RF port of the mixer directly into the IF port, causing interference to the baseband signal.
Another manifestation of even-order distortion is that after the second harmonic of the radio frequency signal is mixed with the second harmonic of the local oscillator output, it is down-converted to the baseband, overlapping with the baseband signal, causing interference. The conversion process is shown in the figure. 5 shown.
Here we only consider the even-order distortion of the LNA . In practice, the RF port of the mixer will encounter the same problem, which should arouse sufficient attention. Because the signal added to the RF port of the mixer is the RF signal amplified by the LNA , this port is the place with the strongest signal amplitude in the RF path, so the even-order nonlinearity of the mixer will cause serious distortion at the output. .
The solution to even-order distortion is to use a fully differential structure in the low-noise amplifier and mixer to offset even-order distortion.
DC offset (DC Offset)
DC deviation is a kind of interference unique to the zero-IF scheme, which is caused by self-mixing (Self-Mixing) . The leaked local oscillator signal can be reflected from the output end of the low noise amplifier, the output end of the filter and the antenna end, or the leaked signal can be received by the antenna and enter the RF port of the mixer. It mixes with the local oscillator signal from the local oscillator port, and the beat frequency is zero, which is DC, as shown in Figure 6(a) . Similarly, the strong interference signal entering the low noise amplifier will also leak into the local oscillator port due to the poor isolation performance of each port of the mixer, and in turn mix with the strong interference from the RF port, and the difference frequency is DC, as shown in the figure. As shown in 6(b) .
These DC signals will be superimposed on the baseband signal and cause interference to the baseband signal, which is called DC deviation. The DC deviation is often larger than the noise of the RF front-end, which makes the signal-to-noise ratio worse. At the same time, a large DC deviation may saturate the amplifiers at all levels after the mixer and cannot amplify useful signals.
After the above analysis, we can estimate the DC deviation caused by self-mixing. Assuming that in Figure 6(a) , the total gain from the antenna to point X is about 100 dB , the peak-to-peak value of the local oscillator signal is 0.63 V ( 0 dBm in 50 Ω ) , and the signal is attenuated when coupled to point A Up to 60 dB . If the total gain of the low-noise amplifier and the mixer is 30 dB , the mixer output will produce a DC deviation of approximately 7 mV . The signal level is useful at this point may be as small as 30 μ Vrms. Therefore, if the DC deviation is directly amplified by the remaining 70 dB gain, the amplifier will enter a saturated state and lose its ability to amplify useful signals.
When the self-mixing frequency changes with time, the DC deviation problem will become very complicated. This situation can occur under the following conditions: When the local oscillator signal leaked to the antenna is emitted by the antenna, it is reflected from the moving object and received by the antenna, and enters the mixer through the low-noise amplifier, and the DC generated by the mixing The deviation will be time-varying.
It can be seen from the above discussion that how to eliminate the DC offset is the key consideration when designing a zero-IF receiver.
AC coupling (AC Coupling)
The baseband signal after down-conversion is coupled to the baseband amplifier by a capacitor blocking method to eliminate the interference of DC deviation. For baseband signals with relatively large energy concentrated near DC, this method will increase the bit error rate and should not be used. Therefore, an effective method to reduce the DC offset interference is to appropriately encode the baseband signal to be transmitted and select a suitable modulation method to reduce the energy of the baseband signal near DC. At this time, AC coupling can be used to eliminate DC deviation without loss of DC energy. The disadvantage is that large capacitors are used, which increases the area of ​​the chip.
Harmonic mixer (Harmonic Mixing)
The working principle of the harmonic mixer is shown as in Fig. 7 . The frequency of the local oscillator signal is selected as half of the frequency of the radio frequency signal, and the mixer uses the second harmonic of the local oscillator signal to mix with the input radio frequency signal. The self-mixing caused by the local oscillator leakage will generate an AC signal with the same frequency as the local oscillator signal, but does not produce a DC component, thereby effectively suppressing the DC deviation.
Figure 8 shows a CMOS harmonic mixer, the second harmonic of the local oscillator signal can be obtained through the inherent square law characteristics of the CMOS transistor. The circuit composed of transistors M3 and M4 converts the differential local oscillator voltages Vlo+ and Vlo- into time-varying currents with second harmonics. The fundamental frequency and odd harmonics of the local oscillator signal are cancelled at the drain connection to generate harmonics. The second harmonic current of the local oscillator signal required by the mixer realizes harmonic mixing.
Flicker noise (Flicker Noise)
The flicker noise in the active device is also called noise, and its magnitude increases with the decrease of the frequency, and it is mainly concentrated in the low frequency band. Compared with bipolar transistors, field-effect transistors are much more noisy. The flicker noise interferes with the baseband signal moved to zero intermediate frequency and reduces the signal-to-noise ratio. Usually most of the gain of the zero-IF receiver is placed in the baseband stage, and the typical gain of the low-noise amplifier and mixer of the RF front-end part is about 30 dB . Therefore, the amplitude of the useful signal after down-conversion is only tens of microvolts, and the influence of noise is very serious. Therefore, the mixer in the zero-IF structure is not only designed to have a certain gain, but also the noise of the mixer should be minimized during the design.
The transistors M1 and M2 in the harmonic mixer shown in Figure 8 are driven by radio frequency differential signals Vrf+ and Vrf- . M1 and M2 are the main sources of noise. The injection current Io is used to reduce the current in the transistors M1 and M2 , thereby Reduce noise.
I / Q mismatch (I / Q Mismatch)
When using the zero-IF scheme for digital communication, if the in-phase and quadrature branches are inconsistent, for example, the gain of the mixer is different, the phase difference between the two local oscillator signals is not strictly 90o , which will cause the baseband I/Q signal to change, namely Produce I/Q mismatch problem. In the past, I/Q mismatch was the main obstacle in digital design. With the improvement of integration, although I/Q mismatch has been improved correspondingly, sufficient attention should still be paid to the design.
Concluding remarks
This paper discusses the characteristics of the two structures of superheterodyne and zero-IF, analyzes the causes of local oscillator leakage, even-order distortion, DC deviation, flicker noise and other problems in the zero-IF structure, and gives the characteristics of the zero-IF receiver. Design methods and related technologies. â–
references:
1 . Behzad Razavi, “ Design ConsideraTIons for Direct-Conversion Receivers ' , IEEE TransacTIons on Circuits and Systems-II: Analog and Digital Signal Processing, Vol. 44, No. 6, June 1997.
2 . Asad A. Abidi, ' Direct-Conversion Radio Transceivers for Digital Communications ' , IEEE Journal of Solid-State Circuits, Vol. 30, No. 12, Dec. 1995.
3 . Zhaofeng Zhang, Zhiheng Chen, Jack Lau, ' A 900MHz CMOS Balanced Harmonic Mixer for Direct Conversion Receivers ' , IEEE 2000.
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Figure 1 Block diagram of superheterodyne receiver structure
Figure 2 Block diagram of the zero-IF receiver
Figure 3 Schematic diagram of zero-IF local oscillator leakage
Figure 4 Interference caused by strong interference signal under even-order distortion
Figure 5 Interference caused by RF signal under even-order distortion
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Figure 6 (a) Local oscillator leakage self-mixing (b) Interference self-mixing
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Figure 7 Working principle of harmonic mixing circuit
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