Zero intermediate frequency radio frequency receiver technology

Pick    To: zero intermediate frequency (Zero IF) or direct conversion (Direct-Conversion) receiver having a small size, low cost and ease monolithic characteristics, the structure is becoming a very competitive radio receiver. Local oscillator leakage (LO Leakage) Based on the Zero-IF configuration and structural performance characteristics introduced superheterodyne (Super Heterodyne), the focus of the present analysis of the zero-IF structure, even-order distortion (Even-Order DistorTIon), DC offset ( DC Offset) , flicker noise (Flicker Noise) and other issues, and gives the design method and related technology of zero intermediate frequency receiver.

introduction

In recent years, with the rapid development of wireless communication technology, wireless communication system products have become more and more popular, becoming 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 choosing the appropriate manufacturing process to improve the performance-price ratio of the system are the directions pursued by RF engineers. Because of its small size, low cost, and ease of monolithic integration, the zero-IF receiver has become a very competitive structure in RF receivers and has received widespread attention in the wireless communication field. Based on introducing 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 gives the design method and related technology of the zero-IF receiver.

Superheterodyne receiver

The Super Heterodyne architecture has been widely adopted since it was invented by Armstrong in 1917 . Figure 1 is a block diagram of the superheterodyne receiver structure. 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 the intermediate frequency band pass filter (IF BPF) to remove the adjacent channel signal, and then performs the second down conversion to obtain the desired 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 image interference and attenuate it to an acceptable level. Using an 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 down conversion is quadrature to generate two baseband signals of 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. Due to the multiple frequency conversion stages, the problems of DC deviation and local oscillator leakage will not affect the performance of the receiver. However, both the image interference suppression filter and the channel selection filter are high- Q bandpass filters, which 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 single chip using the integrated circuit manufacturing process. Therefore, the monolithic integration of superheterodyne receivers is difficult to achieve due to the limitations of process technology.

Zero IF receiver

Because zero-IF receivers do not require off-chip high- Q band-pass filters, they can be integrated on a single chip, and are widely valued. Figure 2 is a block diagram of a zero-IF receiver. Its structure is much simpler than the 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 orthogonal 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, which is completed by the low-pass filter and variable gain amplifier on the chip.

The most attractive part 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. Can be omitted. On the one hand, the elimination of external components 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 radio frequency signals being subject to external interference.

However, the zero-IF structure has problems such as DC deviation, local oscillator leakage, and flicker noise. Therefore, solving these problems effectively is a prerequisite for ensuring the correct implementation of the zero-IF structure.

LO leakage (LO Leakage)

The LO frequency of the zero-IF structure is the same as the signal frequency. If the isolation performance between the LO port and the RF port of the mixer is not good, the LO signal can be easily output from the RF port of the mixer and then pass through the low noise The amplifier leaks into the antenna and radiates into the space, causing interference to the adjacent channel. Figure 3 shows a schematic diagram of the local oscillator leakage. Local oscillator leakage is not likely to occur in superheterodyne receivers because the local oscillator frequency and the signal frequency are very different. Generally, the local oscillator frequency 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 intermodulation. In a zero-IF structure, even-order intermodulation distortion can also cause problems for the receiver. As shown in Fig. 4 , suppose there are two strong interference signals near the desired channel, and 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 shows that two high-frequency interferences will generate a low-frequency interference signal through the LNA with even-order distortion . If the mixer is ideal, after mixing this signal with the local oscillator signal coswLOt , it will be moved to a high frequency without affecting 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 go straight into the IF port from the RF port of the mixer, causing interference to the baseband signal.

Another form of even-order distortion is that after the second harmonic of the RF signal is mixed with the second harmonic output by the local oscillator, it is down-converted to the baseband and overlaps with the baseband signal, causing interference. The conversion process is shown in the figure. 5 shows.

Here we only consider the even-order distortion of the LNA . In practice, the RF port of the mixer will encounter the same problem and should be given sufficient attention. Because the signal added to the RF port of the mixer is the RF signal amplified by the LNA , and this port is the place with the strongest signal amplitude in the RF path, the even-order nonlinearity of the mixer will cause severe distortion at the output .

The solution to even-order distortion is to use a fully differential structure in low-noise amplifiers and mixers to counteract 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 LO signal can be reflected back from the output of the low-noise amplifier, the output of the filter, and the antenna, or the leaked signal can be received by the antenna and enter the RF port of the mixer. It is mixed with the local oscillator signal entering 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, which in turn mixes with the strong interference from the RF port, and the difference frequency is DC, as shown 6 (b) .

These DC signals will be superimposed on the baseband signal and interfere with the baseband signal, which is called DC deviation. The DC deviation is often greater than the noise of the RF front end, which makes the signal-to-noise ratio worse. At the same time, the large DC deviation may saturate the amplifiers at all levels after the mixer and cannot amplify the useful signal.

After the above analysis, we can estimate the DC deviation caused by self-mixing. Assume 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 a 60 dB. If the total gain of the low-noise amplifier and mixer is 30 dB , then a DC offset of approximately 7 mV will occur at the mixer output . 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 of gain, the amplifier will enter saturation and lose the amplification function of the useful signal.

When the self-mixing frequency changes with time, the DC offset problem becomes very complicated. This situation can occur under the following conditions: when the local oscillator signal leaked to the antenna is transmitted through the antenna and then reflected back from the moving object to be received by the antenna, it enters the mixer through the low noise amplifier, and the DC generated by the mixing The deviation will be time-varying.

From the above discussion, it can be seen that how to eliminate DC deviation is a key consideration when designing a zero-IF receiver.

AC coupling (AC Coupling)

The down-converted baseband signal is coupled to the baseband amplifier with a DC blocking method to eliminate DC interference. 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 DC offset interference is to properly encode the baseband signal to be transmitted and select an appropriate modulation method to reduce the energy of the baseband signal near DC. At this time, the method of AC coupling can be used to eliminate DC deviation without losing DC energy. The disadvantage is that a large capacitor is used, which increases the chip area.

Harmonic mixer (Harmonic Mixing)

The working principle of the harmonic mixer is shown in Figure 7 . The frequency of the local oscillator signal is selected to be 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 leakage of the local oscillator will produce an AC signal with the same frequency as the local oscillator signal, but no DC component, thus effectively suppressing the DC deviation.

Figure 8 shows a CMOS harmonic mixer. The second harmonic of the local oscillator signal can be obtained by the inherent square-law characteristic of the CMOS transistor. The circuit composed of transistors M3 and M4 converts the differential local oscillator voltages Vlo + and Vlo- into a time-varying current with second harmonics. The fundamental frequency and odd harmonics of the local oscillator signal are cancelled at the drain connection, generating 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 active devices is also called noise, and its size increases with the decrease of frequency, mainly concentrated in the low frequency band. Compared with bipolar transistors, field effect transistors are much noisier. Flicker noise interferes with the baseband signal moved to zero-IF, reducing the signal-to-noise ratio. Usually most of the gain of the zero-IF receiver is placed at the baseband level, and the typical gain of the low-noise amplifier and mixer in 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 the noise of the mixer should be minimized when designing.

The transistors M1 and M2 in the harmonic mixer shown in FIG. 8 are driven by the RF differential signals Vrf + and Vrf- . M1 and M2 are the main sources of noise. The role of the injected current Io is to reduce the current in the transistors M1 and M2 , thus Reduce noise.

I / Q mismatch (I / Q Mismatch)

When the zero-IF scheme is used 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, that is I / Q mismatch problem occurs . In the past, I / Q mismatch problems were the main obstacles in digital design. With the increase in integration, I / Q mismatch has been improved accordingly, but it should still be given enough attention during design.

Conclusion

This paper discusses the characteristics of superheterodyne and zero-IF structures, analyzes the causes of problems such as local oscillator leakage, even-order distortion, DC deviation, and flicker noise in the zero-IF structure. 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.

 

Figure 1 Block diagram of superheterodyne receiver

Figure 2 Block diagram of a zero-IF receiver

Figure 3 Leakage diagram of zero-IF local oscillator

Figure 4 Interference caused by strong interference signals under even-order distortion

Figure 5 Interference caused by RF signal under even-order distortion

 

Figure 6 (a) Local oscillator leakage self-mixing (b) Interference self-mixing

 

Figure 7 Working principle of harmonic mixing circuit

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