A Design Scheme of CDMA LNA

Mobile communication technology is the product of the high development and mutual combination of modern communication technology and computer technology. With the widespread application of digital information technology, modern communication technology is developing at an unprecedented high speed, and mobile communication is also developing in the direction of multiple access communication. Compared with FDMA (frequency division multiple access) and TDMA (time division multiple access), CDMA (code division multiple access) has the advantages of large system capacity, flexible configuration, high spectrum utilization, soft switching, and good confidentiality. Based on these advantages, CDMA technology has been widely used.

As the first stage of CDMA receiver, CDMA low noise amplifier directly determines the noise figure of the whole receiver, thus largely determines the receiver sensitivity. Therefore, CDMA low noise amplifier with small noise figure, high gain, in-band flatness and high linearity has become an important part of CDMA transceiver.

Research and Design of CDMA Low Noise Amplifier

Basic Principle of CDMA Low Noise Amplifier Circuit

The functional block diagram of the low noise amplifier is shown in Figure 1. In order to meet the high gain of 50dB, the low noise amplifier adopts four-stage amplification AMP1, AMP2, AMP3 and AMP4; in order to obtain a good input-output standing wave ratio, isolator ISOLATOR1 is used at the input end, and isolator ISOLATOR2 is used at the output end. In order to suppress various out-of-band spurious signals, a surface acoustic wave filter SAWFILTER with a high suppression degree working in this frequency band is used; in order to achieve adjustable gain, a digitally controlled attenuator ATT1 is used; in order to achieve automatic output power control, a voltage-controlled attenuator ATT2 is used.

Figure 1 CDMA low noise amplifier block diagram

The circuit can be divided into three small units: power processing circuit, monitoring processing circuit, and radio frequency (RF) link. The power processing circuit uses the switching power supply LM7805 to convert the external 9V voltage into the 5V voltage required for each chip to work. LM7805 has the characteristics of high conversion efficiency and a large dynamic range of input voltage; the monitoring processing circuit realizes the RS485 communication interface with the host computer, output signal power detection, module gain adjustment and automatic level control (ALC).

The working principle of the RF link is shown in Figure 1. First, the input isolator ISOLATOR1 achieves good input standing wave ratio performance, the input signal is then amplified by AMP1 and AMP2, and then the out-of-band signal is suppressed by the surface acoustic wave filter SAW, and then after the third stage of amplification by AMP3, it passes through the digital controlled attenuator ATT1 with adjustable gain, and then through the automatic level control voltage controlled attenuator ATT2, and then through the fourth stage of amplification by AMP4. After AMP4, a small part of the energy is distributed to the detector to realize output power detection, and the main signal is finally output through the output isolator ISOLATOR2.

Key issues and solutions for designing CDMA low noise amplifiers

Design of low noise, high gain first and second stage amplifiers

According to the cascade formula of noise coefficient NFtot(m)=NF1+(N F2-1)/A v1+…+(N Fm-1)/Av1…A v(m-1) (where NFtot (m) is the total noise coefficient of the mth stage, NFm is the noise coefficient of the mth stage, and Avm is the gain of the mth stage), the noise coefficient of the entire amplifier is mainly determined by the first and second stages. The design of the first and second stage amplifiers with low noise and high gain is the difficulty of the entire low noise amplifier design. The S parameter simulation of noise and gain of AMP1 and AMP2 was performed using Agilent's Advanced Design System (ADS), and the results are shown in Figures 2 and 3.

Figure 2 AMPI noise and gain simulation results

Figure 3 Noise and gain simulation results of AMP2

Improve out-of-band rejection and add

Adjustment of in-band fluctuation after SAW filter SAW filter has the characteristics of small size, steep filter band and good selectivity. Adding SAW filter improves the out-of-band suppression performance of low noise amplifier and improves frequency selectivity. At the same time, since the in-band fluctuation and out-of-band suppression of SAW filter are a pair of contradictory indicators, the in-band fluctuation index of the low noise amplifier usually becomes worse after adding SAW filter. Therefore, it is necessary to measure S11 and S22 of SAW filter separately on vector network analyzer, and add input and output matching networks to improve in-band fluctuation.

Linearity and other considerations

In order to ensure that the low noise amplifier has less nonlinear distortion, high OIP3 devices should be selected, especially the last stage amplifier AMP4.

Monitoring should also be considered in the design of low-noise amplifiers, and gain adjustment, automatic level control, and output power detection should be achieved through communication interfaces. Electromagnetic shielding is also an issue that cannot be ignored. Aluminum shielding boxes are used to isolate the interference of electromagnetic fields in space and prevent RF signal leakage. At the same time, the RF circuit, power supply circuit, and monitoring circuit are separated by spacers to prevent them from interfering with each other. The low-noise amplifier designed in the end has the performance of low noise, high gain, high linearity, and good in-band flatness.

Tests and Results

Through the connection debugging of the circuit, the various indicators of the low noise amplifier measured by the digital signal source, spectrum analyzer, vector network analyzer and noise figure analyzer are shown in Table 1. The spectrum analyzer screenshots of the in-band fluctuation and third-order intermodulation in Table 1 are shown in Figures 4 and 5.

Table 1 Performance of low noise amplifier

Figure 4: In-band fluctuation test results of low noise amplifier

Figure 5: Third-order intermodulation test results of low noise amplifier

Conclusion

CDMA (Code Division Multiple Access) cellular mobile communication is an advanced spread spectrum communication technology for digital cellular mobile communication that has developed rapidly in the world in recent years. It has the advantages of low radiation, good call quality, high confidentiality, etc. Almost all major telecommunications equipment manufacturers in the world provide CDMA system equipment, and CDMA low noise amplifiers are extremely important components in CDMA transceivers.