Mobile communication technology is a product of the highly developed and integrated 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 along 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 rate, soft handover, and good security performance. 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 machine, which largely determines the receiver sensitivity. Therefore, a low noise figure, high gain, flat in-band, high linearity CDMA low noise amplifier has become an important part of CDMA transceivers. Research and Design of CDMA Low Noise Amplifier The basic principle of CDMA low noise amplifier circuit Figure 1 CDMA low noise amplifier block diagram 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 two stages of AMP1 and AMP2, and then the out-of-band signal is suppressed by the surface acoustic filter SAW, and then amplified by the third stage of AMP3. Achieve gain-adjustable numerical control attenuator ATT1, after implementing automatic level control voltage-controlled attenuator ATT2, and then through AMP4 fourth-stage amplification, after AMP4 divide a small amount of energy to the detector to achieve output power detection, the main signal finally passes Output isolator ISOLATOR2 output. Key Problems and Solutions of CDMA Low Noise Amplifier Design Design of first-stage and second-stage amplifiers with low noise and high gain According to the cascade formula of noise figure NFtot (m) = NF1 + (N F2-1) / A v1 +… + (N Fm-1) / Av1… A v (m-1) (where NFtot (m) is the total m level Noise figure, NFm is the noise figure of the mth stage, and Avm is the gain of the mth stage.), The noise figure of the entire amplifier is mainly determined by the first stage and the second stage. The design of the first-stage and second-stage amplifiers with low noise and high gain is the difficulty of the entire low-noise amplifier design. Using Agilent's Advanced Design System (ADS) to simulate the noise and gain S-parameters of AMP1 and AMP2, the results are shown in Figure 2 and Figure 3. Figure 2 AMPI noise and gain simulation results
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The functional block diagram of this low noise amplifier is shown in Figure 1. In order to meet the high gain of 50dB, the low-noise amplifier uses four-stage amplification AMP1, AMP2, AMP3 and AMP4; in order to get a good input and output standing wave ratio, the isolator ISOLATOR1 is used at the input, and the isolator is used at the output ISOLATOR2. In order to suppress all kinds of spurious signals out of band, SAWFILTER, a highly suppressed surface acoustic filter working in this frequency band, was used; in order to achieve gain adjustment, a numerical control attenuator ATT1 was used; in order to realize automatic output power control, a Voltage controlled attenuator ATT2. 
The circuit can be divided into three small units: power processing circuit, monitoring processing circuit, radio frequency (RF) link. The power supply processing circuit adopts the switching power supply LM7805, which converts the externally provided 9V voltage to the 5V voltage required for each chip to work. LM7805 has the characteristics of high conversion efficiency and large input voltage dynamic range; the monitoring processing circuit realizes RS485 communication interface with the host computer, output signal power detection, module gain adjustment and automatic level control (ALC). 
