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Programación en general => Programación Web => Base de Datos => Mensaje iniciado por: jingwxs04 en Octubre 10, 2013, 08:56:33 am
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In the modern radar receiver, employs the widest structure to be the superheterodyne structure. In this structure, the one piece system often needs outside slice electric-wave filter remove the image signal, such as SAW electric-wave filter, therefore bring influence to integrated level of the system. In order to reach certain image rejection ratio, and don’t use the electric-wave filter outside slice, usually using the image reject mixer can be offered inhibition degree about 60 dB. But the modern radar receiver needs inhibition degree of 80 dB at least, this increases the difficulty for design of the image reject mixer.
In order to solve this problem, research work mainly concentrates on the mirror image to inhibit on the design of LNA. From the literature, can see that pass LNA and trap filter notch filter Connection,last 20 dB and 75 dB inhibition degree at one piece LNA of it respectively. This text combines the index of LNA in the radar receiver, improve through designing circuit structure and inhibit degree, have connected with image reject mixer of the final-stage and reached higher image rejection ratio, have improved the inhibition degree Current transducer (http://www.transducersensor.com/) to the image signal of whole radar receiver.
A trap filter
In this text, requiring the working frequency band to be S frequency band to low-noise amplifier, the noise coefficient FN is 2- 3 dB, power gain, in above 30 dB, it is not lower than 10dBm to export 1 dB point of compression, working voltage is 5 v. Require, LNA basic structure adopted is the inductance negative feedback of an emitter to the above. The type shows, choose to make the port matched in appropriate sense value, the most important thing is, this kind of structure has avoided the noise resistance on the passway of the signal, has reduced the noise coefficient greatly, as shown in Fig. 1.
The trap filter is divided into structure of active and passive 2 kinds, in order to reduce the complexity designed, in the mirror image inhibits LNA design, adopt the passive structure, as shown in Fig. 2, including C1, C2 and L1 3 passive components parasitic resistance quantity is RL1 in L1 .
2 LNA design analysis
The characteristic of LNA has determined the performance index of the radar receiver to a great extent. In order to meet designing the power gain in the index, actual LNA adopts the cascade connected way of the third grade to design.
In the type: Fn, GAn, IIP3 N is LNA each grades of noise coefficients here n =1,2,3,Power gain and intermodulation parameter of the Third order.
From formula 6 , formula 7 It is easy to obtain:
Reduce the noise coefficient of the module, it is mainly a noise coefficient which reduces the circuit of preceding stage;
2In order to reduce the impact on noise of the whole module of noise of the subsequent circuit, the module of preceding stage needs offering the appropriate gain to noise temperature ratio;
3Should improve the linear scale of the module, besides improving the linear scales in the circuit at all levels, the gain to noise temperature ratio at all levels can’t be too high.
In order to meet the noise requirement for the whole LNA, first step amplifier should mainly face and optimize the noise to design, in order to get the minimum noise coefficient, the noise coefficient of the overall system basically depends on the noise coefficient of first step. Two stage amplifier should offer certain gain to noise temperature ratio and linear scale while facing the noise and is optimized, in order to avoid the gain to noise temperature ratio of the whole amplifier is overlow. The third stage amplifier mainly faces the optimization of the linear scale, through the means such as increasing inductive magnitude and transistor-resistor logic bias current of emitter etc., can improve the output power of point of compression of 1 dB power effectively, but the gain to noise temperature ratio will be influenced by the fact that sure.
The bar structure of adopting the source altogether of the basic amplifier altogether, can already increase the isolation of the I/O, have reduced Miller effect among Ld1 and M2. Meanwhile, the indirectness of M1 and M2 enters the inductance Ld1, contributes to improving gain to noise temperature ratio and noise situation of the amplifier.
In tertiary circuit design, the most complicated one is the third stage circuit. The third stage amplifier should respond to the request that point of compression of 1 dB power is exported and smaller than 10 dBm, so mainly face and optimize the linear scale to design. The circuit diagram of the amplifier is shown as in Fig. 3, it can be by formula 8 that the output power Third order intermodulation of the amplifier is clicked Estimate:
From formula 8 China can find out the linear scale to be improved the amplifier, can pass Vce voltage and collector current Ic of improving the amplifying tube. Because Vce will not be very high, mainly improve the linear scale of the amplifier by improving the collector current Ic, but improve the increase that the electric current will bring the power consumption simply.
Can improve the output power of point of compression of 1 dB power while increasing the emitter inductance, but the gain to noise temperature ratio of the amplifier will be thereupon reducing, the input of point of compression of 1 dB power will increase at the same time, strengthen current sensor (http://www.transducersensor.com/Electrical-Isolation-Transducer/Current-Transducer/Current-Transducer.html) the pressure of the first class amplifier linear scale correctly. So the emitter inductance can only be chosen an appropriate sense value, can’t be too big.
3 LNA simulation result
The simulation result is shown as Fig. 4, Fig. 5.
Near image frequency, the gain to noise temperature ratio about 25 dB when not having trap filter, drops to – 21. Near 9 dB value, play certain inhibit function to the image signal which enters LNA. Table 1 shows the stability state in the frequency band of the circuit, the coefficient is far greater than 1, the circuit is absolutely stable.
In addition, from formula 5 Can find out the q-factor of the trap filter receives the resistor parasitics in the inductance to limit. So, in the cmos process, the inductive characteristic is influencing LNA final characteristic.
Can see LNA is in there are trap filters and not there is not artificial consequential contrast under two kinds of situations of trap filter through Table 2.
4 concludes the speech
In order to reach the requirement that the radar receiver of one piece inhibits degree for the mirror image, adopt CMOS O. The cascade connected mirror image of a third grade of 18 m technological design inhibits LNA. Through inserting the passive wave limitting electric-wave filter in LNA, realize the decay of the image signal, thus has reduced the difficulty of design of the mixer circuit of rear end.
The amplifier to designing carries on emulation in ADS finally, its result is, the service voltage of maximum is under 5 V situation, signal frequency band 3. 0- 3. 2 GHz, intermediate frequency is exported as 225 MHz, power gain is greater than or equal to 31 dB, the noise coefficient is less than or equal to O. 5 dB, the power clicked in I/O 1 dB is – 19 respectively. 5 dBm and 11. 5 dBm, reaches 22 dB to inhibition degree of the image signal. Have avoided using the electric-wave filter outside slice, has improved the integrated level of the system. Because of the present cmos process, inductive quality still remains to improve. The mirror image of the one piece inhibits LNA and wants to reach better characteristic, await further study.
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