Design of Dual-Band LNA for Mobile Radio ETI041 Radio Project 2011

Transcription

1 Design of Dual-Band LNA for Mobile Radio ETI041 Radio Project 2011 Ivaylo Vasilev and Ruiyuan Tian Dept. of Electrical and Information Technology Lund University, Sweden {Ivaylo.Vasilev, Highlights A low noise amplifier with a novel two-stage matching network is designed for mobile radio receivers with applications over two frequency bands of 900 MHz and 1800 MHz. The noise figure is less than 2.5 db and 4 db, whereas the gain is more than 22 db and 10 db for the two operating frequencies, respectively. The 1 db compression point is obtained at P OUT of 3.9 dbm and 6.1 dbm. The two-stage matching network provides good standing wave ratio (< 2) covering more than 100 MHz bandwidth for both frequency bands. 1

2 Abstract In this project, a low noise amplifier (LNA) is designed for mobile radio receivers with applications over two frequency bands of 900 MHz and 1800 MHz, which are commonly used for cellular systems such as GSM and UMTS. The Infineon NPN silicon RF transistor BFP420 is chosen for the purpose of high gain and low noise characteristics. In order to achieve output matching for both frequency bands, a novel two-stage matching network with one open- and one short-stub is proposed. The LNA design is simulated using MATLAB. The prototype is implemented on a PCB and its performance is verified through RF lab measurements. As a result, the noise figure is less than 2.5 db and 4 db, whereas the gain is more than 22 db and 10 db for the two operating frequencies, respectively. The 1 db compression point is obtained at P OUT of 3.9 dbm and 6.1 dbm. The two-stage matching network offers good standing wave ratio (< 2) covering more than 100 MHz bandwidth for both frequency bands. 2

3 Contents 1 Introduction 4 2 LNA Design Transistor Stability, Gain and Noise Design Dual-Band Output Impedance Matching Transistor Biasing PCB Prototype 10 4 Measurement 10 5 Conclusion and Discussion 11 3

4 Figure 1: Block diagram of a radio receiver for digital I/Q demodulation. 1 Introduction In a mobile radio receiver, the low noise amplifier (LNA) is a vital component. For a typical receiver structure with digital I/Q modulation as shown in Figure 1, it is placed very close to the receiving antenna. As the overall noise figure (NF) is determined mostly by the front stages of the receiver circuit, the LNA itself is desired to produce low NF. For the same reason, the LNA is also expected to have high enough amplification for the operation of later stages into the receiver. Last but not the least, the LNA should also have good linearity behavior. These requirements make the design of a LNA one of the most challenging tasks for RF engineers. Furthermore, the multiple frequency usage of cellular systems adds one more degree of complexity to RF design in general. For example, UMTS-FDD is designed to operate over 14 different frequency bands between 700 MHz and 2100 MHz. When it comes to the LNA design, it should achieve desired performance either over this large frequency bandwidth, or for each specific frequency band individually. In this project, the problem is addressed by designing a LNA for operation frequency of 900 MHz and 1800 MHz, which are commonly used for GSM and UMTS systems in Europe and Asia. The conventional design methodology of LNA [1] is extended with special attention to dual-band operation. The remainder of the report is organized as follows. Section 2 describes the LNA design with MATLAB simulations. Section 3 presents its PCB prototype. In Section 4, the design is verified through RF measurements. Section 5 concludes the report. Throughout the discussion, the focus is on the performance of LNA over the two operating frequencies. 2 LNA Design The LNA design is carried out using a standard two-port network representation with MATLAB simulations. Special attention is paid on the performance over the two operating frequencies. 4

5 (a) Gain (b) Noise Figure 2: Gain and noise characteristics of the transistor BFP420 [2]. 2.1 Transistor The performance of LNAs depends largely on the choice of transistor and biasing conditions. Considering the radio frequency range of the application, the Infineon NPN silicon RF transistor BFP420 [2] is chosen based on its gain and noise characteristics, as given in Figure 2. With a typical operating voltage V CE of 2 V and an operating current I C of 12 ma, the gain is above 20 db whereas the NF is below 1.3 db for both 900 MHz and 1800 MHz. It also shows that the gain is increasing whereas the NF is reducing with respect to I C, respectively. As a result, 12 ma is chosen to achieve good compromise. The typical value of DC current gain β is given as 95. Furthermore, the BFP420 package has two emitter pins, which is considered good for common emitter configurations. Using a setup given in [1], the two-port S parameters are measured with a vector network analyzer (VNA) for further design. 2.2 Stability, Gain and Noise Design The design of LNA is facilitated using a two-port network representation [1]. The block diagram is given in Figure 3, which consists of blocks of the transistor BFP420 and the output impedance matching network. Input matching is not necessary since we will design it as matched to the source impedance, 5

6 Figure 3: Two-port network representation with BFP420 and output impedance matching. i.e., Γ S = 0. The design is carried out based on MATLAB simulations. As a result, the design is summarized and presented on a Smith chart shown in Figure 4. Stability In order to design for stable operation of the transistor, the parameter metrics K and are employed. As given in Table 1, conditional stability is achieved since K < 1 and < 1 is obtained for both of the frequency bands. After checking that S 11 < 1 and S 22 < 1 are obtained for both frequencies, it assures that the central part of the Smith chart is stable for both input and output. It also facilities the choice of Γ S = 0. Table 1: Stability test of the LNA design. Frequency 900 MHz 1800 MHz K Noise The noise properties of the transistor BFP420 can be obtained from [2], which are listed in Table 2. In Figure 4, Γ opt as well as the noise circle for F min + 1 db are shown for the two frequencies. It shows that Γ opt is close to the center of the Smith chart, which facilities the choice of Γ S = 0. As a result, given that Γ S = 0, the NF is 0.95 db and 1.30 db for 900 MHz and 1800 MHz, respectively. Table 2: Noise property of BFP420 [2], V CE = 2 V and I C = 12 ma. Frequency F min [db] Γ opt r n / MHz MHz

7 Figure 4: Simulation of the design presented on a Smith chart. Solid and dashed lines denote for 900 MHz and 1800 MHz, respectively. Gain Since ΓS = 0 is desired, the design is for maximum available gain of at least S21 2. The circle for gain design is shown in Figure 4 as well. As a result, given that ΓS = 0, the obtained gain is listed in Table 3. Table 3: Gain obtained from design given ΓS = 0. Frequency Available Gain [db] S21 2 [db] 900 MHz MHz Dual-Band Output Impedance Matching The most challenging task in this work is to design the output impedance matching network for the two frequencies. A simple matching network, either with a LC-circuit using lumped component or a transmission line with a stub, is usually designed for a single operating frequency with a fundamentally limited bandwidth [3]. In the special case of matching for the fundamental frequency and its first harmonic, a solution with two-section Chebyshev transformers has been proposed [3]. However, the approach is more involved and therefore it is left for future work. 7

8 Figure 5: Two-stage output impedance matching network. (a) Smith chart (b) Reflection Coefficienct Figure 6: Result of the two-stage output impedance matching network. On the other hand, a similar problem is usually treated in impedance matching for antennas. In [4], impedance matching for a dual-band antenna is achieved at operating frequency of 900 MHz and 1900 MHz. In this approach, matching for the low band is firstly attempted with only series capacitors CS and parallel inductors LP. These components are considered to affect lower frequencies more than higher frequencies, such that matching to the low band is obtained with minimal effect on the high band. Secondly, the high band matching should be obtained without deteriorating the low band performance. This can be achieved with the use of parallel capacitors CP and series inductors LS, both of which affect higher frequencies more than lower frequencies. Table 4: Solution for output impedance matching network. Frequency 900 MHz 1800 MHz Line l1 = λ l2 = λ Stub d1 = λ d2 = λ 8

9 Figure 7: Transistor biasing. In this work, we will extend this approach to the use of transmission lines with open and short stubs. The design is given in Figure 5, where a two-stage transmission line with one short and one open stub is presented. Transmission line pairs (l 1, d 1 ) and (l 2, d 2 ) are responsible for matching the 900 MHz and the 1800 MHz bands, respectively. The obtained solution is given in Table 4. Results of the two-stage output impedance matching network are shown in Figure 6. We conclude that matching at both frequency bands is acceptable. 2.4 Transistor Biasing The transistor needs to work under proper DC conditions, which are provided by the biasing design. Figure 7 shows the biasing circuit used in this work, which is similar to the one described in [1]. Given V CC = 12 V with V CE = 2 V, I C = 12 ma, V B = 0.7 V and β = 95, the resistors are obtained according to [1]. The available values of the obtained resistors are listed in Table 5. The transmission line at the input stage is chosen as λ/4-transformer at the frequency of 1200 MHz, with a characteristic impedance of 100 Ω. Parameters used for the transmission line at the input are obtained through optimization for both the low and the high band by taking into account the dual-band operation requirement of the LNA. Table 5: Resistors with available values of the biasing design. R C R B1 R B2 R B3 680 Ω 470 Ω 1.2 kω 5.6 kω In addition, several decoupling capacitors are added. All capacitors are of value 10 nf except for C 0 = 12 pf. 9

10 3 PCB Prototype Figure 8: PCB prototype. The circuit schematic shown in Figure 7 is implemented on a PCB with thickness of 1.55 mm and ɛ r = 4.3 (FR4). The transmission line dimensions are listed in Table 6. The PCB prototype is shown in Figure 8. It is noteworthy that, although several via holes were added, unstable and underestimated measurement results were obtained. More via holes are necessary to achieve good grounding and thus enable measured results close to the simulated LNA parameters. Throughout the measurements the standard BNC connector type was used. Table 6: Transmission line dimensions of the PCB prototype. Unit: mm. (W 0, l 0 ) (W 1, l 1, d1) (W 1, l 2, d 2 ) (0.6975, ) (3.0225, , ) (3.0225, , ) The circuit schematic is simulated in MATLAB. Simulation results of output SWR and gain are given in Figure 9. It shows that good output matching of SWR < 2 is obtained for both 900 MHz and 1800 MHz. Regarding the gain, it shows that it is at least S 21 2 for both frequencies, whereas it is about 5 db lower for the high band. 4 Measurement In order to verify the simulation results, RF measurements are carried out on the PCB prototype that is manufactured according to the design. The prototype is first measured with a VNA. The measured results of output SWR and gain are given in Figure

11 (a) SWR (b) Gain Figure 9: Simulation results of the LNA design. (a) SWR (b) Gain Figure 10: Measured results of the LNA design. According to the measured SWR, the design covers a SWR < 2 bandwidth of 130 MHz and 114 MHz centered at 860 MHz and 1825 MHz, respectively. The measured gain is also considered in accordance to the simulation results. As a result, the gain is higher than 22 db for the lower band and higher than 10 db for the higher band. The other parameters measured for verification include NF and 1-dB compression point. According to Figure 11, the NF is shown to be less than 2.5 db for the lower frequency band and less than 4 db for the higher frequency band. The 1-dB compression point readings at P OUT were 3.9 dbm and 6.1 dbm for the two bands, respectively. 11

12 Figure 11: Measured NF of the LNA design. (a) Low band at 860 MHz (b) High band at 1825 MHz Figure 12: Measured 1-dB compression point of the LNA design. 5 Conclusion and Discussion To conclude, a LNA operating at two frequency bands commonly used for mobile radio is successfully designed. The novelty includes a two-stage network for output impedance matching at the two bands. The design is verified through RF measurements. Results show a good agreement with the simulations. As a summary, the measured specifications of the designed LNA are listed in Table 7. Furthermore, the lesson that we learned from the practical work is that good grounding is important in order to achieve proper performance. Therefore, when doing a PCB prototype for RF applications, many via holes are necessary. Copper tape has been used on the PCB to offer better grounding. In addition, it is also noted that the RF measurements have not been per- 12

13 Table 7: Specifications of the designed LNA. Frequency NF Gain Bandwidth (SWR < 2) 1-dB comp. P OUT 900 MHz 2.5 db 22 db 795 MHz-925 MHz 3.9 dbm 1800 MHz 4 db 10 db 1772 MHz-1886 MHz 6.1 dbm formed in a radio shielded environment. Therefore, for the frequency bands that the design is of concern, some RF disturbance can not be totally avoided. This may have resulted in some discrepancies between the measurement and the simulation. 13

14 Acknowledgment We would like to express our gratitudes to Göran Jönsson for his insightful guidance and valuable time. We would also like to thank Lars Hedenstjerna for his help with the PCB prototype. Helpful discussions with Andreas Axholt regarding noise measurements are also gratefully acknowledged. 14

15 References [1] L. Sundström, G. Jönsson and H. Börjeson, Radio Electronics, Department of Electroscience, Lund University (2004) [2] The Infineon NPN silicon RF transistor BFP420, Data sheet, Infineon, [3] S. J. Orfanidis, Electromagnetic Waves and Antennas, ECE Department, Rutgers University (2008), orfanidi/ewa/ [4] P. J. Bevelacqua, Antenna Theory, Online (2009), 15