A Wideband RF Downconverter for the NIJ Public Safety Radio

Transcription

1 Chameleonic Radio Technical Memo No. 16 A Wideband RF Downconverter for the NIJ Public Safety Radio S.M. Shajedul Hasan and S.W. Ellingson December 1, 2006 Bradley Dept. of Electrical & Computer Engineering Virginia Polytechnic Institute & State University Blacksburg, VA 24061

2 A Wideband RF Downconverter for the NIJ Public Safety Radio S.M. Shajedul Hasan and Steven W. Ellingson December 01, 2006 Mobile and Portable Radio Research Group (MPRG) Bradley Department of Electrical and Computer Engineering Virginia Polytechnic Institute and State University Blacksburg, VA

3 Table of Contents Section I... 5 Introduction... 5 Section II... 6 Downconverter Design High Level Design Overview Low Level Design Overview Diplexer Design & Performance MHz Elliptic Filter Design Section III Measurement & Analysis GNI Analysis Measurement Setup Measured Performance Section IV Conclusion References Appendices ii

4 List of Tables Table 3.1: GNI analysis of the RFDC without the first diplexer Table 4.1: Summary of the parameters of RFDC Table 4.2: Summary of the cost for one RFDC board iii

5 List of Figures Figure 2.1: High-level diagram of the downconverter Figure 2.2: Low-level block diagram of the RF downconverter Figure 2.3: Diagram of basic diplexer filter Figure 2.4: Schematic of 1250 MHz diplexer Figure 2.5: Simulation and measurement results of 1250 MHz diplexer Figure 2.6: Schematic of 78 MHz diplexer Figure 2.7: Simulation and measurement results of 78 MHz diplexer Figure 2.8: Schematic of 78 MHz filter Figure 2.9: Simulation results of 78 MHz filter Figure 3.1: Measurement Setup Figure 3.2: Measured downconverter gain Figure 3.3: Measured downconverter gain in 318 to 378 MHz Figure 3.4: Measured downconverter gain in 678 to 738 MHz Figure 3.5: Measured downconverter input 1 db compression point Figure 4.1: Image of RFDC iv

6 Section I Introduction This report describes the design of a downconverter for a multiband radio for public safety applications. This work is performed as part of the project A Low Cost All-Band All-Mode Radio for Public Safety sponsored by National Institute of Justice of the U.S. Dept. of Justice [1]. The RF downconverter (RFDC) design presented here accommodates public safety frequencies ranging from 138 to 894 MHz [2]. The measured gain, noise figure (NF) and third order intermodulation product (IIP3) are 47 db, -37 dbm and 4.5 db respectively. The output of this RFDC is 78 MHz IF with 40 MHz 3 db bandwidth. The downconverter was constructed on a printed circuit board (PCB). Next two sections present the complete design including the gain NF IIP3 (GNI) analysis and measured performance. Conclusions have been made at the end, summarizing all the parameters and presenting suggestions for future improvement. Appendices contain the bill of materials (BOM) with vendor names, image of the PCB, and schematics. 5

7 Section II Downconverter Design This section describes downconverter design. 2.1 High Level Design Overview Fig. 2.1 shows a high-level diagram of the RFDC, defined as the boxed region within this figure. The RFDC is preceded by a frequency preselector, whose role is to route the input RF through one of three filter paths. This preselector has several filters to split the incoming RF into three frequency bands such as low ( MHz), mid ( MHz) and high ( MHz). Any frequency band can be selected from these three by selecting the proper switch position in the preselector. This RFDC uses a dual conversion superheterodyne architecture. The first local oscillator (LO-1) acts as the tuning mixer while the second local oscillator (LO-2) remains fixed. The frequency of the LO-1 varies from MHz and it upconvertes the RF signal to a higher frequency of 1250 MHz. The image frequencies for this stage of conversion are MHz, which are blocked in the subsequent filtering. LO-2 downconverts this 1250 MHz signal to 78 MHz IF signal. The frequency of the LO-2 is fixed to 1328 MHz. This stage of conversion creates the image frequency of 1406 MHz, which is also blocked by the subsequent filtering. The final IF of the downconverter, which is 78 MHz, is sent to an analog to digital converter (ADC). This falls into the second Nyquist zone of an ADC sampling in the range MSPS, as explained in [5]. Also, this IF places the second and third harmonics of the IF far apart so that a simple bandpass filter can be used into the ADC. Any additional filtering can be implemented in post-processing software. To filter the spurious frequency components from the 1250 MHz IF frequency we selected to use an MC series filter from Lark Engineering, which provides 60 MHz 3 6

8 db bandwidth. A 78 MHz elliptic filter was designed by using the discrete components, which provides 40 MHz 3 db bandwidth. Reducing cost is one of the major concerns of this project. Usually, the filters made by using discrete components are cheaper than the commercially available filters. So, the design and the measurements described in this paper used the 78 MHz filter created by using discrete components with 40 MHz 3 db bandwidth. However, our designed PCB for RFDC has an additional space to replace this discrete component 78 MHz filter with a commercial filter from TTE Inc.; specifically Part No. TTE_K /98M , which provides 40 MHz 3 db bandwidth. Figure 2.1: High-level diagram of the downconverter. 2.2 Low Level Design Overview Fig. 2.2 shows the component level diagram of RFDC. As we already described in the previous section, the input of this RFDC is the selected frequency bands from the preselector filter and the output is 78 MHz IF. To offset the losses of the RFDC board due to the extra conversion stages, five ERA-3SM amplifiers, from Mini-Circuits, Inc., were placed before and after the mixers and associated diplexer stages. ERA-3SM has been chosen for its high gain (around 15 db) in our desired frequency regions. It draws 40mA at 9V. The mixers used for all conversion stages of the downconverter are the SYM-11, which is also from Mini-Circuits, Inc. This is a level 7 (dbm) mixer, which covers the frequency 7

9 ranges from 1 to 2500 MHz with IIP3 of +10 dbm. The average conversion loss of this mixer is around 9 db. Since the LO frequency for the first mixer is little bit high compared to the other stages of the RFDC, we used ERA-6SM amplifier, which has wide input frequency range (DC-4 GHz) with 10 db average gain and 70 ma current consumption at 9 V supply. The second mixer, which downconverts the 1250 MHz signal to 78 MHz IF, uses an ERA-3SM amplifier to make the LO level appropriate for the mixer s input. We have used diplexers just after each of the mixer stages. Their main purpose is to mitigate reflections caused by impedance mismatch at the mixer ports which can lead to undesired spurious product generation. The first diplexer used after the first mixer acts as a lowpass filter to pass 1250 MHz and the second diplexer in second mixer s output also works like a low-pass filter to pass 78 MHz IF signal. Next section describes the design and performance of these diplexers in detail. Figure 2.2: Low-level block diagram of the RF downconverter. 2.3 Diplexer Design & Performance RF devices can be highly sensitive to their output matching. This is especially true in the case of mixers, where if the IF port is not properly terminated, signals reflected back into the IF port can cause undesired spurious products to be generated. Since traditional filters are reflective in the stopband (i.e., they do not provide constant impedance as a function of frequency), their use following a mixer is best avoided. Diplexer filters minimize input reflections by offering a constant impedance, Z 0, as a function of frequency; i.e. both the stopband and passband present a low VSWR [3]. A simple design for a diplexer 8

10 would be based on a lowpass and highpass filter in parallel as shown in Fig The stopband filter is terminated in a matched load while the output is taken from the passband filter. The diplexers design used for the downconverter was based on the work of Sabin [4]. Figure 2.3: Diagram of basic diplexer filter MHz Diplexer Design The first mixer stage, as shown in Fig. 2.2, also uses high side injection putting the undesired products above the IF. Thus, the diplexer at this stage operates in the LPF mode. In this stage the IF frequency is 1250 MHz and the image frequencies are 1388 to 2144 MHz. Since the IF frequency is little bit close to the image frequency, it is difficult to get the acceptable cut-off frequency for this diplexer. This diplexer is designed with a crossover frequency of 1550 MHz, which is above the 1250 MHz IF, to ensure little loss in the passband. There are two problems for this crossover frequency firstly, it does not have high loss for some of the image frequencies such as around 1388 MHz, secondly, the component values are very small which can decrease the performance of this diplexer. The schematic is shown in Fig For testing, the diplexer was constructed on a stand-alone PCB. The measurement and simulation results are shown in Fig The measurement result did not match with the simulation result. As we already mentioned in above the potential reasons may be the small values of the components. The measurement result shows that the crossover frequency is around 1000 MHz which is unacceptable. Since we did not get the expected 9

11 performance, we bypassed this diplexer during the actual measurements for the whole RFDC board. Term Term1 Num=1 Z=50 Ohm input L L1 L=8.2 nh C C1 C=3.9 pf L L2 L=10 nh C C2 C=2.7 pf L L3 L=3.3 nh output-1 Term Term2 Num=2 Z=50 Ohm C C3 C=1.5 pf L L4 L=2.2 nh C C4 C=1 pf L L5 L=3.9 nh C C5 C=3.3 pf output-2 Term Term3 Num=3 Z=50 Ohm Figure 2.4: Schematic of 1250 MHz diplexer. Figure 2.5: Simulation and measurement results of 1250 MHz diplexer. 10

12 MHz Diplexer Design The second mixer in our RFDC uses high side injection which places the undesired frequency products well above the IF. It is therefore appropriate to use a diplexer in the LPF mode so that only the IF is passed. Thus, the diplexer at the second mixer stage, as shown in Fig. 2.2, operates in the LPF mode with a crossover frequency of 140 MHz. The crossover frequency was chosen to be slightly high the IF to minimize the losses in 78 MHz. The designed diplexer is also suited enough to reject the image frequency, which is 1406 MHz in this stage of conversion. ADS software was used to analyze the design performance. The schematic and the results are given in Fig. 2.6 and 2.7 respectively, which shows the frequency response of both the HPF and LPF. It also shows the input reflection coefficients. To measure the performance, the diplexer was constructed on a stand-alone PCB. Fig. 2.7 shows the measurement results for the LPF side and the HPF side. As can be seen from the figure, the crossover frequency turned out to be around 130 MHz, slightly below the expected frequency of 140 MHz. The result reveals that there is little loss in the passband ( around 0.7 db at 78 MHz) and significant loss in the stopband. Term Term1 Num=1 Z=50 Ohm input L L1 L=82 nh C C1 C=39 pf L L2 L=100 nh C C2 C=30 pf L L3 L=39 nh output-1 Term Term2 Num=2 Z=50 Ohm C C3 C=15 pf L L4 L=33 nh C C4 C=12 pf L L5 L=39 nh C C5 C=33 pf output-2 Term Term3 Num=3 Z=50 Ohm Figure 2.6: Schematic of 78 MHz diplexer. 11

13 Figure 2.7: Simulation and measurement results of 78 MHz diplexer MHz Elliptic Filter Design A 78 MHz elliptic filter has been designed with 78 MHz center frequency and 40 MHz 3 db bandwidth. A spectrum analyzer has been used to measure the performance of this filter. The schematic and the frequency responses are shown in Fig. 2.8 and 2.9 respectively. Figure 2.8: Schematic of 78 MHz filter. 12

14 Figure 2.9: Simulation results of 78 MHz filter. 13

15 Section III Measurement & Analysis The previous section described the high and low-level designs of the downconverter and provided some performance results for individual diplexers and filter. This section presents a complete set of performance results for the entire RFDC. 3.1 GNI Analysis Table 3.1 shows a GNI analysis of the entire downconverter, which is based on manufacturer device specifications. As we already mentioned in the previous chapter that we bypassed the diplexer 1250 from our measurement due to its poor performance. As can be seen from the Table, the gain of RFDC is 49 db, noise figure is 4.5 db and IIP3 is -37 dbm. Note that these data are based entirely on device specifications and thus do not represent measured data. Table 3.1: GNI analysis of the RFDC without the first diplexer. Stage Parameters Cascade Stage Component Gain (db) NF (db) IIP3 (dbm) Gain (db) NF (db) IIP3 (dbm) 1 ERA-3SM Amplifier SYM-11 Mixer Diplexer-1250 MHz ERA-3SM Amplifier BPF - LARK ERA-3SM Amplifier BPF - LARK ERA-3SM Amplifier SYM-11 Mixer Diplexer-78 MHz ERA-3SM Amplifier BPF - 78 MHz Elliptic

16 3.2 Measurement Setup Fig. 3.1 shows the measurement setup to evaluate the performance of the designed RFDC. We used three signal generators to supply the RF input and LO signals. A DC power source was used to supply the 9 V source and a spectrum analyzer was used to measure the output performance. Since high side LO injection has been used for the mixers, the IF and image frequency is ( f f ) and ( f f ) respectively. As an LO example if the RF input to a mixer is 138 MHz and the mixer uses 1388 MHz LO signal in high-side injection mode then the IF and image frequency would be 1250 MHz and 2638 MHz respectively. The equipment list and the parameters of the LO signals are summarized in Table 3.1. RF LO IF Figure 3.1: Measurement Setup. 3.3 Measured Performance Fig. 3.2 shows the gain as a function of frequency for the entire downconverter. As can be seen from the results, the average gain is around 47 db which is very close to our expected gain of 49 db. For some frequencies the gain is little bit low from the average value; specifically around 350 MHz and 720 MHz. Since these two low gain suck-out 15

17 regions do not fall into the operating bands of this RFDC i.e. the public safety frequency bands [5], we are not concerned about this. Fig. 3.3 and Fig. 3.4 show higher resolution measurements in the frequency region of MHz and MHz respectively. The average gain for the MHz region is around 42 db and for the MHz region is around 46 db. As we already mentioned, these frequencies are not fall into the operating bands of our designed RFDC, but we have taken the high resolution measurements just to analyze the gain-frequency characteristics. The input 1 db compression point shown in Fig. 3.5 follows the trend of the gain quite closely, as expected. That is, as the gain drops the compression point increases. From this figure we can also get some idea about the IIP3, which is as a rule-of-thumb is about 10 db higher than the 1 db compression point. On this basis, the average IIP3 from the 1 db compression point graph is estimated to be -32 dbm, compared to the calculated IIP3 of - 37 dbm. Gain Vs Frequency Gain (db) Freq (MHz) Figure 3.2: Measured downconverter gain. 16

18 Gain Vs Frequency (318 MHz- 378 MHz) Gain (db) Freq (MHz) Figure 3.3: Measured downconverter gain in 318 to 378 MHz. Gain Vs Frequency (678 MHz MHz) Gain (db) Freq (MHz) Figure 3.4: Measured downconverter gain in 678 to 738 MHz. 17

19 Input 1 db Com p. Pt. Vs Frequency -36 Input 1 db comp. pt. (dbm) Freq (MHz) Figure 3.5: Measured downconverter input 1 db compression point. 18

20 Section IV Conclusion The GNI analysis and the performance analyses for the complete downconverter are presented in the previous chapter. The performance analyses were in close agreement with the GNI analysis expectation. Table 4.1 summarized the measured performance of and Fig. 4.1 shows the image of this RFDC. Table 4.1: Summary of the parameters of RFDC. Parameter Measured Units Performance Tuning Range (10 Bands) MHz Instantaneous 3 db 40 MHz Bandwidth Gain 47 (avg.) db 50.5 (max.) 35.4 (min.) IIP3-32 (avg.) dbm -28 (max.) -34 (min.) Power 2.52 (0.28A@9 V) Watt Output Center Frequency 78 MHz Dimensions in 19

21 Figure 4.1: Image of RFDC. Summary of the cost for one RFDC board is given in Table 4.2. Table 4.2: Summary of the cost for one RFDC board. Component Quantity Price (US Dollar) Capacitor Inductor Lark Bandpass Filter MMCX Connector Power Connector Mixer Attenuator Amplifier Resistor RF Choke PCB fabrication Total

22 References [1] S.M. Hasan et al., "A Low Cost Multi-Band/Multi-Mode Radio for Public Safety," SDR Forum, Orlando FL, November [2] S.W. Ellingson, "Requirements for an Experimental Public Safety Multiband/Multimode Radio: Analog FM Modes", Technical Report No. 8, July 27, 2006, [3] G. Matthaei, L. Young, E.M.T. Jones, Microwave Filters, Impedance-Matching Networks, and Coupling Structures, Artech House Publishers, Feb 1980, ISBN [4] Sabin, W.E., Diplexer Filters for an HF MOSFET Power Amplifiier, QEX July/August 1999, pp.20. [5] S.W. Ellingson, "Phase I Technical Report", Technical Report No. 15, October 1, 2006, 21

23 Appendices 22

24 Appendix A: Component List Description Qty Value Package Manufacturer Part No. Distributor Distributor Part No. Part ID CAP CER 10PF 50V C0G p C0603 TDK Corporation C1608C0G1H100D Digikey ND C2, C5, C7, C8, C10, C11, C13, C14, C15, C21, C23, C25, C26, C28, C29, C31, C39 CAP CER 1000PF 50V C0G 5% p C0603 TDK Corporation C1608C0G1H102J Digikey ND C1, C4 CAP CER.10UF 50V Y5V u C0603 TDK Corporation C1608Y5V1H104Z Digikey ND C3, C6, C9, C12, C24, C27, C30 CAP CERAMIC 1.0PF 50V 0603 RFSMD 1 1.0p C0603 AVX Corporation 06035J1R0BBTTR Digikey ND C16 CAP CERAMIC 3.0PF 50V 0603 RFSMD 1 3.0p C0603 AVX Corporation 06035J3R0BBTTR Digikey ND C17 CAP CERAMIC 3.9PF 50V 0603 RFSMD 1 3.9p C0603 AVX Corporation 06035J3R9BBTTR Digikey ND C40 CAP CERAMIC 2.2PF 50V 0603 RFSMD 1 2.2p C0603 AVX Corporation 06035J2R2BBTTR Digikey ND C18 CAP CERAMIC.8PF 50V 0603 RF SMD 1 0.8p C0603 AVX Corporation 06035J0R8PBTTR Digikey ND C19 CAP CERAMIC 2.4PF 50V 0603 RFSMD 1 2.4p C0603 AVX Corporation 06035J2R4BBTTR Digikey ND C20 CAP CER 12PF 50V C0G 5% p C0603 AVX Corporation C1608C0G1H120J Digikey ND C36, C44 CAP CER 15PF 50V C0G 5% p C0603 TDK Corporation C1608C0G1H150J Digikey ND C33 CAP CER 30PF 50V 5% C0G p C0603 Murata Electronics GRM1885C1H300JA01D Digikey ND C35, C41 CAP CER 33PF 50V C0G 5% p C0603 TDK Corporation C1608C0G1H330J Digikey ND C37 CAP CER 39PF 50V C0G 5% p C0603 TDK Corporation C1608C0G1H390J Digikey ND C34 23

25 Description Qty Value Package Manufacturer Part No. Distributor Distributor Part No. Part ID CAP CER 47PF 50V C0G 5% p C0603 TDK Corporation C1608C0G1H470J Digikey ND C45, C46 CAP CER 68PF 50V C0G 5% p C0603 TDK Corporation C1608C0G1H680J Digikey ND C42 CAP CER 100PF 50V C0G 5% p C0603 TDK Corporation C1608C0G1H101J Digikey ND C32, C38 CAP CER 120PF 50V C0G 5% p C0603 TDK Corporation C1608C0G1H121J Digikey ND C43 LARK FILTER 2 LARK_MC125 Lark LARK_MC MC1250 Lark Engineering Inc. LARK_MC MM MM Engineering Inc. 3MM F1, F2 TTE FILTER 1 TTE_K4938- TTE_K /98M-20- TTE_K /98M- 58/98M-20- TTE20 TTE Inc. TTE Inc F3 CONN MMCX JACK RCPT STR SMD Emerson Network Power 6 MMCX MMCX GOLD Connectivity Solutions Digikey J821-ND J1, J2, J5, J6, J7, J8 HEADER.025" SQ PIN SIP 36POS 1 9V 1X02 Mill-Max Manufacturing Corp Digikey ND JP1 Inductor 1 2.2n 0402/0603 Coil Craft 0603CS-2N2X_LU Coil Craft 0603CS-2N2X_LU L6 Inductor 2 2.7n 0402/0603 Coil Craft 0402CS-2N7X_LU Coil Craft 0402CS-2N7X_LU L5, L7 Inductor 2 6.8n 0402/0603 Coil Craft 0603CS-6N8X_LU Coil Craft 0603CS-6N8X_LU L3, L4 Inductor 1 33n 0402/0603 Coil Craft 0603CS-33NX_LU Coil Craft 0603CS-33NX_LU L13 Inductor 1 36n 0402/0603 Coil Craft 0603CS-36NX_LU Coil Craft 0603CS-36NX_LU L21 Inductor 2 39n 0402/0603 Coil Craft 0603CS-39NX_LU Coil Craft 0603CS-39NX_LU L12, L14 Inductor 2 72n 0402/0603 Coil Craft 0603CS-72NX_LU Coil Craft 0603CS-72NX_LU L20 Inductor 1 82n 0402/0603 Coil Craft 0603CS-82NX_LU Coil Craft 0603CS-82NX_LU L24 Inductor 1 91n 0402/0603 Coil Craft 0603CS-91NX_LU Coil Craft 0603CS-91NX_LU L10 Inductor 2 100n 0402/0603 Coil Craft 0603CS-R10X_LU Coil Craft 0603CS-R10X_LU L11, L22 Inductor 1 150n 0402/0603 Coil Craft 0603CS-R15X_LU Coil Craft 0603CS-R15X_LU L18 Inductor 1 330n 0402/0603 Coil Craft 0603CS-R33X_LU Coil Craft 0603CS-R33X_LU L23 24

26 Description Qty Value Package Manufacturer Part No. Distributor Distributor Part No. Part ID INDUCTOR CHIP.82UH 10% SMD 1 820n 0402/0603 JW Miller PM1008-R82K-RC Digikey M8475CT-ND L19 MIXER 2 SYM-11 TTT167 Mini Circuits SYM-11 Mini Circuits SYM-11 M1, M2 PAT 4 3 AF320 Mini Circuits PAT-3 Mini Circuits PAT-3 P1, P2, P3, P4 AMPLIFIER 1 ERA-6SM WW107 Mini Circuits ERA-6SM Mini Circuits ERA-6SM Q2 AMPLIFIER 6 ERA-3SM WW107 Mini Circuits ERA-3SM Mini Circuits ERA-3SM Q1, Q3, Q4, Q5, Q6, Q7 RES 56 OHM 1W 5% 2512 SMD 1 56 R2512 Panasonic - ECG ERJ-1WYJ560U Digikey P56XCT-ND R5 RES 160 OHM 1W 5% 2512 SMD R2512 Panasonic - ECG ERJ-1TYJ161U Digikey PT160XTR-ND R1, R6, R8, R9,R13, R14 RES 51 OHM 1/16W.5% 0603 SMD 2 51 R0603 Susumu Co Ltd RR0816Q-510-D Digikey RR08Q51DCT-ND R7, R15 RF CHOKE 7 ADCH-80A CD542 Mini Circuits ADCH-80A Mini Circuits ADCH-80A U$1, U$2, U$3, U$4, U$5, U$6, U$7 25

27 Appendix B: PCB 26

28 Appendix C: Schematic 27

29 Appendix C: Schematic (Contd.) 28