Application Note Receiver MLX71120/21 Dual Band Scope This application note describes the use of the MLX71120/21 RFICs in a dual-band receiver configuration. Thanks to its two RF inputs the MLX71120/21 receivers can be used with two different antennas to operate in the well known European ISM bands at 433.9MHz and at 868.3MHz. This frequency diversity method leads to a very reliable receiver. If the communication channel in one ISM band is disturbed (e.g. by noise of fading) or just occupied by other services then the user can easily switch to the other one. This can be done without changing any external component around the MLX71120/21 RFIC. The desired frequency band can be selected by the logic level at pin RFSEL (set either to VCC or VEE). This application note provides the detailed application schematic with the complete component list, as well as some measurement results such as sensitivity and co-channel immunity. Applications Highly reliable RF receivers, e.g. for alarms, security, automotive applications Easy to switch multi-band receiver modules Related Melexis Products MLX71120/21 Multi-band RF Receivers Theory of operation Due to the special synthesizer structure of the MLX71120/21 devices, it is possible to receive two different frequencies located in different frequency bands without changing the crystal reference frequency, as is normally the case for single-channel receivers. As shown below, there are two special device-dependent frequency plans controllable via pin RFSEL. This allows the reception of one operating frequency in the 433MHz band and another frequency in the 868MHz band. RFSEL fRF [MHz] fIF1 [MHz] fLO1 [MHz] fLO2 [MHz] 0 433.9225 145.2483 579.17 144.7925 1 868.3000 289.1300 579.17 289.5850 fRO [MHz] 24.132083 Table 1: Frequency plan of MLX71120 (IF2 = 455kHz) RFSEL fRF [MHz] fIF1 [MHz] fLO1 [MHz] fLO2 [MHz] 0 433.25 146.82 580.07 145.02 1 868.30 288.23 580.07 290.03 fRO [MHz] 24.169444 Table 2: Frequency plan of MLX71121 (IF2 = 1.8MHz) Formulas for detailed frequency calculations can be found on page 14 of the MLX71120/21 data sheets. 39011 71121 01 Rev. 001 Page 1 of 6 AN71121_DB April/08 Application Note Receiver MLX71120/21 Dual Band Application Circuit Figure 1 shows the schematic diagram of the dual-band 433/868MHz receiver application using the MLX71120/21 Melexis receivers. FSK ASK VCC RCL IFSEL CLKO ENRX BAND output XTAL ROI 25 TEST 26 IFSEL 27 CLKO 28 VCC 22 MLX71120/21 9 C7 10 11 12 13 14 15 16 CIF CB1 CB2 CP1 DF1 17 CF2 FSK ASK CERFIL CF1 VCC LNAI2 DF2 8 4 CP2 DFO 18 SLCSEL 7 VEE SLC 19 MODSEL L4 32L QFN 5x5 6 LNAO2 CF3 PDP 20 IFAN C6 PDN 21 IFAP VCC 3 LNAO1 5 MIXN L5 6 CINT 23 4 MIXP C9 SAWFIL2 3 50 C5 C4 2 VEE VEE CB3 1 CRS RSSI 24 MIXO L2 L3 RSSI 4 50 3 DTAO 29 1 LNAI1 ENRX 30 L1 6 SAWFIL1 VCC 1 RFSEL 31 32 CX CB0 Figure 1: Dual band application schematic To improve the blocking performance, two surface acoustic wave filters (SAW) are added between the two connectors and the LNA inputs. Two inductors L1 and L4 are used for impedance matching of the two LNA inputs with the SAW filters. The two LNA outputs LNAO1 (pin 3) and LNAO2 (pin 6) are connected to the differential inputs of the first mixer MIXP (pin 4) and MIXN (pin 5) through the capacitors C5 and C6. The LNA outputs are loaded by tuned resonant circuits. L2 and C4 are tuned to 433MHz and L5 and C9 are tuned to 868MHz, respectively. The selection pins LNASEL (pin 32) and RFSEL (pin 31) can be combined into a common BAND control pin. This way only the LNA that is matched to the desired frequency band will be active while the other one will be turned off. 39011 71121 01 Rev. 001 Page 2 of 6 AN71121_DB April/08 Application Note Receiver MLX71120/21 Dual Band Component List of Figure 1 Part Value for MLX71120 Value for MLX71121 Tol. C4 5.6 pF 5.6 pF ±5% LNA1 output tank capacitor C5 100 pF 100 pF ±5% MIX1 positive input matching capacitor C6 100 pF 100 pF ±5% MIX1 negative input matching capacitor C7 1.8 pF 1.8 pF ±5% matching capacitor C9 3.9 pF 3.9 pF ±5% LNA2 output tank capacitor CB0 33 nF 33 nF ±10% decoupling capacitor CB1 330 pF 330 pF ±10% decoupling capacitor CB2 330 pF 330 pF ±10% decoupling capacitor CB3 330 pF 330 pF ±10% Description decoupling capacitor ±10% data low-pass filter capacitor, optimized for data rate of 4 kbps NRZ data low-pass filter capacitor, optimized for data rate of 4 kbps NRZ optional capacitor for noise cancellation filter CIF 1 nF NIP ±10% IFA feedback capacitor CP1 10 nF 10 nF ±10% positive PKDET capacitor CP2 10 nF 10 nF ±10% negative PKDET capacitor CF1 470 pF 470 pF ±10% CF2 220 pF 220 pF ±10% CF3 according to data sheet page 12 CRS 1 nF 1 nF ±10% RSSI output low pass capacitor CX 27 pF 27 pF ±5% crystal series capacitor L1 27 nH 27 nH ±5% matching inductor L2 15 nH 15 nH ±5% LNA1 output tank inductor L3 27 nH 27 nH ±5% matching inductor L4 18 nH 18 nH ±5% matching inductor L5 3.9 nH 3.3 kΩ 3.9 nH 3.3 kΩ ±5% LNA2 output tank inductor ±5% optional CLK output resistor RCL SAWFIL1 SAWFIL2 CERFIL XTAL SAFCC433MBL0X00 (433.92 MHz) SAFCC868MSL0X00 (868.3 MHz) CFUKG455KD4A NIP B6dB = 20 kHz 24.132083 MHz 24.169444 MHz low-loss SAW filter from Murata low-loss SAW filter from Murata ceramic filter IF2 fundamental mode crystal Table 3: Component list 39011 71121 01 Rev. 001 Page 3 of 6 AN71121_DB April/08 Application Note Receiver MLX71120/21 Dual Band Measurement Results In order to validate the functionality of the MLX71120/21 receiver for such a frequency diversity application, the sensitivity and the isolation are measured. The isolation is defined as the sensitivity difference between the desired input and the undesired input. For example, the receiver is set for 868MHz and the signal generator is set for 868MHz. The isolation is the difference between the sensitivity with the RF applied to the 433MHz and the RF applied to the 868MHz input. The measurement results are based on the application schematic described in the previous chapter. The following results are taken under normal operating conditions at TA = 23°C and VCC = 3V. The sensitivity values are measured at a data rate of 4kbps NRZ and they are valid for a bit error rate of 3·10-3. Note that the sensitivity of the MLX71120 in ASK mode is not specified because the narrow IF bandwidth is not suitable for ASK. BAND Frequency Modulation Sensitivity Isolation Low 433.923 MHz FSK -112 dBm 63 dB High 868.300 MHz FSK -108 dBm 38 dB Table 4: Measurement results MLX71120, BIF = 20kHz, Δf = ±5kHz BAND Low High Frequency Modulation Sensitivity Isolation 433.250 MHz FSK -107 dBm 61 dB 433.250 MHz ASK -116 dBm 59 dB 868.300 MHz FSK -104 dBm 39 dB 868.300 MHz ASK -113 dBm 36 dB Table 5: Measurement results MLX71121, BIF = 300kHz, FSK Δf = ±20kHz, ASK 100% mod. index Conclusion Thanks to its two RF inputs the MLX71120/21 RF receiver can be used as a dual-band receiver with one and the same crystal frequency. Such frequency diversity application can be useful when a highly reliable receiver is needed in a noisy environment and to reduce fading effects. The proposed application schematic provides a very good sensitivity and isolation. 39011 71121 01 Rev. 001 Page 4 of 6 AN71121_DB April/08 Application Note Receiver MLX71120/21 Dual Band Your Notes 39011 71121 01 Rev. 001 Page 5 of 6 AN71121_DB April/08 Application Note Receiver MLX71120/21 Dual Band Reference and Related Documents MLX71120/21 data sheets Evaluation Board for MLX71120 Evaluation Board for MLX71121 For the latest version of this document, go to our website at: www.melexis.com Or for additional information contact Melexis Direct: Europe, Africa: Americas: Asia: Phone: +32 1367 0495 E-mail: [email protected] Phone: +1 603 223 2362 E-mail: [email protected] Phone: +32 1367 0495 E-mail: [email protected] ISO/TS 16949 and ISO14001 Certified 39011 71121 01 Rev. 001 Page 6 of 6 AN71121_DB April/08