Application Note Transceiver TH7122x External PA, LNA and TX/RX switch TH7122 and TH71221 with External PA and LNA The output power and the sensitivity of TH7122x transceivers for SRD are enough for many applications. They can deliver up to 10dBm output power and have a sensitivity of up to -105dBm in FSK mode. However, some parameters of the standard TH7122x EVB can be improved. The standard EVB does not have much RF selectivity, so the image rejection of the boards at high frequencies is low. In some cases the leakage from the receiver VCO to the input is also high. For some applications the customer needs a better sensitivity and a higher output power. The aim of this application note is to show how to improve these parameters. One PCB layout has been designed for both TH7122 and TH71221 transceivers. A summary of the measured results is shown below. Measured Results for 433 MHz • • • • • • • • • • • Power supply range: 3V to 5.5V Operating current: 12mA in receive mode and 3.6V power supply Operating current: 16mA in receive mode and 5V power supply Operating current 77 to 87mA in transmit mode and 3.6V power supply Operating current 107 to112mA in transmit mode and 5V power supply Output power +17dBm at 3.6V power supply Output power +20dBm at 5V power supply Sensitivity: -115dBm at FSK with 180kHz IF filter BW Image rejection better than 40dB Improved blocking immunity Max. Input level: –10 dBm for FSK Measured Results for 868 MHz • • • • • • • • • • • Power supply range: 3V to 5.5V Operating current: 12mA in receive mode and 3.6V power supply Operating current: 16mA in receive mode and 5V power supply Operating current 77mA in transmit mode and 3.6V power supply Operating current 107mA in transmit mode and 5V power supply Output power +17dBm at 3.6V power supply Output power +20dBm at 5V power supply Sensitivity: -115dBm at FSK with 180kHz IF filter BW Image rejection better than 25dB Blocking immunity is increased Max. input level: -10dBm for FSK 39011 07122 05 Rev. 001 Page 1 of 14 AN7122x_PA_LNA Feb/07 Application Note Transceiver TH7122x External PA, LNA and TX/RX switch Document Content 1 2 3 Circuit Description .....................................................................................................3 1.1 TX/RX PIN Diode Switch .................................................................................................. 3 1.2 External Power Amplifier .................................................................................................. 4 1.3 External LNA and SAW Filter for the Receiver ................................................................. 7 1.4 Software Settings ............................................................................................................. 9 Complete Board for the PA, LNA and TX/RX Switch.............................................10 2.1 Board Component Values (Fig. 10) ................................................................................ 10 2.2 Component Arrangement Top Side ................................................................................ 12 2.3 Board Layouts ................................................................................................................ 13 References and related documents........................................................................14 39011 07122 05 Rev. 001 Page 2 of 14 AN7122x_PA_LNA Feb/07 Application Note Transceiver TH7122x External PA, LNA and TX/RX switch 1 Circuit Description The basic of the schematic is the standard EVB7122x. Three additional blocks: PA, LNA and TX/RX PIN diode switch have been added. All other components from the standard EVB remain the same. The full schematic of the application is shown in Figure 10. 1.1 TX/RX PIN Diode Switch Since the TH7122x transceivers do not have a dedicated TX/RX control pin we used the voltage on pin PS_PA as the sense signal. This pin is used to set the output power of the transmitter and is connected to ground via resistor RPS. The voltage drop on the resistor RPS is 0 V in receive mode and up to 500mV in transmit mode. The maximum voltage depends on the value of the resistor RPS and the power supply, but the change is enough to use with the voltage comparator, U2. A dual voltage comparator is used also to produce control signals for a PIN diode switch. The PIN diode switch needs complementary signals as is shown in Fig. 1. The first part of U2 works as a voltage comparator and the second part as an inverter. The same control signals can be used to bias the external LNA and the external PA. For a reliable operation of the comparator, the resistors R5 and R6 provide a voltage above 0V and below the voltage on resistor RPS. This voltage is applied to the non-inverting input pin of the comparator U2_1. Thus, during receive mode the output 1 of the comparator is low and the output 7 is high. This switches PIN diode D1 off and the PIN diode D2 on. During transmit mode the control signals are reversed so that D1 is on and D2 is off. The resistors R7, R8 and R9 provide necessary current for proper operation of the PIN diode switches. Fig. 1: 39011 07122 05 Rev. 001 Schematic of the PIN diode switch Page 3 of 14 AN7122x_PA_LNA Feb/07 Application Note Transceiver TH7122x External PA, LNA and TX/RX switch 1.2 External Power Amplifier The output power of the standard TH7122x EVB is up to 10dBm with a 3V power supply. It is enough for SRD and low power devices. If more output power is desired, an external power amplifier can be used. The transistor amplifier operates together with the PIN diode TX/RX switch. The schematic of the external PA and the TX/RX switch is shown in Fig. 2. A low cost transistor BFP450 is used. The base of the transistor Q1 is biased from the TX/RX switch through the resistor RTX2 in transmit mode only. In receive mode the transistor is off. The collector bias is provided via resistor RTX1 and inductor LTX2. The load for the collector is the output matching network. The output matching network is a low pass filter which suppresses all harmonics about 3540dB. The output power is about 20dBm with a 5V power supply and 17dBm with a 3.6V power supply for the both 433 and 868MHz bands. CTX3 Fig. 2: Schematic of the external PA with TX/RX switch Some spectrum plots are shown below. The plots have been taken for the 433 and 868MHz bands. Fig. 3 to 5 show the spectrum plots for 433MHz and 5V power supply. Fig. 6 to 8 show the same parameters for the 868MHz application. The harmonic emissions for 433MHz are better than -35dBc while for 868MHz values of about -42dBc can be achieved. Note that these measurements are made conductive, into the 50Ω load of a spectrum analyzer. Harmonic emissions can be further suppressed by using an antenna. 39011 07122 05 Rev. 001 Page 4 of 14 AN7122x_PA_LNA Feb/07 Application Note Transceiver TH7122x External PA, LNA and TX/RX switch Fig. 3: Fig. 4: 39011 07122 05 Rev. 001 TX output signal at 433MHz, CW mode TX output signal at 433MHz, FSK modulation Page 5 of 14 AN7122x_PA_LNA Feb/07 Application Note Transceiver TH7122x External PA, LNA and TX/RX switch Fig. 5: 39011 07122 05 Rev. 001 TX output signal and spurious emissions, CW mode at 433MHz Page 6 of 14 AN7122x_PA_LNA Feb/07 Application Note Transceiver TH7122x External PA, LNA and TX/RX switch Fig. 6: Fig. 7: 39011 07122 05 Rev. 001 TX output signal at 868MHz, CW mode TX output signal at 868MHz, FSK modulation Page 7 of 14 AN7122x_PA_LNA Feb/07 Application Note Transceiver TH7122x External PA, LNA and TX/RX switch Fig. 8: 39011 07122 05 Rev. 001 TX output signal and spurious emissions, CW mode at 868MHz Page 8 of 14 AN7122x_PA_LNA Feb/07 Application Note Transceiver TH7122x External PA, LNA and TX/RX switch 1.3 External LNA and SAW Filter for the Receiver When the receiver requirements call for better image rejection and sensitivity, an external LNA with SAW filter can be used to significantly improve the performance. The TH7122x transceivers are a single conversion super-heterodyne with no inhered suppression of the image frequency. An RF front-end filter, usually a SAW filter, is required for good image rejection. Another advantage of a SAW filter is the improvement of the blocking immunity of the receiver, but the disadvantage is that filter has some insertion loss which decreases the sensitivity of the receiver. An external LNA can compensate the loss of the filter and improve the sensitivity of the receiver because of its lower noise figure. The sensitivity of the standard EVB is about -105dBm, and it becomes about -115dBm for both 433 and 868MHz bands with the external LNA. The schematic is shown in Fig. 9. For the LNA a BFP405 transistor is chosen. It is used as a common emitter amplifier. At low frequencies, an emitter resistor is important for stability, but at high frequencies, a bypass capacitor in parallel with emitter resistor can produce oscillation and make the amplifier unstable for some frequencies. Furthermore, an nonbypassed emitter resistor can degrade the performance of the amplifier. Therefore, in this design the emitter of the transistor is grounded. The components CRX0, CRX1, LRX2 and LRX4 are input/output matching networks for different types of SAW filters. For the 50 ohms Murata filters the inductor LRX2 is not in place. The external LNA works together with the TX/RX switch and the TX/RX control unit. The base of the transistor Q2 is biased at receive mode via resistor RRX2. In the transmit mode the transistor is off. The capacitor CRX2 and the inductor LRX5 represent the input matching network for the LNA transistor. The image rejection is between 25dB and 50dB for different frequency bands. The blocking immunity in this application is increased significantly. Also the VCO leakage in receive mode is further decreased in this circuit. CTX3 Fig. 9: External LNA with TX/RX switch 1.4 Software Settings fRO = 7.1505MHz CPCUR VCOCUR Channel frequency RR NR RT NT RX TX RX TX 433.92 MHz 32 1894 32 1942 260µA 260µA 500µA 500µA 868.3 MHz 16 1919 16 1943 260µA 260µA 900µA 900µA 39011 07122 05 Rev. 001 Page 9 of 14 AN7122x_PA_LNA Feb/07 Application Note Transceiver TH7122x External PA, LNA and TX/RX switch 2 Complete Board for the PA, LNA and TX/RX Switch The basic of the schematic is the standard EVB7122x. Three additional blocks: PA, LNA and TX/RX PIN diode switch have been added. All other components from the standard EVB remain the same. The full schematic of the application is shown in Fig. 10. CTX3 Fig. 10: Complete FSK application circuit 2.1 Board Component Values (Fig. 10) Part Size Value @ 433 MHz Value @ 868 MHz U1 LQFP32 QFN 5x5 TH7122 TH71221 TH7122 TH71221 Melexis transceiver U2 D1 D2 Q1 Q2 C0 C1 SO8 SOD323 SOD323 SOT343 SOT343 0603 0603 TS3702ID BAR63 BAR63 BFP450 BFP405 NIP 4.7 pF TS3702ID BAR63 BAR63 BFP450 BFP405 NIP 1.8 pF rail to rail voltage comparator PIN diode PIN diode PA transistor LNA transistor VCO tank capacitor LNA output tank capacitor C2 C3 C4 C5 C6 CB0 0603 0603 0603 0603 0603 1210 1.5 pF 10 nF 330 pF 1.5 nF NIP 10 µF 1.5 pF 10 nF 330 pF 1.5 nF NIP 10 uF MIX input matching capacitor Data slicer capacitor Demodulator output low pass capacitor RSSI output low pass capacitor PKDET capacitor de-coupling capacitor CB0' CB1 CB2 CB3 CB4 0805 0603 0603 0603 0603 100 nF 10 nF 330 pF 100 pF 10 nF 100 nF 10 nF 330 pF 100 pF 10 nF de-coupling capacitor de-coupling capacitor de-coupling capacitor de-coupling capacitor de-coupling capacitor 39011 07122 05 Rev. 001 Page 10 of 14 Description AN7122x_PA_LNA Feb/07 Application Note Transceiver TH7122x External PA, LNA and TX/RX switch Part Size Value @ 433 MHz Value @ 868 MHz CB5 CB6 CB7 CB8 CB9 0603 0603 0603 0603 0603 100 nF 100 pF 100 nF 100 pF 100 pF 100 nF 100 pF 100 nF 100 pF 100 pF de-coupling capacitor de-coupling capacitor de-coupling capacitor de-coupling capacitor de-coupling capacitor CB10 CF1 CF2 CP0 CPS CX1 0603 0603 0603 0603 0603 0805 100 pF 1 nF 100 pF NIP NIP 12 pF 100 pF 1 nF 100 pF NIP NIP 12 pF de-coupling capacitor loop filter capacitor loop filter capacitor CERDIS loading capacitor power select capacitor RO capacitor for FSK CX2 CRX0 CRX1 CRX2 CTX0 CTX1 CTX2 0805 0603 0603 0603 0603 0603 0603 82 pF 100 pF 100 pF 100 pF 100 pF 6.8 pF 4.7 pF 56 pF 100 pF 100 pF 100 pF 100 pF 3.3 pF 3.3 pF RO capacitor for FSK RX coupling capacitor RX coupling capacitor RX coupling capacitor TX coupling capacitor TX impedance matching capacitor TX impedance matching capacitor CTX3 CTX4 CTX5 CTX6 CTX7 CTX8 0603 0603 0603 0603 0603 0603 100 pF 4.7 pF 2.7 pF 100 pF NIP 470 pF 100 pF 2.2 pF 2.7 pF 100 pF NIP 470 pF output coupling capacitor TX impedance matching capacitor TX impedance matching capacitor TX coupling capacitor TX impedance matching capacitor PA feedback capacitor L0 L1 LTX0 LTX1 LTX2 LTX3 LRX4 0603 0603 0603 0603 0603 0603 0603 33 nH 15 nH 15 nH 33 nH 10 nH 39 nH 82 nH 4.7 nH 4.7 nH 3.9 nH 10 nH 6.8 nH 10 nH 82 nH VCO tank inductor LNA output tank inductor TX impedance matching inductor TX impedance matching inductor TX impedance matching inductor TX impedance matching inductor RX impedance matching inductor LRX5 RB1 RF RP RL0 RPS 0603 0603 0603 0603 0603 0603 22 nH 150 33 k NIP NIP 47 k 10 nH 150 33 k NIP NIP 47 k RX impedance matching inductor protection resistor loop filter resistor CERDIS loading resistor CERFIL loading resistor, optionally power select resistor RS1 RS2 RS3 RRX1 RRX2 RTX1 RTX2 0603 0603 0603 0603 0603 0603 0603 10 k 10 k 10 k 100 82 k 6 3.9 k 10 k 10 k 10 k 100 82 k 6 5.1 k protection resistor protection resistor protection resistor protection resistor LNA bias resistor protection resistor PA bias resistor RTX3 R1 R3 R4 R5 R6 0603 0603 0603 0603 0603 0603 560 NIP NIP 100 k 100 k 4.7 k 560 NIP NIP 100 k 100 k 4.7 k Negative feedback resistor PKDET resistor demodulator output loading resistor voltage comparator input resistor voltage comparator divider resistor voltage comparator divider resistor R7 0603 1.2 k 1.2 k PIN diode bias resistor 39011 07122 05 Rev. 001 Page 11 of 14 Description AN7122x_PA_LNA Feb/07 Application Note Transceiver TH7122x External PA, LNA and TX/RX switch Part Size Value @ 433 MHz Value @ 868 MHz R8 R9 R10 0603 0603 0603 SMD 3.45x3.1 1.2 k 1.2 k NIP SFECF10M7HA00 1.2 k 1.2 k NIP SFECF10M7HA00 SMD type SMD 3x3 CDSCB10M7GA136 CDSCB10M7GA136 SAFCC433MBL0X00 (f0 = 433.92 MHz) SAFCC868MSL0X00 (f0 =868.3 MHz) CERFIL CERDIS SAWFIL XTAL Note: HC49 SMD 7.1505 MHz ±20ppm calibration, ±20ppm temperature Description PIN diode bias resistor PIN diode bias resistor demodulator output loading resistor ceramic filter from Murata, or equivalent part ceramic discriminator, from Murata, or equivalent part low-loss SAW filter from Murata, or equivalent part fundamental-mode crystal, Cload = 10 pF to 15pF, C0, max = 7 pF, Rm, max = 70 Ω NIP – not in place, may be used optionally 2.2 Component Arrangement Top Side Board size is 58.5mm x 39.4mm 39011 07122 05 Rev. 001 Page 12 of 14 AN7122x_PA_LNA Feb/07 Application Note Transceiver TH7122x External PA, LNA and TX/RX switch 2.3 Board Layouts • Board layout data in Gerber format is available, board size is 58.5mm x 39.4mm. PCB top view PCB bottom view 39011 07122 05 Rev. 001 Page 13 of 14 AN7122x_PA_LNA Feb/07 Application Note Transceiver TH7122x External PA, LNA and TX/RX switch 3 References and related documents • • • • • TH7122 data sheet TH71221 data sheet Evaluation Board for TH7122 Evaluation Board for TH71221 Guillermo Gonzalez: “Microwave Transistor Amplifier Analysis and Design” For the latest version of this document, go to our website at: www.melexis.com Or for additional information contact Melexis Direct: Europe and Japan: Phone: +32 1367 0495 E-mail: [email protected] All other locations: Phone: +1 603 223 2362 E-mail: [email protected] ISO/TS16949 and ISO14001 Certified 39011 07122 05 Rev. 001 Page 14 of 14 AN7122x_PA_LNA Feb/07