TransmitterReferenceDesignfora900MHzFull-DuplexRadio

Transmitter Reference Design for a 900MHz Full-Duplex Radio
  Abstract: This reference design covers the design of a transmitter inside a 900MHz full-duplex radio using the MAX2902 single-chip transmitter. The MAX2902 is designed for use in the 869MHz to 915MHz frequency band, and complies with FCC CFR47 part 15.247 902MHz to 928MHz ISM band specifations. The reference design is capable of delivering more than +19dBm with VCC = 2.9V and ICC = 170mA, and +20dBm with VCC = 3.3V and ICC = 190mA, while meeting 14dB of CNR (1.22Mbps BPSK) at 10:1 VSWR (all angles).
  
  
  
  
  Figure 1. Reference design board features the MAX2902 single-chip transmitter, which is designed for use in 868MHz to 915MHz frequency band. The device complies with FC CFR47 part 15.247 902MHz to 928MHz ISM band specifications.
  
  
  
  More detailed image (PDF, 222kB)
  Figure 3. Output power and supply current at various supply voages as a function of PA gain (VASK).
  Note that the reference design incorporates a 900MHz BPF (approx. 0.6dB IL) at Tx output. Reference design and EV kit provide almost identical output power at the PA.
  
  Supply current for the reference design is less than 10mA higher than the EV kit at full power. This is due to the increased bias to the PA that decreases the VCO pulling.
  
  
  
  
  More detailed image (PDF, 219kB)
  Figure 4. Output power and EVM at various VSWRs, as a function of PA gain (VASK).
  Recommended operating points reflect VASK levels to keep EVM below 20% at a desired max VSWR (worst-case phase).
  
  Reference design allows a higher nominal output power for same EVM.
  
  
  
  More detailed image (PDF, 218kB)
  Figure 5. Output power and EVM at various VSWRs as a function of VCC.
  VASK is optimized in the design according to the worst-case VSWR the system can tolerate. Lowering VASK drops POUT, but keeps EVM in check for increasing VSWR. Use the following VASK settings to limit EVM to 20%:
  Design for 1:1 VSWR tolerance ≥ 2.1V
  Design for 4:1 VSWR tolerance ≥ 1.7V
  Design for 8:1 VSWR tolerance ≥ 1.6V
  
  
  Detailed Description
  The MAX2902 is single-chip transmitter designed for use in 868MHz to 915MHz frequency bands. It complies with FC CFR47 part 15.247 902MHz to 928MHz ISM band specifications.
  
  The transmitter IC offers a high level of integration while minimizing external components. The transmit modulator, power amplifier, RF VCO, 8-channel frequency synthesizer, and baseband PN sequence lowpass filter are all integrated. By filtering BPSK modulation, the spurious emissions are reduced, enabling up to 8 independent transmit channels in the U.S. ISM band. Inputs are provided for spread-spectrum BPSK, ASK, and OOK. FM can be achieved by directly modulating the VCO. The device is intended primarily for use with external differential antenna.
  
  In this reference design the MAX2902 is applied as the transmit IC in the 900MHz band. It is controlled by a National Semiconductor LMX2353 fractional-N PLL.
  Kyocera is a registered trademark of Kyocera Corporation.
  Murata is a registered trademark of Murata Manufacturing Co., Ltd.
  National Semiconductor is a registered trademark of National Semiconductor Corp.
  SPI is a trademark of Motorola, Inc.
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