Design of output matching network for MAX1472 ASK transmitter
Design of output matching network for MAX1472 ASK transmitter
The MAX1472 is a phase-locked loop transmitter with a crystal as a reference clock and is designed to transmit OOK / ASK data in the 300MHz to 450MHz frequency band. The device can support data rates up to 100kbps. When the system is matched at 50Ω, the MAX1472 PA can provide output power greater than + 10dBm and maintain efficiency greater than 43%. The crystal-based architecture allows the MAX1472 to avoid many of the problems faced by SAW transmitters, providing greater modulation depth, faster frequency setting, higher transmit frequency tolerance, and reduced temperature effects.
The power amplifier (PA) of the MAX1472 is a high-efficiency, open-drain Class C amplifier. With an appropriate output matching network, the PA can drive a wide range of impedances, including small-sized loop PCB antennas and any other 50Ω antennas. Figure 1 shows the recommended matching network for a 50Ω system. The matching network converts the 50Ω load into the higher impedance of the PA output port and forms a band-pass filter that attenuates higher-order harmonics. 
Figure 1. MAX1472 application circuit
Table 1 shows the component values ​​of the output matching network at four common frequencies. These values ​​are used in the MAX1472 evaluation kit (EV kit). In practical applications, considering the parasitic inductance and capacitance of the PCB, these values ​​need to be fine-tuned.
Table 1. Element values ​​of the output matching network
| - | fRF = 315MHz | fRF = 345MHz | fRF = 385MHz | fRF = 433.92MHz |
| C1 | 15pF | 10pF | 10pF | 7pF |
| C2 | 22pF | 18pF | 15pF | 12pF |
| C3 | 15pF | 15pF | 12pF | 10pF |
| L1 | 27nH | 27nH | 24nH | 22nH |
| L2 | 22nH | 18nH | 18nH | 15nH |
| fXTAL | 9.84375MHz | 10.78125MHz | 12.03125MHz | 13.56000MHz |
Figure 2, Figure 3 and Figure 4 show the output power and efficiency curves of the MAX1472 under the above matching network configuration.
Figure 2. Relationship between efficiency and power supply voltage under continuous wave
Figure 3. 50% OOK signal, efficiency vs. supply voltage
Figure 4. Relationship between output power and supply voltage
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