设计输出匹配网络,MAX1472 ASK的发射机-Desig

通信设计应用

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描述

The MAX1472 is a crystal-referenced phase-locked loop transmitter designed to transmit OOK/ASK data in the 300MHz to 450MHz frequency range. It supports data rates up to 100kbps. When matched to a 50Ω system, the MAX1472 PA is capable of delivering more than +10dBm of output power while maintaining an effective efficiency greater than 43%. The crystal-based architecture of the MAX1472 eliminates many of the common problems with saw transmitters by providing greater modulation depth, faster frequency settling, higher tolerance of the transmit frequency, and reduced temperature dependence.

The power amplifier (PA) of the MAX1472 is a high-efficiency open-drain class-C amplifier. With a proper output matching network, the PA can drive a wide range of impedances, including the small loop PC board trace antenna and any 50Ω antenna. Figure 1 shows a recommended matching network for a 50Ω system. The matching network transforms the 50Ω load to a higher impedance at the output of the PA in addition to forming a bandpass filter that provides attenuation for the higher order harmonics.

MAX1472
Figure 1. MAX1472 application circuit.

Table 1 shows the recommended output matching network component values for four widely used operating frequencies. These values were used with the MAX1472 evaluation kit (EV-kit). Some fine tunings may be required due to parasitic inductance and capacitance on PCB.

Table 1. Output Matching Network Component Values
-
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

Figures 2, 3, and 4 illustrate the output power and effective efficiency of the MAX1472 when configured with the above matching networks.

MAX1472
Figure 2. Efficiency vs. supply voltage with CW tone.

MAX1472
Figure 3. Efficiency vs. supply voltage with 50% OOK signal.

MAX1472
Figure 4. Output power vs. supply voltage.

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