Datasheet MAX1472 (Maxim) - 7

300MHz-to-450MHz Low-Power, Crystal-Based ASK Transmitter MAX1472 Crystal Oscillator Output Matching to 50
Ω The crystal oscillator in the MAX1472 is designed to When matched to a 50Ω system, the MAX1472 PA is present a capacitance of approximately 3.1pF between capable of delivering more than +10dBm of output the XTAL1 and XTAL2 pins. If a crystal designed to power at VDD = 2.7V. The output of the PA is an open- oscillate with a different load capacitance is used, the drain transistor that requires external impedance crystal is pulled away from its intended operating fre- matching and pullup inductance for proper biasing. quency, thus introducing an error in the reference fre- The pullup inductance from PA to VDD serves three quency. Crystals designed to operate with higher main purposes: It resonates the capacitance of the PA differential load capacitance always pull the reference output, provides biasing for the PA, and becomes a frequency higher. For example, a 9.84375MHz crystal high-frequency choke to reduce the RF energy cou- designed to operate with a 10pF load capacitance pling into VDD. The recommended output-matching net- oscillates at 9.84688MHz with the MAX1472, causing work topology is shown in the Typical Application the transmitter to be transmitting at 315.1MHz rather Circuit. The matching network transforms the 50Ω load than 315.0MHz, an error of about 100kHz, or 320ppm. to a higher impedance at the output of the PA in addi- In actuality, the oscillator pulls every crystal. The crys- tion to forming a bandpass filter that provides attenua- tal’s natural frequency is really below its specified fre- tion for the higher order harmonics. quency, but when loaded with the specified load
Output Matching to PC Board Loop
capacitance, the crystal is pulled and oscillates at its
Antenna
specified frequency. This pulling is already accounted In most applications, the MAX1472 PA output has to be for in the specification of the load capacitance. impedance matched to a small-loop antenna. The Additional pulling can be calculated if the electrical antenna is usually fabricated out of a copper trace on a parameters of the crystal are known. The frequency PC board in a rectangular, circular, or square pattern. pulling is given by: The antenna has an impedance that consists of a lossy component and a radiative component. To achieve ⎛ 1 1 ⎞ high radiating efficiency, the radiative component C f m p = ⎜ − ⎟ × 106 should be as high as possible, while minimizing the 2 C + C C + C ⎝ ⎠ case load case spec lossy component. In addition, the loop antenna has an inherent loop inductance associated with it (assuming where: the antenna is terminated to ground). For example, in a typical application, the radiative impedance is less than fp is the amount the crystal frequency is pulled in ppm. 0.5Ω, the lossy impedance is less than 0.7Ω, and the Cm is the motional capacitance of the crystal. inductance is approximately 50nH to 100nH. Ccase is the case capacitance. The objective of the matching network is to match the C PA output to the small loop antenna. The matching spec is the specified load capacitance. components thus transform the low radiative and resis- Cload is the actual load capacitance. tive parts of the antenna into the much higher value of When the crystal is loaded as specified, i.e., Cload = the PA output, which gives higher efficiency. The low Cspec, the frequency pulling equals zero. radiative and lossy components of the small loop anten- na result in a higher Q matching network than the 50Ω network; thus, the harmonics are lower.
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