LMX9830 Bluetooth™ Serial Port Module

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LMX98309.0 Digital Smart Radio (Continued)Based on crystal spec and equation:C L = C int + C TUNE + Ct1//Ct2C L = 8pF + 2.6pF + 6pF = 16.6pF16.6pF is very close to the TEW crystal requirement of16pF load capacitance. With the internal shunt capacitanceC total :C total = 16.6pF + 5pF = 21.6pFCLK+LMX9830C intC TUNECLK-TEW CrystalThe LMX9830 has been tested with the TEW TAS-4025Acrystal, reference Table 19 on page 22 for specification.Since the internal capacitance of the crystal circuit is 8 pFand the load capacitance is 16 pF, 12 pF is a good startingpoint for both Ct1 and Ct2. The 2480 MHz RF frequencyoffset is then tested. Figure 14 on page 23 shows the RFfrequency offset test results.Figure 14 on page 23 shows the results are -20 kHz off thecenter frequency, which is –1 ppm. The pullability of thecrystal is 2 ppm/pF, so the load capacitance must bedecreased by about 1.0 pF. By changing Ct1 or Ct2 to 10pF, the total load capacitance is decreased by 1.0 pF. Figure15 on page 23 shows the frequency offset test results.The frequency offset is now zero with Ct1 = 10 pF, Ct2 = 10pF.Reference Table 20 on page 22 for crystal tuning valuesused on Mesa Development Board with TEW crystal.Ct1CrystalFigure 12. LMX9830 Crystal RecommendedCircuitR1 C1 L1C0Figure 13. Crystal Equivalent CircuitCt22. Crystal PullabilityPullability is another important parameter for a crystal,which is the change in frequency of a crystal with units ofppm/pF, either from the natural resonant frequency to aload resonant frequency, or from one load resonant frequencyto another. The frequency can be pulled in a parallelresonant circuit by changing the value of loadcapacitance. A decrease in load capacitance causes anincrease in frequency, and an increase in load capacitancecauses a decrease in frequency.Table 19. TEW TAS-4025ASpecificationValuePackage4.0x2.5x0.65 mm - 4 padsFrequency13.000 MHzModeFundamentalStability > ±15ppm @-40 to +85CC L Load Capacitance 16pFESR80 Ω max.C 0 Shunt Capacitance 5pFDrive Level50 ±10uVPullability 2 ppm/pF minStorage Temperature -40 to +85CTable 20. TEW on LMX9830 DONGLEReferenceLMX9830Ct112pFCt212pF3. Frequency TuningFrequency Tuning is achieved by adjusting the crystal loadcapacitance with external capacitors. It is a Bluetoothrequirement that the frequency is always within ±20 ppm.Crystal/oscillator must have cumulative accuracy specificationsof +15 ppm to provide margin for frequency drift withaging and temperature.www.national.com 22
9.0 Digital Smart Radio (Continued)LMX9830Figure 14.Frequency Offset with 12 pF//12 pF CapacitorsFigure 15.Frequency Offset with 10 pF//10 pF Capacitors23 www.national.com
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Page 13 and 14: 8.0 Functional Description8.1 BASEB
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LMX9830 Bluetooth™ Serial Port Module

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