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LMX2522LQ1635 Datasheet(PDF) 7 Page - National Semiconductor (TI) |
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LMX2522LQ1635 Datasheet(HTML) 7 Page - National Semiconductor (TI) |
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7 / 18 page  Functional Description GENERAL DESCRIPTION LMX2522/32 is a highly integrated frequency synthesizer system that generates LO signals for PCS, Cellular CDMA and GPS systems. These devices include all of the func- tional blocks of a PLL, RF VCO, prescaler, RF phase detec- tor, and loop filter. The need for external components is limited to a few passive elements for matching the output impedance and bypass elements for power line stabilization. In addition to the RF circuitry, the IC also includes IF fre- quency dividers, and an IF phase detector to complete the IF synthesis with an external VCO and loop filter. Table 4 summarizes the counter values to generate the default IF frequencies. Using a low spurious fractional-N synthesizer based on a delta sigma modulator, the circuit can support 10 kHz chan- nel spacing for PCS, Cellular CDMA and GPS systems. The fractional-N synthesizer enables faster lock time, which reduces power consumption and system set-up time. Addi- tionally, the loop filter occupies a smaller area as opposed to the integer-N architecture. This allows the loop filter to be embedded into the circuit, minimizing the external noise coupling and total form factor. The delta sigma architecture delivers very low spurious, which can be a significant prob- lem for other PLL solutions. The circuit also supports commonly used reference frequen- cies of 19.20 MHz and 19.68 MHz. FREQUENCY GENERATION RF-PLL Section The divide ratio can be calculated using the following equa- tion: LMX2522 – PCS CDMA: f VCO = {8 x RF_B + RF_A + (RF_FN / fOSC)x10 4}xf OSC where (RF_A < RF_B) LMX2532 – Cellular CDMA: f VCO = {6 x RF_B + RF_A + (RF_FN / fOSC)x10 4}xf OSC where (RF_A < RF_B) where f VCO: Output frequency of voltage controlled oscillator (VCO) RF_B: Preset divide ratio of binary 4-bit programmable counter (2 ≤ RF_B ≤ 15) RF_A: Preset divide ratio of binary 3-bit swallow counter (0 ≤ RF_A ≤ 7 for LMX2522 or 0 ≤ RF_A ≤ 5 for LMX2532) RF_FN: Preset numerator of binary 11-bit modulus counter (0 ≤ RF_FN < 1920 for f OSC = 19.20 MHz or 0 ≤ RF_FN < 1968 for f OSC = 19.68 MHz) f OSC: Reference oscillator frequency GPS-PLL SECTION The divide ratio can be calculated using the following equa- tion: LMX2522 – PCS CDMA: f VCO = {6 x RF_B + RF_A + (RF_FN / fOSC)x10 4}xf OSC where (RF_A < RF_B) LMX2532 – Cellular CDMA: f VCO = {8 x RF_B + RF_A + (RF_FN / fOSC)x10 4}xf OSC where (RF_A < RF_B) where f VCO: Output frequency of voltage controlled oscillator (VCO) RF_B: Preset divide ratio of binary 4-bit programmable counter (2 ≤ RF_B ≤ 15) RF_A: Preset divide ratio of binary 3-bit swallow counter (0 ≤ RF_A ≤ 5 for LMX2522 or 0 ≤ RF_A ≤ 7 for LMX2532) RF_FN: Preset numerator of binary 11-bit modulus counter (0 ≤ RF_FN < 1920 for f OSC = 19.20 MHz or 0 ≤ RF_FN < 1968 for f OSC = 19.68 MHz) f OSC: Reference oscillator frequency PCS CDMA applications using the LMX2522, if the GPS frequency is 1355.04 MHz, Table 1 provides the proper register settings: TABLE 1. Settings for GPS (1355.04 MHz) in LMX2522 PCS CDMA application Reference Frequency RF_B RF_A RF_FN 19.20 MHz 11 4 1104 19.68 MHz 11 2 1680 Cellular CDMA applications using the LMX2532, in which the GPS frequency is 1490.04 MHz, then Table 2 provides the proper register settings: TABLE 2. Settings for GPS (1490.04 MHz) in LMX2532 Cellular CDMA application Reference Frequency RF_B RF_A RF_FN 19.20 MHz 9 5 1164 19.68 MHz 9 3 1404 Cellular CDMA applications using the LMX2532, in which the GPS frequency is 1391.82 MHz, then Table 3 provides the proper register settings: TABLE 3. Settings for GPS (1391.82 MHz) in LMX2532 Cellular CDMA application Reference Frequency RF_B RF_A RF_FN 19.20 MHz 9 0 942 19.68 MHz 8 6 1422 IF-PLL SECTION f VCO = {16 x IF_B + IF_A} x fOSC / IF_R where (IF_A < IF_B) where f VCO: Output frequency of the voltage controlled oscillator (VCO) IF_B: Preset divide ratio of the binary 9-bit programmable counter (1 ≤ IF_B ≤ 511) IF_A: Preset divide ratio of the binary 4-bit swallow counter (0 ≤ IF_A ≤ 15) f OSC: Reference oscillator frequency IF_R: Preset divide ratio of the binary 9-bit programmable reference counter (2 ≤ IF_R ≤ 511) From the above equation, the LMX2522/32 generates the fixed IF frequencies as summarized in Table 4. www.national.com 7 |
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