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CLC5903 Datasheet(PDF) 16 Page - National Semiconductor (TI) |
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CLC5903 Datasheet(HTML) 16 Page - National Semiconductor (TI) |
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16 / 29 page  www.national.com 16 response through to the second filter. The latter can then be programmed as a Nyquist (typically a root-raised-cosine) fil- ter for matched filtering of digital data. The complete channel filter response for standard coeffi- cients is shown in Figure 25. Passband flatness is shown in Figure 26. The complete filter response for GSM coefficients is shown in Figure 27. GSM Passband flatness is shown in Figure 28. The mask shown in Figure 27 is derived from the ETSI GSM 5.05 specifications for a normal Basestation Transceiver (BTS). For interferers, 9dB was added to the carrier to inter- ference (C/I) ratios. For blockers, 9dB was added to the dif- ference between the blocker level and 3dB above the reference sensitivity level. Channel Bandwidth vs. Sample Rate When the CLC5903 is used for GSM systems, a bandwidth of about 200kHz is desired. With a sample rate of 52MHz, the total decimation of 192 provides the desired 270.833kHz output sample rate. This output sample rate in combination with the FIR filter coefficients create the desired channel bandwidth. If the sample rate is increased to 65MHz, the decimation must also be increased to 65MHz/270.833kHz or 240. This new decimation rate will maintain the same output bandwidth. At 78MHz, the decimation must increase again to 78MHz/270.833kHz or 288. The output bandwidth may only be changed in relation to the output sample rate by creating a new set of FIR filter coefficients. As the filter bandwidth Figure 23. F2 STD frequency response 0 0.1 0.2 0.3 0.4 0.5 −100 −90 −80 −70 −60 −50 −40 −30 −20 −10 0 Frequency Response of F2 Using STD Set Frequency Normalized To Filter Input Sample Rate Figure 24. F2 GSM frequency response 0 0.1 0.2 0.3 0.4 0.5 −50 −45 −40 −35 −30 −25 −20 −15 −10 −5 0 5 Frequency Response of F2 Using GSM Set Frequency Normalized To Filter Input Sample Rate Figure 25. CIC, F1, & F2 STD frequency response 0 500 1000 1500 2000 2500 3000 −150 −100 −50 0 Combined Frequency Response of CIC/F1/F2 Using STD Set Frequency (KHz) FCK = 52MHz Decimation = 192 OFS = 270.83kHz Figure 26. CIC, F1, & F2 STD Passband Flatness 0 50 100 150 200 250 −0.1 −0.08 −0.06 −0.04 −0.02 0 0.02 0.04 0.06 0.08 0.1 Combined Frequency Response of CIC/F1/F2 Using STD Set Frequency (KHz) Figure 27. CIC, F1, & F2 GSM frequency response 0 500 1000 1500 2000 2500 3000 −100 −80 −60 −40 −20 0 Combined Frequency Response of CIC/F1/F2 Using GSM Set Frequency (KHz) FCK = 52MHz Decimation = 192 OFS = 270.83kHz Detailed Description (Continued) |
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