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EVB71111 Datasheet(PDF) 5 Page - Melexis Microelectronic Systems |
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EVB71111 Datasheet(HTML) 5 Page - Melexis Microelectronic Systems |
5 / 14 page EVB71111 868/915MHz Receiver Evaluation Board Description 39012 71111 01 Page 5 of 14 EVB Description Rev. 011 June/07 By using the internal PLL synthesizer of the TH71111 with the fixed feedback divider ratio of N = 16 (DIV_32), two types of down-conversion are possible: low-side injection of LO and high-side injection of LO. The following table summarizes some equations that are useful to calculate the crystal reference frequency (REF) and the LO frequency, for a given RF and IF. Injection type low high REF (RF – IF)/32 (RF + IF)/32 LO 32 • REF 32 • REF IF RF – LO LO – RF RF image RF – 2IF RF + 2IF 1.6.1 Selected Frequency Plans The following table depicts crystal, LO and image signals considering the examples of 868.3 MHz and 915 MHz RF reception at IF = 10.7 MHz. Signal type RF = 868.3 MHz RF = 868.3 MHz RF = 915 MHz RF = 915 MHz Injection type low high low high REF / MHz 26.80000 27.46875 28.25938 28.92813 LO / MHz 857.6 879.0 904.3 925.7 RF image / MHz 846.9 889.7 893.6 936.4 The selection of the reference crystal frequency is based on some assumptions. As for example: the image frequency should not be in a radio band where strong interfering signals might occur (because they could represent parasitic receiving signals), the LO signal should be in the range of 800 MHz to 930 MHz (because this is the optimum frequency range of the VCO1). Furthermore the IF should be as high as possible to achieve highest RF image rejection. The columns in bold depict the selected frequency plans to receive at 868.3 MHz and 915 MHz, respectively. 1.6.2 Maximum Frequency Coverage Parameter fmin fmax Injection type high low RF / MHz 789.3 940.7 REF / MHz 25.0 29.0625 LO / MHz 800 930 IF/ MHz 10.7 10.7 |
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