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TH71101 Datasheet(PDF) 4 Page - List of Unclassifed Manufacturers |
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4 / 20 page TH71101 315/433MHz FSK/FM/ASK Receiver TH71101 Data Sheet Page 4 of 20 Nov. 2001 3901071101 Rev. 005 Frequency Planning Frequency planning is straightforward for single-conversion applications because there is only one IF that might be chosen, and then the only possible choice is low-side or high-side injection of the LO signal (which is now the one and only LO signal in the receiver). The receiver’s single-conversion architecture requires careful frequency planning. Besides the desired RF input signal, there are a number of spurious signals that may cause an undesired response at the output. Among them is the image of the RF signal that must be suppressed by the RF front-end filter. By using the internal PLL synthesizer of the TH71101 with the fixed feedback divider ratio of N = 16 (DIV_16), two types of down-conversion are possible: low-side injection of LO and high-side injection of LO. The fol- lowing table summarizes some equations that are useful to calculate the crystal reference frequency (REF), the IF and the LO frequency respectively, for a given RF. Injection type low high REF (RF – IF)/16 (RF + IF)/16 LO 16 •REF 16 •REF IF RF – LO LO – RF RF image RF – 2IF RF + 2IF The following table depicts crystal, LO and image signals considering the examples of 315 MHz and 433.6 MHz RF reception at IF = 10.7 MHz. Signal type RF = 315 MHz RF = 315 MHz RF = 433.6 MHz RF = 433.6 MHz Injection type low high low high REF / MHz 19.01875 20.35625 26.43125 27.76875 LO / MHz 304.3 325.7 422.9 444.3 RF image / MHz 293.6 336.4 412.2 455.0 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 300 MHz to 430 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 315 MHz and 433.6 MHz, respectively. |
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