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EVB71122C-915-C Datasheet(PDF) 7 Page - Melexis Microelectronic Systems |
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EVB71122C-915-C Datasheet(HTML) 7 Page - Melexis Microelectronic Systems |
7 / 32 page PR EL IM INA RY EVB71122 300 to 930MHz Receiver Evaluation Board Description 39012 71122 01 Page 7 of 32 EVB Description Rev. 001 Sept/06 2 Functional Description 2.1 Frequency Planning Because of the double conversion architecture that employs two mixers and two IF signals, there are four different combinations for injecting the LO1 and LO2 signals: • LO1 high side and LO2 high side: receiving at fRF(high-high) • LO1 high side and LO2 low side: receiving at fRF(high-low) • LO1 low side and LO2 high side: receiving at fRF(low-high) • LO1 low side and LO2 low side: receiving at fRF(low-low) As a result, four different radio frequencies (RFs) could yield one and the same second IF (IF2). Fig. 2 shows this for the case of receiving at fRF(high-high). In the example of Fig. 2, the image signals at fRF(low- high) and fRF(low-low) are suppressed by the bandpass characteristic provided by the RF front-end. The bandpass shape can be achieved either with a SAW filter (featuring just a couple of MHz bandwidth), or by the tank circuits at the LNA input and output (this typically yields 30 to 60MHz bandwidth). In any case, the high value of the first IF (IF1) helps to suppress the image signals at fRF(low-high) and fRF(low-low). The two remaining signals at IF1 resulting from fRF(high-high) and fRF(high-low) are entering the second mixer MIX2. This mixer features image rejection with so-called single-sideband (SSB) selection. This means either the upper or lower sideband of IF1 can be selected. In the example of Fig. 2, LO2 high-side injection has been chosen to select the IF2 signal resulting from fRF(high-high). Fig. 2: The four receiving frequencies in a double conversion superhet receiver It can be seen from the block diagram of Fig. 1 that there is a fixed relationship between the LO1 signal fre- quency fLO1 and the LO2 signal frequency fLO2. LO2 LO1 LO2 f f N LO2DIV = = (1) The LO1 signal frequency fLO1 is directly synthesized from the crystal reference oscillator frequency fRO by means of an integer-N PLL synthesizer. The PLL consists of a dual-modulus prescaler (P/P+1), a program counter N and a swallow counter A. tot PFD PFD RO LO1 N f A) P (N f A) P (N R f f ⋅ = + ⋅ = + ⋅ = (2) f LO2 f LO2 f LO1 f RF f RF f RF f RF |
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Similar Description - EVB71122C-915-C |
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