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发射电路、时钟、电源.ppt

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发射电路、时钟、电源.ppt
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发射电路设计、时钟、电源,郑振耀,内容,Digital-To-Analog Converter Band-Pass Filters Frequency Up-Converter RF Attenuator and Gate Tune Mode,Digital-To-Analog Converter,U8 (DAC5687) is an ultra-fast dual-channel 16-bit DAC converting digital signal to RF signal. Two input channels get real-time digital signals from FPGA involving 4-phase selection, amplitude modulation, phase modulation, and frequency jumping. The digital input updating rate is 80 MHz. Inside the DAC, the digital signal is processed in 160 MHz. The process includes interpolation, frequency shift, digital filter, quadrature modulator correction, gain adjustment, and unbalanced amplitude correction. All parameters are configurable by software through its serial channel. The DAC output updating rate is 160 MHz. The central IF frequency from two DAC’s outputs is 20 MHz. Two channels are working quadraturely.,,,Band-Pass Filters,Two RF transformers T1 and T2 convert the current outputs to voltage. The two channel signals are filtered by two 3-pole symmetric band-pass filters to remove the harmonics and noise. The input and output impedance is 50Ω to match the DAC output and the frequency modulator input.,,,Frequency Up-Converter,U9 (AD8345) is a quadrature frequency modulator. The operating frequency is from 140 MHz to 1000 MHz. It mixes the external frequency fLO with IF frequency fIF (20MHz) and generates the final transmitter frequency fTX.fTX = fLO - fIF. The quadrature modulator has single sideband output. The other sideband (fLO + fIF) is eliminated by choosing correct input phases. The external frequency source (PTS, for example) has to be set tofLO = fTX + fIF = fTX + 20 MHz.,,,,The input level to the modulator is important to guarantee the minimum harmonics in its output. Therefore the external frequency source should keep its output level within a range of +7dB (+/-2dB). T3 increases the common mode rejection, and D1 and D2 are for the input limiter. U3 is a broadband RF amplifier with 12dB gain. C413, C414, C415
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