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Key issue for Low IF RF Transceiver Design --原创  

2007-05-10 21:48:16|  分类: 无线通信 |  标签: |举报 |字号 订阅

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1.) Specification design:

TX: PA power and power rise/fall, spectrum PSD, spurious, carrierfrequency tolerance, symbol frequency tolerance, constrllation error, EVM ... ...

RX: sensitivity, max. input level, jamming resistance, chain gain, IP3/IP2, nf ... ...

for example: to get nf, nf=C/I-Pin-Pnoisefloor+margin

chain simulation needed to get all spec trade-off, it's very important step.

2.) How to select IF frequency?

The criteria is to minimize in band noise power, the in band noise power

Pn(fi)=Int(Nq(f)+Nt(f))df=Nw(2fb+fc*ln((fi+fb)/(fi-fb)))+4Erms^2*(2fb/fs-sin(2pi*fb/fs)/pi*cos(2pifi/fs))

(fi: IF frequency, fb: BW, fs: sampling frequency, Nw: write noise power spectral density, Erms^2=delta^2/12)

To solve dPn(fi)/d(fi), that's get the IF frequency you want.

Based on the required att. at 2nd order, the filter order get by:log(att2nd-1)/2*log(fir/Bp)

3.) irr: image rejection is important step for IF receiver, iir caused by impedance mismatch or impefect hybrid directivity or low vswr...

The spectrum view of iir: S=I+jQ+delta(I-jQ), if you get counterpart of delta(I+jQ), the image rejected.

The general methodology is to adopt irr filter to make signal notch. The advanced one is to integrate into delta-sigma ADC:

iir=-10log((1+amp^2-2amp*cos(theta))/(1+amp^2+2amp*cos(theta))/))-->

iir=abs((aa(k+l)+jbb(h-l))/(aa(1+k*l)+jbb(1-h*l))), to construct architecture based on the coefficient, that's get the solution, the low pass delta-sigma take advantage of flick avoiding and 3dB improvement achived.

The above three represents one of the key issue in low IF design, more issue includes pll/phase noise, PA/efficiency, dc-offset, agc/lpf etc., for 2G band(11b/15.4/rfid etc.) the low IF is robust for implementation and got verified by writer, for more information would be write in series for publishing.

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