why the CDMA receiver BB filter bandwidth is 615KHz

[helen.liu]

cdma baseband filter bandwidth

Hi,

In the CDMA receiver, the input signal at antenna port is 1.23MHz, in case of use ZIF architecture, when downconverted the input signal, why the BB filter is 615kHz, I don't know where does the other half band signal power? and why the bandwidth of 1.23MHz signal changed to 615kHz? Does this influence the quality of the signal?

look forward your replay, thanks.

 

[rmf_rncperf]

Remember the baseband consists of both + and - frequencies. When the baseband signal is multiplied by a carrier the entire baseband signal is shifted to the passband. That's why the "actual" passband is 2x the "actual(+ frequencies, ie: what you would see on a spectrum analyzer)" basband.

You can look at it this way. Use the double angle formula. Sat Wb is the upper end of the BB signal (~615Khz) make Wc the carrier signal Wc>>>>>Wb, than
Cos(Wbt)*Cos(Wct) = the passband signal = Cos((Wc-Wb)t)+Cos((Wc+Wb)t). Note the passband signal is now 2x the bandwidth of the BB.

Hope this helps