Jerry_yuan
New Member
XR2211 is an FSK Demodulator/Tone Decoder chip. I have made a demudulater as described in its datasheet, but i am not quite clear how it worked. according to its circuit, it simply contains 2 mixers, and how can a mixer output an DC level after low passing? I mean if there is an input signal with a frequency f1, and the LO is also f1, the mixer will output an cos(phi) after low passing, the phi is the phase difference between f1 and f2, theoretically, if f1 and f2 are quardrature(90 deg. phase difference), the output is 0, only if they have little phase difference could the mixer output an recognizable DC voltage, so how can the mixer output an stable DC level as an indicator of the tone decoding?