Who knows how XR2211 works?

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?

A simple google provides this in the first hit:

"The XR2211 is a monolithic phase-locked loop (PLL) system"

Which is what you'd expect of a tone decoder.

no, it is not a PLL, it is a chip like LM567, it has a VCO, and 2 mixers, if you want to build a PLL, you have to add a phase detector and a loop filter as well as frequency divider if necessary. In its datasheet, there is a tone decoder application schematic and it really works. what I would like to know here is the principle for the application, the problem is posted above.

Jerry_yuan said:

no, it is not a PLL, it is a chip like LM567, it has a VCO, and 2 mixers, if you want to build a PLL, you have to add a phase detector and a loop filter as well as frequency divider if necessary. In its datasheet, there is a tone decoder application schematic and it really works. what I would like to know here is the principle for the application, the problem is posted above.

Click to expand...

You mention the datasheet. Here's the first sentence in the General Description:

The XR-2211A is a monolithic phase-locked loop (PLL)
system especially designed for data communications
applications.

Click to expand...

The datasheet has a pretty good description of how it works.

I must appologize for not noticing its general discription in its datasheet, but how can it be a PLL since there is no phase-comparator on the chip , ? as far as I know, PLL should include a phase-comparator in the loop(usually a PFD or XOR gate).

Jerry_yuan said:

no, it is not a PLL, it is a chip like LM567, it has a VCO, and 2 mixers, if you want to build a PLL, you have to add a phase detector and a loop filter as well as frequency divider if necessary. In its datasheet, there is a tone decoder application schematic and it really works. what I would like to know here is the principle for the application, the problem is posted above.

Click to expand...

The LM567 is a PLL as well, you need to study the different types of phase detectors!.

Jerry_yuan said:

I must appologize for not noticing its general discription in its datasheet, but how can it be a PLL since there is no phase-comparator on the chip , ? as far as I know, PLL should include a phase-comparator in the loop(usually a PFD or XOR gate).

Click to expand...

Look at Figure 1 in the datasheet. The block called Loop Θ-Det is the phase detector for the PLL. See also P.5:

The main PLL within the XR-2211A is constructed from an input preamplifier, analog multiplier used as a phase detector and a precision voltage controlled oscillator (VCO).

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Do you have a mental block when it comes to reading the datasheet?

now It is clear, thanks guys!