I was going through the documentation of TPMS sensor FXTH87 and I am trying to understand better how the low frequency receiver works. Here is a scheeme of it :
I understand that the incomming signal is modulated, and that we have to get rid of the carrier and so to get just the envelope, we use a rectifier. But why are they 4 rectifiers?
As I understand it, the incoming signal goes trough Amp1 and then there is a buffer (why? to keep the signal intact?), then this signal from one side is rectified and on the other side it goes into a second amplifier (Amp2) and then a second buffer (why?),...
The only way I can understand it is that the signal is rectified 3 different times and summed and averaged to minimize the error. This seems weird since the signal rectified by the second rectifier seems to be signal*Amp1*Amp2 and the signal rectified by the 3rd rectifier seems to be signal*Amp1*Amp2*Amp3.
Also why is the signal clamped at the beginning? Is it this operation that they mean(see image below)?
As for what is within the yellow box I am quite lost. In the data-sheet it says you can select 2 threshold for the detection of the carrier: a high sensitivity and a low sensitivity. It seems for the first sensitivity, the incoming signal in the yellow box will be the one after buff1 and for the other sensitivity, it will be the signal after the buff2. I guess if we select the low sensitivity, they will amplify the signal just once and take the output right at buff1. If we select high sensitivity, it means we want to be able to detect weaker signals so we amplify the signal once more before going into the carrier detector, is that the idea? What is the small arrow that looks like a current source?
Also, why is the entry of the Logic Block 2 is 129KHz? If something, it should be 125KHz in my opinion because this is the frequency of the carrier. Why is this frequency needed to decode the data? Are we somehow mixing the 2 frequencies?
One last thing : 125KHz is a low frequency, it has a wave-length of 2.4km, so as I understand it, we are not transmitting electro-magnetic wave here but we are just modulating the magnetic field. Is that correct? The receiver has some tune circuit that resonate with the magnetic field produced by the tranceiver and this is how we get the data?
Thanks for your help and please bear with me, I don't know much about electronics, I just like to learn it.