I'm planning to retrieve the AM modulated sign from Vr.
When I connect an oscilloscope I see the 134.2 kHz carrier signal, but not the modulating signal.
As a side note, I'm not sure what frequency the modulating signal is. According to the company that builds the rfid microchip used on the rfid reader, they have filtering pole frequencies of 16Hz and 1.5 kHz. I'm assuming high pass and low pass, but I'm not entirely sure. Will I have to build a filter before viewing the modulating signal on a scope?
Any other suggestions/information I should provide?
The way RFIDs work is that the receiver chip+antenna sits in the near-field
of the transmitting loop and switchs on and off a short circuit across the
receiver's antenna/tuned circuit at a particular data-rate. This then cancels
(or not) a tiny fraction of the near field's amplitude (fractions of 1%).
The transmitting chip is constructed to amplify changes in the Q of the transmitting
antenna/tuned circuit (the same thing as changes in the voltage amplitude), at the
data-rate frequency (filtering out both low frequencies and the carrier frequency).
Wthout that filtering the tiny changes are impossible to see by eye!
The data rate might be a few kHz, the carrier 150kHz (or many MHz), and
low frequency interference from the mains is in the 100's of Hz range (and
pre-filtered by the tuned circuit).
Correct me if I have interpreted what you said incorrectly, but all I should need to do is filter out the carrier signal (low pass at 60 kHz). Other interference in the hz range is already filtered out.
I don't understand what you are trying to achieve - you really need an RFID chip or
module to talk to an RFID tag, because it has all the right signal processing built-in.
The chip I looked at used an external diode/cap to rectify the signal, and internal
filters and amp to make it large enough to output as a logic signal. Sensitivity of
10mV. U2270B is the chip I've used.
How would one phase modulate without using significant power?
The normal modulation is by shorting/opening the antenna tuned circuit
on the RFID chip at the baudrate, which has maximal effect on the near
field for minimum power consumption as its purely passive. The effect
is still <0.1% or suchlike, but that's enough to detect riding on the 100V
ac of the transmitter LC tank circuit.
Think of a metal-detector encountering a small piece of strange metal that
oscillated between conductive and insulating at 1200 baud to convey a
message - that's basically the same technology.