Capacitive coupling over the human body - designing a receiver - Page 1
Forum section: electronics / rf-microwave

author: mhmart
author_rank: Newbie
timestamp: February 28, 2023, 02:19:15 pm
content: HiI am trying to communicate over the human body using battery powered devices.On both wrists, I wear an elastic band with a copper strip attached to it, squeezed to my body and an pin to it so I can hook up probes and stuff- Yes, it looks funny.- No, I have not yet electrocuted myself... (well technically I might have but I have not hurt myself)To do so, I have an ESP32 powered by batteries on the one side, sending out (for now just to test) a PWM signal of 2 MHz.On the other side I did hook up the probe (check the cool image). I Indeed get a (small) 2 MHz signal. (Probe was in 10x attenuation mode so Vpp is about 150mVNow. I would like to make a circuit that can get those pulses and amplify it for another ESP to read.What would be a good circuit battery powered that could do this?The thing to keep in mind is that there will not be a shared ground.I tried a few things and one of the circuits is in here. But honestly, after trying for a while, I no longer have a clue of anything I'm doing and what I should be doing.Somehow I think I can also approach this receiver as an AM receiver with the antenna just being the human body, right?Could one maybe help me with some tactics to keep in mind or comment my circuit?I would like to give you an argument of why such a circuit should, or should not, work - but - I don't know whyThe configuration with 3 opamps does worst.The configuration with a simple opamp does allright but It amplifiers about 23MHz for some reason so I cannot do much of amplification there. What could I try to do to filter? Adding any series resistors (LPF With resistor in series and cap in parallel) will cause the signal to drop on the input of the opamp.What else could I add or try in the receiver to make it work?Also, I did have a suggestion of adding an bias to the input of the opamp config but doing so removes the signal all together for some reason. Suggestions on that?

author: profdc9
author_rank: Frequent Contributor
timestamp: March 02, 2023, 04:01:32 am
content: The device is trying to design a low powered, near-field radio transmitter and receiver, using the capacitive coupling or surface waves of a body of salt water.It's not clear to me what kind of communications bandwidth or data rate is intended here. This would very much affect the approach one would use and how complicated it would be.The easiest modulation I think to use would be frequency shift keying. You could output one of two frequencies, say 50 kHz and 100 kHz, for example, to indicate a zero or one bit.The ESP32 is probably fast enough that you could directly implement a demodulator in software. You could sample at 400 kHz, and then use quadrature demodulation to get I and Q signals. The demodulation sequence for 100 kHz would be 1,1,-1,-1 for I and 1,-1,-1,1 for Q, and for 50 kHz it would be 1,1,1,1-1,-1,-1,-1 for I and 1,1,-1,-1,-1,-1,1,1 for Q.You could then look at the sum of the absolute values of I and Q to figure out which of the tones is present.This method does not require any multiplication, only addition and subtractions of integers.