near-field magnetic induction communication

[Daniel Kalny]

Hello everybody,

I’d like to ask you who are more experienced with ferrite antennas and LF transmissions for a help with the following problem. I have a circuit for near-field magnetic inductive communication consisting of a microcontroller generating a square-wave signal (carrier, 131 kHz). An amplitude modulation (on/off keying) is used for the communication. A schematic is given in the figure 1 (schematic_transceiver.png).

The signal is fed into a non-inverting op amp U1A. The output from the op amp drives a series LC circuit, the inductor L1 being a ferrite antenna. Receiver is similar in that it is a parallel LC circuit whose output is fed to another op amp U1B. Both op amps have an asymetric power supply (Vdd = 1.8V and GND).
The signal transmitted is a pulse of 128 cycles of the carrier frequency. A received signal is shown in the figure 2 (signal.png).

The horizontal axis correspond to samples taken by an ADC. The vertical one is an arbitrary number corresponding to voltage (the higher the number on the axis the lower the voltage; GND = 5450, Vdd = 3500). The signal is rectified by the second op amp, therefore it is a half sine wave. I would expect to see an envelope of the received signal similar to the transmitted one (a square pulse) except for an exponential attack and decay of the envelope. However, as can be seen in the figure, there are other oscillations at roughly 1/12 of the carrier frequency, mainly at the beginning of the pulse. Could anybody provide an explanation of the fenomena and/or suggest what should I do to remove/decrease these dramatic changes in the amplitude of the envelope? Any suggestion is welcome.

Thank you.

 

[moty22]

Try 100pF across the 10M resistor.

 

[RadioRon]

I get the impression that you expect those op amps to operate as linear amplifiers. In that case, your DC bias is wrong. When operating from an asymmetric DC supply of 1.8V and ground, it is necessary to bias the feedback network to mid-VCC so that the output voltage centers at that same voltage and so that the output swings around that center voltage. To fix this, disconnect the bottom of R5 from ground and connect it to 1/2 of VCC (or 0.9V in your case). Do the same for R3.

 

[blueroomelectronics]

What sort of range are you expecting?

 

[Daniel Kalny]

To moty22: I think that would suppress higher frequencies, not the low ones.
To RadioRon: Yes, you're right. The op amp is intended for asymetric power supply. Actually, if I did what you suggest I would get a full signal from the receiving op amp, not the rectified one. However, would it have an impact on the "slow" oscillations? I don't see how that would change.
To blueroomelectronics: Do you mean communication range? This is a near field comm. Currently I can reach a few centimeters. From literature I know the achievable range is in the order of several decimeters or meters at most.

 

[MikeMl]

You are delving into Peeps, the system for finding buried skiers. You will find that the relative orientation of the transmitting and receiving coils makes a huge difference.

 

[RadioRon]

Without really delving too deeply into it, I'm wondering if the transient behavior that you refer to as the slow oscillations is simply either the transmitter or receiver op amp suffering a shift in bias point as a result of the presence of the rectified signal, which carries with it a different average dc voltage than was present the moment before the transmission occurred. In other words, the DC operating point of the op amp is shifting during the pulse, but with a relatively slow higher order response (ie. underdamped).

If so, one way to deal with this might be to reduce the values of your resistors by three orders of magnitude to reduce the time constants of the circuit.

 

[Daniel Kalny]

Hi RadioRon, that makes sense to me. I'll definitely try to vary the resistor values. Thanks.

 

[MrAl]

Hi,

Near field is interesting. The drawback, as you probably know, is that the signal strength drops off fast with distance. If you want to calculate, you dont have to calculate the whole field really, just line of site to get an idea what is happening. I had posted some info on Biot Savart right here on ETO some time ago with a full explanation of the technique to calculate the field at a point in space at any distance. I didnt get a chance to post the techniques for a circular ring or ellipse yet, but i could probably just show the result, and maybe develop the equation for the helix too (coil of wire, with or without core). That would be interesting i think.

Are you having trouble deciding what transmit frequency to use?

 

[Daniel Kalny]

On the other hand an advantage is it's not attenuated significantly by metals, water or human body. I think it's a good medium for body area network. Being able to reach within one meter is sufficient. I'l have a look at your article and definitely yes, it would be interesting to see calculation of the field of a coil.
As for the frequency, I have chosen 131 kHz, the same as IEEE 1902.1 protocol uses. I don't plan to transfer huge amount of data. The intention is to be able to send few bytes from time to time; the point is to use as little energy as possible.