Need help with communication through a resonant inductive coil

[Plecto]

Hello I'm making an inductive power transfer system that is in the need of a way of communicating through the inductive link. My first thought was to modulate the current draw on the receiving side and modulating the voltage on the transmitting side, but it then became apparent that we need a way higher bit rate than what can be achieved with this method. My thought is to add a high frequency modulated signal on top of the power transfer with a modulator and demodulator in each end. I've never worked with radio or modulation techniques of this kind before although I do remember the principles from university. The baud rate requirement is 115200 baud/s minimum although 1 Mbaud/s is desirable. I was able to easily transfer 10Mhz through the resonant coils, but it started to get attenuated at 15Mhz (the highest my function generator can go). I'm not looking to delve into and get a depth understanding of all of the different modulation schemes, I'm rather hoping that someone could help me and point me in the right direction of what kind of IC's that accomplish this? What kind of modulation scheme would be appropriate for this kind of application?

 

[betwixt]

Tell us what kind of power levels you are transferring through the link and at what frequency it is at. The signal presently going through the coil will influence our advice on how it, or some other signal, can be modulated. If you need high speed it may be better to use a second coil with lower inductance and a carrier frequency well away from the power link.

Brian.

 

[Plecto]

Thanks for the reply The power would be about 60W or so. Resonant frequency about 170khz.

With my limited experience with this in mind, I was thinking of some 'finished' solution. An IC with SPI input, some modulated waveform out the other and the same on the receiving side, leave it to me to get the frequencies from one end to the other. Am I over-simplifying things? I'm thinking that the ways of transferring the signal through the coil is an issue for later as we might use this for other applications as well.

 

[betwixt]

You certainly are over simplifying things. Modulating the power signal cannot give you the data speeds you need. The absolute minimum "only works under ideal conditions" power frequency would have to be twice the data frequency so you need at least 250KHz for the lowest data rate and more than 2MHz for the highest. For safety you would have to use considerably higher frequencies.

I would seriously consider using a completely different data link co-located with the power transfer. You haven't said whether the data is bi-directional or not but for short distances and serial in to serial out, I think I would consider an ESP8266 IC and link by Wi-Fi. They cost almost nothing and with one at each end you can transfer close to 1Mb/s over reasonable distances. Configured as Telnet server/client they would look like a wired connection from a data point of view. They work in the 2.4GHz band so interference from the power coil should be minimal.

There are many other wireless link devices on the market but watch out for their highest serial rates.

Brian.

 

[Plecto]

I'm just looking for ways of modulating and demodulating the digital information. Lets say I don't have an inductive link, but rather just a piece of copper as the transmission line. How do I convert the 1s and 0s from a MCU to a modulated DPSK, QPSK or GMSK waveform within the frequency range of 5-20Mhz or there about?