I'm currently designing a transmitter for NFMI to transmit an audio signal to the built in telecoil of a hearing aid. As I understand it my antenna calculations are correct and I can achieve an induction of 2.09uH with 6 turns of coil around my ferrite core. However, I don't know what the rest of the circuit includes. I know that NFC and NFMI are very similar, are their circuits the same? Do I need an NFMI chip to build a transmitter? Does anybody have a circuit diagram I could use?

 

NXH2280

Seems it is fairly patented and made by only one OEM.

The only entries I found showed out of stock and one had a 770 day factory backorder wait list.

 

I suspect the TS is thinking of a simple induction loop system that just uses a loop connected to an audio amplifier. I think all that he needs is an audio amplifier connected to a coil via a resistor. The resistor is just to swamp the inductance of the coil so the received signal is not attenuated at higher frequencies. The coil would just need to be placed close to the hearing aid. (Some telephones have this built into the ear piece.)

Les

 

Per Les, is this what you are considering?
Build you own T-Loop system

 

Hi Dick,
That is how it is done to cover a large area. I am assuming that the TS wants to couple a signal from a coil placed close to the hearing aid. He does not make it clear in his question if this is the type of system he wants or if it is a system where the audio is modulated onto a carrier at some frequency. I don't understand how he is doing the inductance calculation for the coil. 2.09 uH seems to be a very low inductance if it is just straight audio that he wants to transfer. We will have to wait for him to give some more details of what he is trying to do.

Les.

 

What is the resonant frequency of the telecoil?

Is the telecoil, the same thing as the 6 turns coil with ferrite core?

Do you have a hearing aid or are you building one?

Does the receiving circuit have to be contained in the ear bud....................or will a corded earphone, with a circuit board the size of a credit card be ok?

 

Appologies for the delayed response and thanks for getting back to me. I'm not building the hearing aid I've bought one with a built in telecoil, so I don't have to worry about the receiver. Unfortunately there isn't a specification for the telecoil. The protocol for NFMI states that my system must have an operating frequency of 13.56MHz and a resonance less than 35MHz. The transmitter will only have to send the signal half a metre to the hearing aid. As I've found little information about NFMI online I used NFC as a guideline for the inductance and have read that an inductance between 300nH and 3uH is required. Would a higher inductance value be required for audio transfer?

 

The small amount of materials I looked at did talk of one circuit with single digit microhenry inductance value. Don't be to quick to rule that out.

 

Pardon my confusion. Do you want to use straight magnetic induction, or do you want a carrier?

Does the position of the transmitter or receiver ever change?

 

Don't worry about it, I'm new to this. I think I want a carrier. The transmitter and receiver might move but it'll be proportionally. They'll always be the same distance and angle apart.

 

I don't think I can offer much more without knowing EXACTLY what you are doing.

One must know the complete application to decide which principle to use.