Building a radio to recieve a ~20khz audio signal

I am trying to build a radio to receive a 200 hz - 20 kHz audio signal. The signal frequency is the most important thing for this project.

Can anyone give me a starting point as to what specific parts I would need (breadboard, operational amplifier, capacitors, transistors, resistors, antennae, etc..) for this frequency range?

So, do you mean that you want to receive an RF signal that is modulated with a 200 hz – 20 kHz audio signal [because you can't receive audio energy with an RF receiver]?

If so, what type of modulation? What carrier frequency?

If not, then do you mean receive an RF signal in the 200 hz – 20 kHz range?

If so, what kind of signal? Continuous Wave [CW], modulated? Or is that irrelevant?

As Steve said are you transmitting these frequencies via a high frequency carrier - or just want to receive an audio signal - which in that case just build an audio amplifier with a mic input. If you are then I would look for circuits that can do AFSK - at least that way you would cut down on sound wave interference - somewhat

Hi Alex,
It would help if you can give more details of the nature of your project. When we can see what you are trying to accomplish, we can give more detailed advice.

When you say, "The signal frequency is the most important thing for this project.", do you mean the audio signal or the radio signal ?

Are you trying to build a high fidelity radio receiver ? This would usually go down to about 20 Hz. And 20 kHz is a frequency that very few adults can hear.

Your starting point is to clearly define the functions you require the radio to perform, such as to be receiving speech or music or digital data, The second is to obtain a circuit diagram, then you will have a list of parts to buy.

Some background -

I have a cochlear implant. It is a transmitter and receiver. It transmits the external audio signal from my surroundings and I receive audio from my medical team. The reception signal falls between the 20 hz - 20000 hz range, because this is the audible range of the cochlea.

http://bit.ly/1952vmo

So essentially what I am trying to do is broadcast this signal over an external speaker. I do not wish to transmit any signal, just receive the audio signal from this range of frequencies.

So, how does the audio get from the implant, to whatever the "medical team" is using to receive it?

And/or what do you mean by "receive audio from my medical team"? Receive it how?

This may be useful to help everyone in this discussion :

http://en.wikipedia.org/wiki/Cochlear_implant#Parts_of_the_cochlear_implant

Alex, does your implant operate something like the one shown in Wikipedia ?

If it does, I think what is needed is something like a loop antenna over your ear to pick up the signal from the external part (the signal that goes to the inner implant), and amplify it, possibly broadcasting it by radio like bluetooth to another receiver, so that it can be heard by other people.

Have I got the basic idea of what you want ? Please correct any misunderstanding I may have.

Essentially, it does not work on Bluetooth though. It works by transmitting and receiving *longwave* radio frequencies, somewhere in the 20 - 20000 hz range

(the medical team is located in Brazil, so shortwave frequencies like Bluetooth wouldn't transmit far enough. Also, my implant is wireless, differing from the wiki page. But the idea is the same).

So I was hoping to just intercept, for lack of a better word, the radio signal and broadcast it over a speaker on my makeshift radio, which would be receiving the same frequency signal as my implant.

I know its complicated, but any help would be appreciated.

Thanks Alex, here is a diagram of how I understand the situation.
Please correct any mistakes.

P.S. You say your medical team is in Brazil, so where are you ?

Hi Alex.
The other coments are important for us to know, but (assuming HF carrier), you will need a band pass filter, and you can do this at 455kHZ, with a few standard AM IF cans from transistor radios. I have built such a receiver for Hi Fi AM (10 kHz band width). If you can use SSB, then a filter of 20 kHZ bandwidth is easy, but you may need to go somewhat higher in frequency for an AM signal as you wil need a filter of 40 kHz bandwidth. I will watch this space for your comments.

For an implant it would make more sense to use a magnetic coupling rather than rf to avoid picking up radio stations all day. In that case you just need a coil on an amplifier to do the job. For the receiving end same thing. A coil on a microphone input.

Thanks to everyone for their help. Here is where I am right now.

This is the radio I decided to build. http://www.stormwise.com/project1.htm.

It is capable of receiving ULF frequencies. I've built and mounted the circuit. But I am not sure what the next step is, or if this circuit will even serve my purpose.

1.) What are the input and output used for? Are they just grounds, or do I need some input to be able to receive ULF waves?

2.) Where do I connect the antenna - and how do I build a ULF antenna?

3.) How do I tune into specific frequencies - can I add a tuning mechanism to the circuit?


Thanks!

And to the curious, I am located outside of Philadelphia. The waves are not AM or FM, they are ULF waves which can be transmitted longer distances and are propagated through the Earth's surface. Again, I just need to receive the audio from the implant.

The input terminals connect to the aerial (antenna) that picks up the signal, the output terminals are for an auiliary speaker or (possibly with attenuation) for recording the signal.

The home page of the site (http://www.stormwise.com/) sells the antennas, or go down to the "LIST OF Do-It-Yourself VLF RADIO PROJECTS:" section. If you contact the site operator he might provide help.

Also, you could google "ULF antenna construction" (or similar terms) for home build designs.

This circuit is intended to receive all signals in its frequency range. If your transmitter only broadcasts at audio frequency, this receiver might do what you want without tuning.

We would need further information on your transmitter to figure out any other circuit details, but I would think that either your medcal team or the implant manufacturer would be able to supply that information. They might even be able to give you a suitable circuit diagram to build.

Perhaps you could use an "Active Tuneable Notch Filter" to add frequency selection to this circuit:

http://books.google.com/books?id=-sE7JVywygQC...SMy4GwBw&ved=0CGIQ6AEwBQ#v=onepage&q&f;=false

Would a normal copper wire antenna suffice? Just a length of copper wire ~100 ft?

There are two antenna terminals, so the antenna will be some kind of loop or coil.

Try googling "low frequency antenna construction" or "low frequency aerial design" or similar.
For short ranges, a fairly small loop or coil might work.

Ideally, the antenna in your implant would be the ideal shape, if you can get the specification from the manufacturer or your medical team. I would expect that they have aerials and conventional receivers for testing the transmitter/implant system.
The manufacturer's design team would probably be delighted to help you, and quite possibly provide a circuit and antenna design inforomation.

Could I use a length of copper wire and solder a dual wire output to the end?

My general knowledge of antennas is minimal, but I would think it needs to be a loop from one terrninal to the other at low frequencies. You can read up on it.
But, wire is cheap, experiment and see what happens !

I used to work with cochlear implants. The signal used by the implant we were using between the microphone and the implant was a 1.8MHz carrier with amplitude coded digital modulation on it. The carrier was used to power the implant and the digital codes encoded which electrode, out of 16, was to be stimulated and at what current level. Not anything that would be directly processable by the human ear by listening to the signal.