Frequcency Modulation

[mutant_aliem]

I was wondering if anybody knows of a simple way to perform frequency modulation on a low frequency (around 1Hz) AC signal using discrete components (BJT's, Op-amps etc). I have no idea how possible this is as I can't find any schematics.

If this isn't possible, are there any (cheap - only a few dollars) IC's that will do this? I have been told that the MC1496P chip can be used for freq. mod., but from it's datasheet, I can only see that it has a frequency doubling 'function,' which doesn't really look suitable (I want the 1Hz signal modulated to around 20kHz or higher).

Thanks.

 

[Nigel Goodwin]

I suggest you try posting EXACTLY what you are wanting to do, and not how you think it needs to be done - your post doesn't make any sense to me?.

In particular you say "(I want the 1Hz signal modulated to around 20kHz or higher)", you don't modulate 'to' something, you modulate 'by' something. So I've no idea what you're asking?.

 

[mutant_aliem]

Sorry about that. I'll try to explain it better.

Basically, the circuit has to be able to tell you if someone is breathing or not. That signal has a frequency of about 1Hz (since that is about the rate you breathe at). The problem is, the breathing signal has to be combined with another signal that can have a frequency from 10Hz to 10kHz. So my group were going to modulate the breathing signal by a 20kHz signal. This is so that the breathing signal and the 10Hz-10kHz signal can be combined and then later seperated with some RC filters.

The question was, is it possible to build our own modulator, or is there a cheap IC that can do this?

I hope this is not as confusing as my previous post.

 

[Nigel Goodwin]

Sorry about that. I'll try to explain it better.

Basically, the circuit has to be able to tell you if someone is breathing or not. That signal has a frequency of about 1Hz (since that is about the rate you breathe at). The problem is, the breathing signal has to be combined with another signal that can have a frequency from 10Hz to 10kHz. So my group were going to modulate the breathing signal by a 20kHz signal. This is so that the breathing signal and the 10Hz-10kHz signal can be combined and then later seperated with some RC filters.

The question was, is it possible to build our own modulator, or is there a cheap IC that can do this?

I hope this is not as confusing as my previous post.

You've got it the wrong way round, you need a 20KHz carrier, and modulate THAT with the 1Hz signal - you could easier do this by using the 1Hz signal to alter the frequency of a 20KHz oscillator. At the receiver end, use a PLL (Phase Locked Loop) tuned to 20Khz to recover the original 1Hz signal.

Is the 1Hz an analogue signal?, or just a digital (ON and OFF) signal, an analogue one would make it slightly more complicated as you would require linear modulation.

 

[JimB]

Still rather confusing I would say.

It is not possible to modulate a low frequency with another higher frequency, it just does not make practical sense.
You can modulate a high frequency with a low frequency, in your case you would modulate the 20kHz with the 1 hz.

You then say that you want to filter the resulting signal, why? what do you expect that to achieve?

You also say that you want to frequency modulate one signal with the other. Are you sure you dont want to amplitude modulate?

Sorry but your method does not make sense. As Nigel said tell us what you want to do, not how you think you can achieve some unknown (to us) goal.

JimB

 

[RadioRon]

Impressing the 1 Hz signal onto a 20KHz carrier is easily done using a CD4046 phase locked loop IC. In fact, you can use one for generating the signal and another 4046 (or LM565) for retrieving (or demodulating) it at the other end. To transmit, the easist way is to just use the VCO function of the 4046 by itself and ignore the phase detector. Create a stable and noise-free DC bias and then impress your 1 Hz AC waveform onto that bias, and then attach this composite voltage to the control voltage input of the VCO. Since the VCO frequency of oscillation is directly controlled by this control voltage input, when you put an AC signal into it, you get FM on the VCO output. You tune the carrier frequency by adjusting the DC bias, and you tune the amount of modulation (or as we like to say, the "FM deviation") by increasing or decreasing the amplitude of the AC.

To receive, just hook up an LM565 or similar chip as an FM demodulator operating at 20KHz. You willl get the 1Hz ac signal on the VCO control voltage and can wire this out. Data sheets usually show the hookup for this part as an FM demodulator.
It is worth mentioning that if you design it right, this circuit has "selectivity" and will not respond to signals outside of its "passband" so the need for filters to remove the energy below 10KHz is reduced, which is quite handy.