Some help with Low Frequency FM

Hi guys,

I'm attempting a project where i will be transmitting voice through an underwater wireless link. Basically what i'm trying to do is use fm to transmit the voice between the transducers, effectively the same idea as a radio. Although the only problem in this is that there seems to be very little info out their on fm using frequencies up to about 40 khz, they all seem to be in the MHz range. Would it be as simple as to just recalculate the capacitor/inductor/resistor values on a standard fm transmitter circuit? Also if anyone else would have advice on transmitting through water then it'd be appreciated.

Cheers

Water generally attenates RF quite rapidly. How far do you need to transmit and is it fresh or sea water?

Sorry i should've said, i'm not using antenna, it's piezoelectric transducers. So it transmits as a pressure wave in the water. RF would be no use at all, would've attenuated really quickly. That's why i'd be usin really low frequencies, because it's dependant on the speed of sound in water. Basically using the transducers as a replacement for the antenna

The problem with using FM is that the modulating frequency isn't going to be much lower than the carrier - which is likely to be a problem.

I think that FM would work fine. I would not begin with an FM radio transmitter however. It would be simpler to start with a VCO like that inside the 4046 CMOS logic chip and follow that with an op amp. Driving the piezo transducer will be a bit more challenging as it is highly capacitive, so you might want to follow the op amp with a special op amp designed to push a fair bit of current and tolerate capacitance. Check out the LM8272. How much power do you think you need?

Don't expect a large bandwith, as it's for voice, restrict it to 3kHz like an AM radio.

RadioRon said:

I think that FM would work fine.

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There are formulas for working out deviation and the resultant audio bandwidth - which I have long ago forgotten

But you can't get much audio bandwidth without substantial carrier deviation, and an ultrasonic transducer is most likely to only work on a very specific frequency, FM'ing it is likely to lose the sidebands.

You are perfectly correct about NOT using VHF radio techniques, just a simple voltage controlled audio oscillator is all that's needed, such as the PLL you suggested.

Nigel Goodwin said:

There are formulas for working out deviation and the resultant audio bandwidth - which I have long ago forgotten

But you can't get much audio bandwidth without substantial carrier deviation, and an ultrasonic transducer is most likely to only work on a very specific frequency, FM'ing it is likely to lose the sidebands.

You are perfectly correct about NOT using VHF radio techniques, just a simple voltage controlled audio oscillator is all that's needed, such as the PLL you suggested.

Click to expand...

Actually, that is a pivotal point which escaped me. The transducer itself may be very selective, perhaps too much. Equalization may help, but investigating the transducer bandwidth is the first step I think.

A typical ultrasonic transducer is the Murata MA40S4. It works in air and has a range of 1m to 2m.
It resonates so it has a narrow bandwidth.

Rapid don't sell them but the catalogue (see page 47) on their site lists a 200kHz underwater transducer with a maximum 50W power input and a bandwidth of 25kHz.

**broken link removed**

I think you need to go up in frequency, 40kHz is far too low, 200kHz is perfect.