fm body worn transmitter?

[Jeebus]

I need to make a small body worn fm transmitter that will transmit a modulating frequency of not audio hertz rates but rather 7.49mhz. I then need a fm receiver that will demodulate the signal from its 88.1mhz and leave the 7.49 mhz signal.

was wondering if their are any circuit about or transmitters that I could modify to fit these needs.

On the receiving end, I was wondering if a special receiver is needed to output higher modulating signal frequencies?

Could I honestly tap into a radio frequency ...like 88.1 and receive the 7.49mhz signal and then run it through an O-scope?

thanks for any info

 

[Jeebus]

im guessing the fm transmitter needs to be crystal controlled so that the modulating frequency is exactly what I want it at....anyone know of some sites with info or circuits on making crystal controlled transmitters?

 

[JimB]

To modulate a 88Mhz transmitter with 7.5Mhz is a ridiculous idea.

The resulting signal out of the transmitter will be at least 15Mhz wide, depending on the type of modulation.

You need to think again.
As is so often asked here, what are you REALLY trying to do?

JimB

 

[Jeebus]

ok ridiculous... Im pretty new at this and I'm trying to flesh out this idea of mine about wearing a transmitter that sends a 7mhz signal(which one wavelength is 40 meters) then I would receive this by two receivers(who are within one wavelength) and phase compare the two receiver phases and pop out with a voltage. I thought that FM modulating them would give me a cleaner signal or something.

what are your thoughts? do i not need to FM it? just make a 7 mhz transmitter? Do you know any good resources that I could read up on this?

 

[dknguyen]

What JimB means is that we need to know the specs and a actual goals of the application, not just a repeat of your description with a couple more details. And correct me if I am wrong, but your description of the two receivers being apart (and the use of two receivers) for phase compare doesn't sound quite right.

 

[Papabravo]

Things just don't work the way you think they do. There is no theoretical basis for your assumptions. Just because you think a thing is possible does not mean that it must be so. What in the name of heaven and earth makes you believe that receivers can measure the distance to a transmitter by comparing the phase of independent reconstructions of the modulating signal withing one wavelength of the modulating signal?

This question seems familiar, have you also had this thread on other boards. I guess if you don't get the answer you want you ask the question again.

 

[Jeebus]

you are probably right that things dont work the way i think they do...but that is why i ask questions and hopefully with your knowledge you could help me out. As for the fact that I have posted something like this before is yes, but I have more info, threads die, and I didnt get an answer....why are you so exasperated? Im just asking some questions and I havent insulted you nor demanded answers.

Anyways, I am new at RF so if I ask some naive question....

My application was to see if I could view two 7mhz sine signals after demodulating them from an FM receiver through an o-scope.
The phase compare thing was for later.

Could you point me in a direction about what 15mhz wide means and why this is bad would it be impossible to demodulate, can you only use audio frequencies to modulate a mhz signal?? I only need the transmitter to have a range of say 100meters.

If it is possible to modulate a signal then receive it and demodulate it leaving a 7mhz signal, do any of yall know of some schematics to make such receiver and transmitter?

Im getting that Arrl handbook for radio amatures so hopefully Ill know a little more...

 

[Papabravo]

Since you ask about the source of my exasperation it comes down to a single thing. You are not just a naive questioner. You have asked a question, you have gotten an answer that should have been sufficient. You did not like the answer that you got. So rather than expending your own effort to understand the answer and learn more, you skulk off to another board to ask the same question expecting a different answer. You should try and learn to crawl and walk before you try to run a marathon.

To modulate means to combine two things which are both changing. This combination process is generally non-linear. Multiplication is a non-linear process. In Amplitude Modulation(AM) two signals are combined in a multiplier. If you know trigonometry you can work out that if you multiply two sine waves of different frequencies, call them f1 and f2, you will get four different signals out. The four outputs will be at frequencies f1, f2, f1 + f2, f1 - f2. The signals at the sum of the two frequencies and the difference of the two frequencies are called sidebands. Once you understand AM modulation you can tackle Frequency Modulation(FM). The detailed math is a bit harier and involves Bessel functions.

In your original example the 7.49 MHz. signal was not changing, and it could not be a modulating signal. FM does not mean that you change one frequency with another frequency. Typically FM uses the amplitude of the voice to change the frequency of the carrier. In order to modulate the 88.1 MHz carrier you have to have something that changes. Using voice to modulate an FM carrier, it is the amplitude of the voice that changes the frequency. Understandable speech has a bandwidth of 3kHz. In a narrow band FM transceiver the 3kHz. bandwidth creates an FM modulated output with a much wider bandwidth, as much as 25 kHz. An FM broadcast station which wants to produce high fidelity music to 20 KHz. is spaced 200 KHz. from nearby stations to avoid interference. FM modulation consumes way more bandwidth than the modulating signal.

 

[audioguru]

FM stations broadcast audio frequencies to only 15kHz so that any frequencies near the stereo pilot at 19kHz are suppressed.
Then a 19kHz pilot tone is broadcast and AM carrier-suppressed dual stereo sidebands from 23kHz to 53kHz. SCA "restaurant music" used to be modulating the carrier at around 67kHz but now is used for other things. Then there is still lots of unused space in a station's bandwidth.

 

[Papabravo]

I think you are agreeing that the bandwidth of the FM output signal is wider than the bandwidth of the modulating signal(s). Are you?

 

[dknguyen]

Maybe you should get a textbook on the math behind AM and FM radio before you try and build the circuit.

 

[rigdoctor99]

Also think about the frquency you wish to use...... 7MHZ is, as you say, 40 meters........... In order to get your transmitter to work you will need a very long antenna........ even at 1/4 wave the antenna is 10 meters long.......

I have 85feet of antenna in my attic and still need to use an antenna tuner to work 40meters.....

Re-think, read, and do some more math before trying such a strange experiment.........