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Radio questions

  1. Nov 21, 2006 #1
    Hey i was wondering if someone can help me out with radios and whatnot...i know making transmitters are illegal but i was just curious as to how they work as well as receivers. The thing that confuses me is how do you get information to and from the signal? I mean if i made a tank circuit that would oscillate at a certain frequency, how would that actual resonate with the electromagnetic wave? I to make it resonate you need a inductor and a capacitor but if i didn't apply power to it, would it be possible just to send that to a step-up transformer? would that be enough to listen to the audio encoded in the carrier wave (considering its am) after sending it through a speaker? Also can somone explain to be the whole thing about calculating indutance? The whole thing about finding the values with ought proper equipment is a bit hard to do heh. Sorry i am not the best at electronics but i would really like to become better. If possible can someone draw a quick schematic? I have seen some but they look like they are designed for quality rather then theory. Thanks a lot. Hopefully ill be smart one day haha.
     
    Last edited: Nov 21, 2006
  2. jcsd
  3. Nov 21, 2006 #2
    You are basically asking for an explanation of how everything works from microphone to transmitting antenna and then from receiving antenna to speaker. I would recommend going to wikipedia. The main thing you need to remember in just about ALL electronics is that complex circuits are made up of simpler circuits. A radio transmitter is made up of oscillators, amplifiers, filters, etc. The receiver is made of similar circuits, just optimized to end up with different results. After you read wiki, I'd be glad to help you with any other specific questions.
     
  4. Nov 21, 2006 #3

    berkeman

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    Good advice from supernova -- start with wikipedia and read about the multiple subjects you are interested in. There's a lot of good introductory info at that website.

    Beyond that, can you give us an idea of your background and current educational status? Like, are you in high school and interested in all this electronics stuff, or are you out of school and interested in learning about things that you are curious about? If we understand where you are coming from, we can try our best to point you to other resources and experiments that you can try.
     
  5. Nov 21, 2006 #4
  6. Nov 21, 2006 #5

    Danger

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    I'm not sure how it works in the States, but up here building transmitters is not illegal. The legal aspect comes in when a transmitter excedes a certain wattage, at which point you have to buy a licence in order to use it. You don't need a licence for a toy car or a small walkie-talkie, you might need one for a model aeroplane, and you definitely need one to be a pilot or operate a CB. When I was flying, the primary licence was in order to operate the on-board radio. It applied to everyone from first-flight students to commercial airline pilots.
     
  7. Nov 21, 2006 #6

    berkeman

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    Yeah, here down south :biggrin: it's pretty much the same. You can build a transmitter in any band as long as the transmit power is low enough not to exceed the "unintentional radiator" limits for the band. If you want to build an intentional radiator transmitter, you have to adhere to the FCC regulations for that band. For some bands, that means getting a license (like the HAM or police or fire or other public service bands), and in the ISM (Industrial, Scientific and Medical -- 2.4GHz microwave oven) band, it just means adhering to the transmit spectra power and bandwidth limits.

    But in general, the OP should be very careful about fooling around with independent RF transmission experiments. You absolutely do not want to step on a police or fire freq (for obvious reasons), and it's a bad idea to wander into the HAM bands (because we are practiced at direction finding drills to find interfering sources).
     
  8. Nov 22, 2006 #7
    You may want to start with AM radio transmitters/receivers. I thought i would answer one question though

    The terms "local oscillator" and "envelope detector" might be of use if you're considering how do you demodulate an AM signal. Though I couldn't tell you if it's used in practice, it's easy to understand.
     
    Last edited by a moderator: Nov 22, 2006
  9. Nov 22, 2006 #8

    berkeman

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  10. Nov 22, 2006 #9
    Hey thanks for the responses. So yeah in a freshman in college and i took some electronic courses in high school but it was really basic. I am taking some physics right now (its not heavy on math which sucks but it covers wider topics). But yeah i have been to A LOT of sites about radios and whatnot and currently reading stuff at http://www.allaboutcircuits.com/ ( i soon realized ac has to deal with alot more math then dc does =p). Um as for electronics i have used some kits right now, i have Electronics Learning lab but its mainly says a transistor does this, then wire this up to that and it will work. Only downside is that it doesn't mention stuff about how much voltage does it take to burn parts out..but i guess i can go search for data sheets online. But yeah so i am familiar IC's, resistors,transistors,relays,transformers,etc etc. Mainly what i know about radio waves is that when a electron moves around it creates not only a electric wave but also a magnetic wave thats perpendicular to it, which moves at the speed of light. By how fast the electron shakes determines the frequency and wavelength of the electromagnetic wave. What i don't get is how does the circuit KNOW that the wave has hit it? just because of resonance and then the amplitude of it increases? Thanks again. By the way, i am not just wondering about transmitting audio...i mean if its possible to just create a wave and create another circuit to detect the wave thats good enough for me. Thanks again! (finally i found a good active forum haha)
     
    Last edited: Nov 22, 2006
  11. Nov 22, 2006 #10

    berkeman

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    Never anthropomorphize electronic circuits. They hate it when you do that. :rolleyes:

    It's all about forces. You drive an AC current into the transmitting antenna (by varying the terminal voltage of the antenna), and when the resulting EM wave passes by the receiving antenna, the AC field generates forces on the electrons in the antenna. An efficient antenna resonates at the receive frequency, which gives the best conversion of energy into the conducted signal that appears at the receive antenna terminals.
     
  12. Nov 22, 2006 #11

    NoTime

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    I just sprayed coffee all over my keyboard :tongue2:
     
  13. Nov 22, 2006 #12
    So does the receiving antenna need power? I mean if i had a metal rod is it safe to say its resonating with all electromagnetic waves passing through it?
     
  14. Nov 22, 2006 #13

    berkeman

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    The antenna is a passive device. You can hook it up to a dummy 50 Ohm load (instead of into a 50 Ohm input amplifier), and there will be voltages present across the load from the pickup of the antenna. Small voltages in general, but they're there. And what it picks up is dependent on what is in the air, and the size and construction of the antenna.

    For example, it is common in the building controls network business for the control network wiring to pick up AM radio signals when the lengths of network wire are close to a multiple of a quarter wavelength (75 meters or so for the US AM band). So our network transceivers have to be able to continue to operate reliably (with about 1Vpp differential comm signals) in the face of many volts of common-mode noise picked up from nearby AM radio transmitters.
     
  15. Nov 22, 2006 #14
    But do you need an antenna? I mean i read stuff where for transmitters you don't need a antenna because the circuit acts as one..but what about a receiver? Technically you wouldn't need one either right? And also higher frequency means its better to have a smaller antenna?

    *edit*
    I made a week transmitter and brought it near the radio ,basically it had a potentiometer to change the voltage across it. What i don't get is that i heard it from a am band but it was changing its frequency, not its amplitude...can someone explain this?
     
    Last edited: Nov 22, 2006
  16. Nov 22, 2006 #15
    RF radio frequency is a vast branch of electronics dealing with manipulation of radio waves.


    Bascially receivers and transmittes are made of few basic componets.

    Antennas, Filters, Amplifiers, Local Oscillators, Mixers, And Detectors.

    That's it, learn what those are and you know how radios work.

    I'll be brief here. A rule of thumb is it's alot easier to process lower frequency than higher frequency.

    Most receivers down convert an incoming signal to a lower frequency.

    This is how its done

    A local oscillator is actually a small low power transmitter. It generates a radio signal at a particular frequency and is fed into the mixer.

    The mixer takes the incoming radio wave from the antenna, (which is amplified and filtered) and subtracts its frequency from the local oscillator's frequency. The output from the mixer is the intermediate frequency or IF which is then filtered again and amplified and is fed into a demodulator.

    For example, FM radio have an IF filter of 10.7 MHz, to tune to your favorite station you have to adjust the frequency of the local oscillator.

    In my area a good station Q101 is at 101.1 MHz, in the radio I'm actually tuning the LO to 101.1 MHz - 10.7 MHz = 90.4 MHz, to pick up the station.

    What is does is 101.1 MHz is converted to 10.7 MHz in the receiver.

    10.7 MHz is much more easily processed and demodulated to produce audio.

    Transmitters work the same way, an audio or data carrier of lower frequency is up converter to higher frequency of interrest using mixers and local oscillators.
     
  17. Nov 23, 2006 #16

    berkeman

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    Thanks waht.
     
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