Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Radio numbers

  1. Aug 9, 2004 #1
    I have come to understand the meaning of the terms "am" and "fm" as different kinds of radio emissions. Sadly, I have not understood the meaning of the numbers used to identify certain radio stations. If someone could tell me, it would be fantastic. :smile: I've been dying to know. :tongue2:
  2. jcsd
  3. Aug 10, 2004 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Both kinds of radio signals use a base signal called a "carrier" to encode information. The carrier itself is nothing too special -- it's just a simple sine wave of a given frequency. The radio station Live 105.3 in San Francisco, for example, broadcasts with a carrier frequency of 105.3 MHz.

    If the radio station were broadcasting nothing but silence, it'd be broadcasting a perfect 105.3 MHz sine wave.

    The carrier by itself contains no information -- it's just a sine wave. If you want to convey information, you have to modulate the carrier with the music you want to broadcast. In the AM (amplitude modulation) scheme, the carrier frequency stays fixed, and you vary the amplitude (intensity) of the sine wave to encode the amplitude (intensity) of the music. A high amplitude means the radio should produce a high sound pressure, and a low amplitude means the radio should produce a low sound pressure. Your speaker moves back and forth to reproduce those pressures, and you hear the music.

    In the FM (frequency modulation) scheme, the carrier's amplitude stays fixed, and the frequency is varied by a few hundred kilohertz. A higher frequency means high pressure, a lower frequency means low pressure. Your speaker moves back and forth and reproduces those pressures and you hear the music.

    Here are some pictures of the results of applying AM or FM modulation to a carrier: http://hyperphysics.phy-astr.gsu.edu/hbase/audio/bcast.html#c3

    Let me know if you have any more questions.

    - Warren
  4. Aug 11, 2004 #3
    How are sine waves superimposed?
  5. Aug 11, 2004 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    In general, you use a device known as a "mixer" to add two signals. A mixer can be as simple as a plain ol' resistor.

    - Warren
  6. Aug 11, 2004 #5
    Thank you so much chroot!! I've been dying to know.

    How does a mixer add two signals? What are the signals for?
  7. Aug 11, 2004 #6


    User Avatar
    Science Advisor
    Gold Member

    The "primary" signal is that which your radio tunes into, here, BBC Radio 1 is at around 99.5 MHz, (the primary signal kinda vibrates this many times each second). This is the frequency that you 'tune' your radio into. Think of this one as being like a big wave on the sea, just up and down, up and down (the sine wave Chroot mentioned).

    The other signal (the superimposed one) is much smaller, and this one broadcasts the content of the actual radio station, so this is what you hear. Think of it as being a load of small ripples on the surface of the big wave you imagined earlier. The difference in FM and AM (again as Chroot explained) is the difference between the way that these small waves manifest themselves, - for FM the ripples get faster and slower to reproduce the speech and music, for AM they get bigger and smaller.
  8. Aug 12, 2004 #7

    Ummmm, not quite. A mixer or modulator requires a NON-LINEAR device. Using resistors will form an adder which is NOT a mixer.
  9. Aug 12, 2004 #8


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I beg to differ. Many people use a simple resistor as a mixer in non-critical applications. Sure, it's a really horrible mixer, but it's doing the mixing.

    - Warren
  10. Aug 12, 2004 #9
    Nope, a linear circuit is incapable of true modulation. You will not get sum and difference frequencies out of a circuit that you describe. It may be capable of adding 2 signals together to send down the same wire or something but this is not a mixer. I've worked on alot of RF equipment and I have never seen a mixer/modulator that didn't use some sort of non-linear device.
  11. Aug 17, 2004 #10

    The 'ideal' mixer has nothing to do with resistors it is a square law device
    vo = ( a.sin(w1.t) + b.sin(w2.t) )^2
    This produces harmonics and sum and difference terms
    the required frequency can be selected by filtering or by using a more complex mixer in which some terms cancel , this is the preferred method in chip radios , and chip mixers.
    If a 'mixer' is used on one signal it works as a rectifier which after filtering harmonics produces DC .
    A mixer could be one diode , but is usually made of several bipolar transistors in a ' balanced' form look up any application notes on chip devices .
    Last edited: Aug 17, 2004
  12. Aug 23, 2004 #11
    Everyone here is right to some extent. No one it seems is old enough to remember vacuum tubes or old AM radio. This is before balanced mixers.

    All AM modulation was done by adding voltage, or subtracting, from the average voltage of the final of the RF amp. You had to prevent over modulation by not allowing the audio voltage to go lower that zero.

    100% modulation is when the audio voltage is just at zero and twice the average RF voltage. Any method that is able to add or subtract from the average RF voltage is able to modulate the RF.

    FM is easy. All you need to do is shift the frequency of your oscillator. For the most part this has always been done at low level.
  13. Aug 23, 2004 #12
    Who's talking about balanced mixers? Incidentally, how is old AM radio different than new AM radio? Many AM transmitters on the air have been there for many years. I would be willing to bet even the new ones still run class C and modulate the power supply on the PA.
  14. Aug 23, 2004 #13
    >how is old AM radio different than new AM radio? Many AM transmitters on the air have been there for many years. I would be willing to bet even the new ones still run class C and modulate the power supply on the PA.<

    None of the posts said anything about high level modulation I thought it was worth while to mention it. Old AM radio is before TV and before DSB and carrier injection and low-level modulation. I agree there are still many old final modulators out there but very few on this site think about them or know they exist. It would be interesting to find out who is still using them. If you built a system today it would cost 1.5 to 2 times the cost of other methods. Also old radio receivers had tubes and the portables where 1 to 2 cu ft in size. Do you remember old radio?
  15. Aug 23, 2004 #14
    Well I am old enough , and it's still not addition but multiplication no matter how it's done ( for AM mod , or frequency changing ) , The point is that modulation is a NON linear process compared to linear mixing of signals as in a sound studio .
  16. Aug 25, 2004 #15

    Am I old enough? That's not really the point. I have restored 6 volt tube radios with vibrator contacts that run the high voltage power supply. Age means nothing. My grandparents were plenty old enough to remember it but couldn't tell you the first thing about modulation. I'll say it again, old AM radio is no different than new AM radio concerning the AM broadcast band unless you count AM stereo. You also mention double sideband (DSB). AM is inherently defined as double sideband. Amplitude modulate the carrier and you WILL end up with a double sideband signal with a carrier. When radio evolved is when they started to figure out how to remove parts of the signal such as the carrier and or one or a partial sideband before it went out on the air.

    To the OP: Any of this making sense? You probably have new questions now? Like what is a sideband?
  17. Aug 25, 2004 #16


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member


    "Sidebands" refer to the frequencies alongside the carrier that contain non-zero power. In AM, the frequency is not modulated, so there are no sidebands. The entire signal is only at one specific frequency, that of the carrier.

    - Warren
  18. Aug 26, 2004 #17
    To all:

    I don’t think any of you are old enough to understand the term old AM radio. The term does not refer to the physics of AM it is in reference to a period of time, The early days of radio. If you were old enough you would not think of Old AM Radio in any other way.

    >Modulation is a NON linear process<

    I don’t know how you arrive at that conclusion. The intent of AM is to increase and decrease the peak voltage of the RF in a linear manner proportional to the applied audio. Many transmitters include feed back from the RF output to make the modulated output linear. All AM demodulators are intended to be peak detectors that follow the peak of the RF. A good demodulator will be linear.

    >AM is inherently defined as double sideband. Amplitude modulate the carrier and you WILL end up with a double sideband signal with a carrier.<

    It was found that if you eliminate the carrier and start just with the DSB you might then inject a carrier with less power and produce a more effective output. As you can see this method has many advantages and you are still able to receive this output with a standard AM radio.

    >there are no sidebands<

    I am sorry to disagree with you. The sidebands do contain energy and are located plus and minus the center frequency equal to the frequency of the modulation. This is why you are able to eliminate the carrier in DSB or SSB two sidebands are not necessary one sideband works fine for the transfer of information.
    Look at a spectrum analyzer if you still doubt me.
  19. Aug 26, 2004 #18


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member


    You are correct -- I wasn't thinking about the fact that modulating the carrier's amplitude is the same as convoluting it with a lower-frequency signal. Sorry about that.

    - Warren
  20. Aug 26, 2004 #19
    Still not sure what that means. It has nothing to do with the original posters questions. If you wanna start something with 'old AM' then do it someplace where it doesn't confuse people.

    Yes, what you are saying is correct. Modulators and demodulators should be linear in the sense that the output should represent the input very closely. That is why there is feedback in the transmitter as you describe. But the output is then only as accurate as the demodulator that demodulates the output in order to compare the output to the input. As far as I knew feedback is mostly used in transmitters to keep the output clean of harmonics. For example in UHF transmitters they will do this but the feedback doesn't go all the way back to the audio. They are mainly concerned with avoiding harmonics of the main carrier. But the FUNCTION of modulation is not linear. A true linear amplifier will sum two signals and not create new frequencies. A non-linear amplifier will sum them AND create new frequencies. Creating new frequencies is what modulation is all about.

    Yep. Note to chroot: A modulator and a mixer are the same thing. Not the type of mixer used in recording studios, the mixer used in radio work. They are optimized for the frequencies being processed though.
    Last edited: Aug 26, 2004
  21. Aug 28, 2004 #20
    What's an RF output?
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Radio numbers
  1. Crystal radio (Replies: 9)