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AM vs CW

  1. May 11, 2007 #1
    Hello, have a question...

    A friend and I have been discussing the difference between an AM signal modulated at 100% and a CW key. The AM transmitter would be modulated with a square wave, and the CW key would be turned on and off at the same rate. The debate is if the spectrum will look the same... In CW the carrier is turning on and off, but in AM the carrier is a constant amplitude with sidebands. He argues that the two are the same. Who is right? This thread https://www.physicsforums.com/showthread.php?t=79452&page=2&highlight=am+modulation
    touches the question, but still doesn't answer it fully, thoughts?
  2. jcsd
  3. May 12, 2007 #2
    How would you key the CW at the same rate? The best way is to AM modulate a CW with a square wave, as AM modulators control the amplitude of the output signal you might as well null it.
  4. May 12, 2007 #3
    Say a relay which switches the PTT lines of a radio. The relay coil is hooked to the square wave generator. For the sake of argument say the relay was perfect and is able to switch at the exact same rate as the relay coil. The square wave would be 0 to + voltage.
  5. May 13, 2007 #4
    In ptt, the local oscillator is always on, you are just routing it to a transmit or a receive mixer. But a mixer itself is an AM modulator, so by key switching the transmit IF you are modulating the modulator (or transmit mixer). Any form of amplitude control is AM modulation.

    But the confusion arise in the definition of CW keying. Ideally in AM modulation you need to have a stable CW and alter its amplitude. But keying can imply you are turning off and on the local oscillator generating a CW. That would introduce transients in the oscillator which would cause all sorts of sidebands as you increase the keying rate. I've never seen an application for turning the CW on and off other than in ham amateur rigs.

    Hope that helps.
  6. May 13, 2007 #5
    Say the local oscillator is always on and you are just switching the routing between tx and rx mixers. Will the spectrum analysis look the same as a AM transmitter modulated at 100% with a square wave? I would think its different since you never switch off the AM carrier, you only add sidebands, which after mixing in the receiver gives a net result of 0.
  7. May 13, 2007 #6


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    Several things to think about here. First, any time you do ANYTHING AT ALL to a carrier, you will generate sidebands. In CW circuits in amateur transmitters the pulse is shaped to prevent what is known as key clicks. Key clicks is energy that is put into extra sidebands in the spectrum. If we had a filter narrow enough we could determine that there is energy at more than just the exact carrier frequency of the CW transmission. By shaping the pulse we can keep any sidebands generated very close to the carrier frequency. The only way to prevent all signals except the main carrier is to not change the amplitude whatsoever which pretty much eliminates conveying any information.
  8. May 14, 2007 #7
    @ 100% square wave modulation, the whole carrier is goes on and off.

    Look at the 100% sine wave modulation,

    http://www.rfcafe.com/references/electrical/amplitude_modulation.htm [Broken]

    Every 180 degrees the carrier is 0

    Same thing with a square wave as it is basically a sum of infinite odd harmonics of the fundemental frequency.

    Hope that helps.
    Last edited by a moderator: May 2, 2017
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