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Old 03-04-2008, 11:33 AM
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That's Amplitude Modulation, I didn't forget an "H" at the beginning

Is it possible to suppress the carrier only but still have the two sidebands for the "full frequency" effect? I know it doesn't gain you any bandwidth, but I didn't know if the power savings while keeping the full bandwidth had any advantage...

Notch filter? Maybe this is covered in the General book and I haven't gotten to it yet, and it's probably a stupid question but I'm wondering...
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Old 03-04-2008, 12:08 PM
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Dave?
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Old 03-04-2008, 12:10 PM
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That's what I was thinking... Or Ken's a EE so maybe he knows too...
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Old 03-04-2008, 12:15 PM
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Originally Posted by subzali View Post
Is it possible to suppress the carrier only but still have the two sidebands for the "full frequency" effect? I know it doesn't gain you any bandwidth, but I didn't know if the power savings while keeping the full bandwidth had any advantage...
Yes, check pages 5-10 through 5-13. The output of the Balanced modulator (hint=it's "balanced" 'cuz it has both side bands), form the combined inputs of the Speech Amplifier + the Carrier oscillator = DSB. If sent to the linear amp from there the signal would have better fidelity BUT it would exceed the FCC bandwidth regs. A filter between the Balanced Modulator and the Linear is used to strip 1 side band and decrease the bandwidth.
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Old 03-04-2008, 12:31 PM
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It's called suppressed carrier, double sideband. Completely legitimate modulation technique.

So why bother with this? If you suppress the carrier you reduce the total power envelope transmitted, since in a typical double side band, full carrier AM signal 50% of the power is in tired up in the carrier. The problem with doing this is that the demodulation is much tougher, much, much tougher. BTW, you can suppress the carrier (or reduce it) for single side band, too. Doing SSB with a suppressed carrier is much more efficient use of power. In any case recovering a carrier is needed on the receiving end to demodulate the information and that's the tough part. But it's all certainly perfectly possible.

Why are you wondering, anyway? If you want the math, I can write it up and bring it tomorrow.

You can't really do this unless both ends are aware of it. If you suppress the carrier on regular CB radios, for example, the receiving end would get noise or maybe just nothing. It probably would break the squelch, but the transmitting information would likely sound like garbage. The detector and mixer on the receiving radio need to know how to deal with the lack of a carrier for the demodulation to work. A regular AM detector is very simple and the suppressed carrier, not as much.

Also when you hear someone talk about PEP (peak envelope power) and carrier power, this is what they are talking about. A CB can do 4W PEP AM and 12W PEP SSB. That means you need to only dead key 2W (i.e. key the mic, generate a carrier with no information modulated on it) on AM for a CB and can only do 4W maximum total envelope power when you are talking.
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Last edited by DaveInDenver; 03-04-2008 at 12:54 PM.
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Old 03-04-2008, 12:33 PM
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I have a much more basic question. In 2M, when do you do FM and when do you do AM? How does this work out in practice?? How do you know?
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Old 03-04-2008, 12:56 PM
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I have a much more basic question. In 2M, when do you do FM and when do you do AM? How does this work out in practice?? How do you know?
So far the club has mainly messed with 2M. 2M is easy and there are repeaters all over the band plus many places for simplex. I continue to snoop around on 70CM and find it is less crowded but there is quite a bit of noise in some areas and some commercial communications too. I think it might entirely be possible to find a quite channel on 70cm that we can use for a common RS simplex net. The problem AGAIN comes in that those who chose to go with inexpensive 2m only machines would be left out, which is exactly what we are trying to avoid!
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Old 03-04-2008, 12:59 PM
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I have a much more basic question. In 2M, when do you do FM and when do you do AM? How does this work out in practice?? How do you know?
You get very little information or very distorted information if you are receiving the wrong modulation type. You will detect a station, but it will be very distorted, abnormally low or high volume, etc.

More practically, though, the band plan usually has ranges of frequencies dedicated to various operation. So there will be a range of FM, AM, SSB, CW, etc. In the case of 2m most of the band is set up for FM simplex and repeaters.

Here's ours in Colorado:
http://www.ccarc.net/images/CCARC_FUP_144MHz_revB.pdf
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Old 03-04-2008, 01:08 PM
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Sweet. I'm not wondering for any specific purpose, just stirring the pot, because the pot of my brain is stirred when I don't think I'm getting the full story (they're leading me down the primrose paths in the books), my engineering brain says it wants to know the full story and then determine whether or not it will ever be retained, remembered, or used again
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Old 03-04-2008, 01:11 PM
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I have a much more basic question. In 2M, when do you do FM and when do you do AM? How does this work out in practice?? How do you know?
FM is more commonly used in the areas of the band where repeaters and simplex are used. SSB comes in second. FM gives a much more clear, voice-sounding sihnal than SSB. However, FM requires much more power to go the same distance traveled. AM is not used much at all in these areas.

As a rule of thumb for VHF/UHF, FM uses a vertically polarized antenna, SSB uses horizontal.
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