| Author |
Message |
    
Keithinatlanta
Advanced Member
Username: Keithinatlanta
Post Number: 987
Registered: 3-2003
| | Posted on Thursday, February 20, 2014 - 8:12 pm: | 
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I don't know electronics, but I have heard for years about clipping the limiter in your cb radio. What is that all about and what is the purpose of it? I have been told that it is not a good thing to do. I have heard that for 20 years. I am just curious. Thanks.
Keith in Atlanta
CEF 150 |
Magnum
Junior Member
Username: Magnum
Post Number: 13
Registered: 1-2009
| | Posted on Friday, February 21, 2014 - 7:59 pm: |
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Keith, clipping the limiter in a radio will cause it to overmodulate.when you look at the sinewave of a radio on a scope the peaks are flattopping,this means the audio is distorting and talking on adjasent frequencys.If you want to destroy the audio of a radio clip the limiter,turn the power all the way up and let it rip.by the way this makes you all kinds of friends on other freqs too.73's best wishes. |
Keithinatlanta
Advanced Member
Username: Keithinatlanta
Post Number: 989
Registered: 3-2003
| | Posted on Friday, February 21, 2014 - 8:26 pm: |
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Thanks Magnum, especially for spelling it out in easy to understand terms. I cannot stand to talk to someone who has their radio so overmodulated. You might as well talk to yourself. I have never had that done to any of my radios, and now I am even more glad. Thanks again.
Keith in Atlanta
CEF 150 |
Tech237
Moderator
Username: Tech237
Post Number: 1751
Registered: 4-2004
| | Posted on Saturday, February 22, 2014 - 11:18 am: |
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As a tech I strongly recommend against clipping the limiter.It is there for a very good reason.
What Magnum didn't mention that an overmodulated radio is prone (I'd guestimate 90% chance or better) of causing RFI, adjacent channel interference and a whole heap of other nasties.
Tech237
N7AUS
God made me an athiest, who are you to question his wisdom?
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Dale
Senior Member
Username: Dale
Post Number: 2037
Registered: 12-2002
| | Posted on Saturday, February 22, 2014 - 4:07 pm: |
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magnum is correct. if ya ever hear of
cb shops clipping the limiter. or thier
high tech terms disabling the modulation pot
or bypassing it. stay away from this it will
look good on a watt meter, but sound overmodulated
over the air and co-channel many channels . leading cause to tvi interference imho
dale/a.k.a.hotrod
cef426
cvc#64
454 [dx numbers]
38lsb
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Keithinatlanta
Advanced Member
Username: Keithinatlanta
Post Number: 990
Registered: 3-2003
| | Posted on Tuesday, February 25, 2014 - 9:37 pm: |
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I stay away from all those butcher shops. I was taught a long time ago to "run" when I heard them say they would clip the limiter.
At least I know that much on the tech side  .
But I appreciate the info here, as I am sure I am not the only one who did not know exactly what it means. Thanks again guys.
Keith in Atlanta
CEF 150 |
Sitm
Advanced Member
Username: Sitm
Post Number: 507
Registered: 1-2004
| | Posted on Tuesday, February 25, 2014 - 11:52 pm: |
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The Radio Amateur's Handbook, 1978
A-M and DSB, p.370
If the amplitude of the modulation on the downward swing becomes too great, there will be a period of time during which the rf output is entirely cut off. The shape of the downward half of the modulating wave is no longer accurately reproduced by the modulation envelope, consequently the modulation is distorted. Operation of this type is called overmodulation.
The distortion of the modulation envelope causes new frequencies (harmonics of the modulating frequency) to be generated. These combine with the carrier to form new side frequencies that widen the channel occupied by the modulated signal. These spurious frequencies are commonly called "splatter."
It is important to realize that the channel occupied by an amplitude-modulated signal is dependent on the shape of the modulation envelope. If this wave shape is complex and can be resolved into a wide band of audio frequencies, then the channel occupied will be correspondingly large. An overmodulated signal splatters and occupies a much wider channel than is necessary because the "clipping" of the modulating wave that occurs at the zero axis changes the envelope wave shape to one that contains "high-order harmonics" of the original modulating frequency. These harmonics appear as side frequencies separated by, in some cases, many kilohertz from the carrier frequency.
Because of this clipping at the zero axis, it is important that care be taken to prevent applying too large a modulating signal in the downward direction. Overmodulation downward results in more splatter than is caused by most other types of distortion in a phone transmitter. |
Tech237
Moderator
Username: Tech237
Post Number: 1758
Registered: 4-2004
| | Posted on Wednesday, February 26, 2014 - 11:26 am: |
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Yes Sitm - 100% true, and the limiter in a radio limits the modulation BEFORE it is added to the carrier to prevent such a situation as your quote describes.
Not sure if you posted that to support not clipping the limiter, or to clip the limiter.
Tech237
N7AUS
God made me an athiest, who are you to question his wisdom?
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Sitm
Advanced Member
Username: Sitm
Post Number: 509
Registered: 1-2004
| | Posted on Wednesday, February 26, 2014 - 6:47 pm: |
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Never clip the limiter, never.
I am a licensed amateur extra, would you think I would promote clipping the limiter? |
Tech237
Moderator
Username: Tech237
Post Number: 1760
Registered: 4-2004
| | Posted on Thursday, February 27, 2014 - 11:31 am: |
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Sitm _ I didn't think so, but wasn't sure with the way the ARRL quote is actually worded. It's wording, to me, was a little ambiguous.
Mind you, I have known a few "qualified" radio techs who have advocated doing it.
Tech237
N7AUS
God made me an athiest, who are you to question his wisdom?
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Keithinatlanta
Advanced Member
Username: Keithinatlanta
Post Number: 992
Registered: 3-2003
| | Posted on Friday, February 28, 2014 - 6:12 pm: |
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Guys, thanks for the information. I did not know I would get this much response, but I am glad, as that is what makes this forum tick every day. And thanks again for the input, as I learned from it!
Keith in Atlanta
CEF 150 |
