Question about Cobra 29 LTD CB Radio

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School project how deos a cb work im doing a school project on radio waves

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When you talk into the microphone your voice which is sound pressure is converted to electrical voltages then it is increased in power to about half of the power of what the CB will sen out to the antenna. To make the CB radio transmit, there is a circuit that creates the proper frequency or channel. It is then increased to about twice the power of the voice circuit. They are combined together just before it goes into the antenna.It leaves the radio and goes into the antenna where it leaves the antenna as an electromagnetic wave. Electricity in the air with a certain value of frequency and strength. When the CB receives, The antenna picks up a signal, an electromagnetic wave, as it passes by. The electromagnetic wave, which is a small voltage, with a certain frequency and strength, enters the radio where it is processed . It is then increased in power . It is now connected to a speaker. The speaker is also a converter. It converts electrical voltage back to sound pressure so we can hear it. As the electrical voltages make the cone of the speaker move in and out. There is a lot more detail than this but this should help. Many things happen inside the radio that cant be explained in a few paragraphs.

Posted on Dec 13, 2011

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  • Iron
    Iron Dec 13, 2011

    No mention of crystals, filters. IC chips or anything. Typical espert.

  • Ernest Taylor Dec 14, 2011

    Ok you want crystals? here it is! In the early days of CB they used crystals. One for transmit and one for receive. that is what it took just to have one channel. Then as time progressed they moved up to 23 channel CB radios. Still using crystals but in a different fashion. They had what was called an oscillator. matter of fact they had two of them just to create the frequency needed for each channel. When transmitting, the two oscillators, each running at their own frequency set by the crystals, would be mixed together in what is called a mixer. The result would be the difference between the two. For example only, say one was running at 20MHz at the other at 7.125 MHZ. The result coming out of the mixer would be 27.125. a CB frequency. You could hack the radio and get another channel other than CB frequencies by changing a crystal, so that when the two mixed together would be the new frequency. I am sure that is why the question must have been asked,where are the crystals. After the mixer the next couple of stages would be radio frequency amplifier stages until the last stage the output stage where it went to the antenna. The output stage had a couple of coils and capacitors which would need to tuned for the center of the band you were using. In this case for about channel 12. You could tweak on these to get a bit more power and improve antenna matching and audio performance. Most CB radios had an automatic gain control pot at the final stage area. It would send a portion of the signal back to the input to cut the power down. By tweaking on that control you could bypass that function. Now when it came to receiving the signal came in through the antenna to the radio where it was first amplified and some even had an "RF Gain" control which at this point is what it operated, the gain of this stage. The output of this stage went into,you got it, another mixer. The signal coming in is mixed with an oscillator to get a frequency coming out of the mixer of 10.7 MHz. The 10.7 MHz is then amplified by a couple of amplifying stages called "IF" stages. stands for Intermediate Frequency. Then the output from the IF stage would go into yet another mixer with another oscillator"called a local oscillator, the result coming out of the mixer was 455 KHz another standard frequency radios use for the IF stage. This is also another IF stage. The mixer did what was called a conversion, hence double conversion radio. From the last IF stage it went through a rectification stage to turn the radio signal into audio, from there to an amplifier stage so you can hear it. By the way the squelch was placed in the audio circuit to eliminate background noise. Setting the squelch control knob sets the level that a signal has to be more powerful than before it trips it. It is like a comparator in that when the voltage level reaches the trip point it turns on the audio stage so you can hear it.
    Now when the new 40 channel radios came out they eliminated most all the crystals.
    They really only needed one crystal for the transmit stage to do all 40 channels. Yeah right you say? The circuit uses a phase lock loop to come up with the right frequency. How it works Is somewhat like this: the crystal is attached to an integrated chip that has a job of dividing the frequency by a certain number. Like say divide by 11,456. Just an example. The end result would be a frequency which was fed back to the input and then it would lock itself on frequency. Its a lot more than that and complicated. Now to get more channels other than 40 channels all you had to do is figure out the combination. You the 40 channel selector knob on your radio is actually selecting different combinations of the chip input leads. By changing those combinations you could go above channel 40 to 41,42,,,,,60 or more. Which of course is illegal. other than that the new ones are equipped with noise blankers to get rid of power line noise and other noises. I think that is enough detail as I am not writing a book. If any of you care to elaborate feel free. Please no more sarcasm. I am only trying to help. not everyone is at the same learning level. It takes time to answer questions in such detail, time I could spend doing other things. I don't mind helping but I am limited to how much time I can spend. By the way you spelled expert wrong, you spelled it "espert".

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    Laron Aug 24, 2012

    ALL HAIL THE MIGHTY IRONFIST109!!!!!!!!!!!!!! I am a fan not to be confused with stalker. His material makes for a informative and accurate on subject read. Can't help but rate his comments as helpful.

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It is a simple transmission between magnetic wave forms exchanged by two consistent receivers using the sam frequency and modulation. For example, when you key in on a CB microphone, the CB is sending electronic pulses to the mic and when you talk, the waves are sent to the CB and transformed into radio waves by the transmitter and are sent to other components of the CB like if you had an echo board on the CB, if you had the echo activated, the wave would pass through the echo board and would be transformed dramatically depending on how you set the echo, then after the transformation of the waves, they would be sent through your antenna, through the air and then into other people's CB receivers.

Hope this helps!

Posted on Jul 01, 2008

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