Radio Discussion Community

Hiya all again,
If you have been following my last few posts, Ive been given a a box of old CB radio bits n bobs.
Some of witch I've never really messed with before, so I've posted said items in this forum to get a bit of info on
what they do, and why you would use them.

This post is no diffrent I'm afraid

In this box of CB radio goodies ive now found a FM CB Deviation/SWR meter.

The SWR meter side of things is straight forward.ie. FWD/REF,calibration knob.

But there is also a Deviation side to this meter, again it has the two PL259 ANT/TRANS plug sockets on the rear, but also a power lead,(12V) leading from the back to power up.

The meter it self shows a range left to right 0-3KHz.

Am I right in thinking its a gadget to monitor modulation or somewere around that field?

As always any info on this little box would be most appreciated.

yes you are right should read about 2 for fm higher than this and it over modulates

If you want a bit of the background "theory" type of stuff to go with Stampman's practical answer....

Legal CB radios use a type of FM called Narrow Band FM (NBFM) as opposed to Wide Band FM as used by broadcasting and so forth.
The difference between the two types of FM is, "the maximum allowable frequency shift of the carrier when the transmitter is fully modulated ". That shift is called the Deviation.

Apparently (I didn't know this until I just looked it up ), legal CB has a maximum permissable deviation of 3kHz - which will be why your meter is scaled up to 3 - That's a legal max with whatever law (FCC or Euro) and would be one of many compliance mandates for a radio to get a "PR27/95" (or one of the others) "Type approval", or the FCC equivalent thereof. It doesn't mean that a radio will sound right on 3kHz, just that it's legit and operating "within band"

Aah, got ya! 8)

So it could be a good tool for when ajusting your mic gain, or say a new power mic as such, or anything that could create that shift!

Thanks again guys for helping me through this box of boxes! =D>

With out you guys I would be definatly giving it some of this...

Last of the boxes is somthing called a FM super crunch! Dont ask, Ive not a clue!

Sounds painfull to me

In a bit guys..

Billybob..*

Billybob wrote:
Last of the boxes is somthing called a FM super crunch!

It's a speech processor of some sort. I saw one on the auction site and wondered what it was and Offplanet kindly sent me a link to a YouTube video of someone demonstrating the "FM Super Krunch" (I think it was Krunch with a "K" in the vid).

Of course, I can't find the link now, nor the video on YouTube.

Maybe Offplanet can find the link again?

Here 'tis

http://www.youtube.com/watch?v=UwsGd67yOtY

I couldnt really tell any diffrence from when it was in/off line

So I take it's surppose some sort of squelch box in effect!

I dont think this one here of mine is working

Anyhoo, thanks for the video link offplanet, and to steve, for helping sort out these gadgets.

It's starting to get a bit of a CB radio junkyard over here!

'Not complaining', just stating

Boys n toys ay!

Billybob..*

Hi All
Narrow Fm on cb bands is 2.0khz peak deviation for 1khz test tone applied to radio generally.
speech peaks 2-2.2khz acceptable for 10khz spacing channels.
Dont confuse mic gain with fm deviation its not the same.

Fm deviation is where it limits no matter how much input you drive to microphone input on radio.

If you do not have a test tone generator,set mic gain to max and whistle or say "Five" into microphone ,then set Deviation for about 2.2khz peak.
I would not exceed 2.5khz max.

If audio still sound low on transmit the microphone drive level can be increased,either thru a pre-amp mic or by delving in the radios mic pre-amp circuit.

Export radio commonly have the fm deviation set way to high type 4-5khz,this is because USA,North American hams are still using Wide FM 5khz peak systems, not narrow FM max 2.5khz in europe and UK on 10\11m band,in most cases personally I turn them down to meet UK\EU specs as they can over deviate and cause splashing of audio on normal cb radio radio or worse your radio squelch can chop if receiving wide deviation (5khz)from these radios,I get this problem locally in my area with peeps using ham gear on Cb band,generally some some can be switched to narrow fm mode for better compatibility with legal cb radios.
Fm filters on NFM are 6 to 9khz wide sot setting your deviation to high to make it sound louder can cause it to splash wider than the receive filters on other persons radio receiving your fm transmission.
This its why important to set it no further than 2.5khz peak max.
Hope that explains the difference ,I could go a bit deeper if requested.

have fun setting fm dev up!