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Technical Question: Why are CB radios always off frequency?

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SCPD

QRT
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Seems like a lot of CB'ers that I see on the SDR are always 450hz, 300hz, 150hz off frequency. Are the radios *that* bad or are some users just using older radios?

Do the newest radios have provisions for calibration?
 

KI4RDO

Silent Key
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Have you adjusted the frequency correction on your SDR? Might want to check that first

Sent from my SAMSUNG-SGH-I337 using Tapatalk
 

fuzzybr

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off freq

I think what you may be seeing is ,on ssb cb radios the upper and lower portion is 1.5 from center freq. such as 27.405 am 27.4065 upper and 27.4035 lower .
 

SCPD

QRT
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I think what you may be seeing is ,on ssb cb radios the upper and lower portion is 1.5 from center freq. such as 27.405 am 27.4065 upper and 27.4035 lower .
Okay that's good to know. Actually, I was referring to just the standard AM channels... just seems like a re-occurring theme that most CB'ers are off frequency.

In fact, most of the SSB users are generally on-frequency.

PS. You can hear these guys right now on 27.025 (well, +/- 1000hz)
 

n9mxq

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Import radios, or radios with tied clarifiers.. Without a freq counter, it would be hard for them to tell unless they were way off.
 

majoco

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Perhaps the older AM rigs are off-frequency due to the master PLL crystal ageing. 450Hz in 27MHz is not unreasonable after say ten years and no-one would notice any loss in signal. Some early PLL synthesizers used strange algorithms to generate the required frequency which always were 'slightly' off! How many CB'ers actually have the test gear to diagnose anyway?
 

ko6jw_2

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You are talking about an error of less than 2 parts per million. My Yaesu FT-817 is 4ppm without a TXCO. Two ppm for a CB radio is not bad. What to you want WWV?
 

Dawn

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K06jw and majoco beat me to the post while I was typing this.

Look at your numbers. CB radio last time I checked was spec'd at within +/- .005% of the assigned channel carrier freqeuency. If you're getting those numbers, those are well within spec.

Keep something else in mind. Many old radios out there and crystals drift with age. Most AM sets of 23 channel days had no means of trimming the synthesizer mixing crystals unless they were high end. Most sideband units did. Later 40 channel PLL units have only the chip reference and a mixer rock for AM only sets. Less probable of those units being off all over the place, but off in the same direction. Given that a bulk of the radios out there are 40 channel sets around 20+ years old with no maintainance, your observations are quite conservative.

Your SDR probably has a high accuracy TCXO in it and can maintain very high stability with all conversions at rf locked to the reference.

Add the effects of a hot vehicle in summer sun or the cold of the winter on unheated, unovend, or TCXO crystals or reference/mixer crystals in the transmitter, you have a recipe for a lot of slop one way or the other.

Also take into account that many of these radios are tampered with, especially the clarifier on SSB sets that provides almost a complete channel to channel slide that's relatively unstable by comparison to the original design and that virtually no cber will take a radio in for periodic frequency checks, numbers like that are quite justified. CB was designed to be a compatively inexpensive communications medium unlike land mobile with relatively lax specifications. 450hz is about half the required minimum spec. Considering the variables involved, that's not too bad for a channelized medium with 10kc spacing.
 
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