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Author Topic: Why 2 Crystals in older CB for TX and RX?  (Read 9203 times)

Posts: 83

« on: May 25, 2012, 05:37:15 PM »

I have some old CB radio's that fell into my possession. Junk but fun to tear apart.  Now... I understand about frequency mixing and how most CB's used to work but for some reason I can't wrap my headaround this one.
Why did it take 2 crystals, one for TX and one for RX, to make these old things work on just 1 channel?  I'm not sure why the RX crystal was at a lower frequency.  Can someone simple explain this as I feel I am trying to hard on what I assume is a simple explanation.



Posts: 4450

« Reply #1 on: May 25, 2012, 06:04:32 PM »

Do some research on the inner workings of a superheterodyne receiver and you'll find the answer...

In many CB's of the era the 1st IF (or only IF) was at 1650 kHz. This means the local oscillator (usually) ran 1650 kHz higher than the intended receive frequency. In other cases 455 kHz. This means the RX crystal determines the local oscillator frequency and is chosen with an offset equal to the 1st IF. Meanwhile, on the TX side, there is no IF nor any need to heterodyne. The TX crystal was cut for the actual RF frequency with no concern about an offset.

So... The RX crystal usually runs 1650 kHz higher than the TX crystal for any given channel. It's also possible for the RX crystal to run lower as the 1st mixer can go low side or high side injection. It's the differential that matters. If the crystals are marked by frequency as well as channel, calculate the frequency difference between the two and that's the RX IF.
« Last Edit: May 25, 2012, 06:08:42 PM by AC5UP » Logged

...says a lot about our society that Martin Shkreli went to prison for defrauding investors but not for price gouging lifesaving medication   -   Ken Klippenstein

Posts: 2475

« Reply #2 on: May 25, 2012, 06:05:24 PM »

The mixing scheme for transmit and receive in the older CB rigs required one crystal for each function for each band.

Dick  ADU

Posts: 404

« Reply #3 on: May 25, 2012, 08:35:43 PM »

This lesson is not for 23 channel CBs. Hetrodyning

Take an one of the radios you will see the two banks of crystals. now look around for other crystals by it them self. Most of the older CBs were dual conversion. The difference of the two banks minus the IF crystal frequency should yield about 28 channels the mixer crystal for the A or rc channels not in the bank of crystals. If you look at the frequencies on the crystals the pairs will have a difference  total somewhere between 26.965 and 27.255 MHZ  if memory serves me correct. the TX/RX frequency is what 455 KHZ ? or in your case 1650 KHZ . I seem to remember just a couple of mixer sets for am, now SSB had a whole different set of rocks.

Now think of it Rock bound VHF and UHF radios have formulas also. Figure the 11m radio out and the same methodology works fo the FM radios.
het·er·o·dyne  (htr--dn)
Having alternating currents of two different frequencies that are combined to produce two new frequencies, the sum and difference of the original frequencies, either of which may be used in radio or television receivers by proper tuning or filtering.
tr.v. het·er·o·dyned, het·er·o·dyn·ing, het·er·o·dynes
To combine (a radio-frequency wave) with a locally generated wave of different frequency in order to produce a new frequency equal to the sum or difference of the two.
[hetero- + -dyne, power, frequency (from Greek dunamis, power; see dynamic).]
The American Heritage® Dictionary of the English Language, Fourth Edition copyright ©2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.
heterodyne [ˈhɛtərəʊˌdaɪn]
(Electronics) Electronics to combine by intermodulation (two alternating signals, esp radio signals) to produce two signals having frequencies corresponding to the sum and the difference of the original frequencies See also superheterodyne receiver
(Electronics) produced by, operating by, or involved in heterodyning two signals
Collins English Dictionary – Complete and Unabridged © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003

Posts: 83

« Reply #4 on: May 25, 2012, 08:52:51 PM »

Do some research on the inner workings of a superheterodyne receiver and you'll find the answer...

I know about a superheterodyne but it just didn't hit me for some reason.  Thanks.  Yes, after doing some research on the CB it uses a 455khz IF on the receive and hence why the TX and RX are separated in frequency by 455khz (RX being low side injection). Just seems like such an odd way of doing things.  Figured they would just as the same crystal to TX/RX as a cost savings and/or for simplicity.



Posts: 1757

« Reply #5 on: May 26, 2012, 01:48:43 AM »

When two frequencies mix, you end up with 4 frequencies.

2 are each fundamental.

1 is the difference between the two fundamentals.

1 is the sum of the two fundamentals.

This mixing of two frequencies is called Heterodyning.

In the case of your CB radio, the crystal frequency is mixed with the received RF in order to create the intermediate frequency which is the difference between the two. If the crystal is 455khz below the rx frequency, the resulting mix creates a difference at 455khz. The next stage filters out the other three frequencies (and all other frequencies around 455khz) leaving only the 455khz RF on the output. This is then amplified and filtered, typically three times total.

Typically you have a bandpass filter, your first RF amplifier, the mixer which creates the 455khz IF (intermediate Frequency) and typically three tunes stages which continue to refine and remove all the other frequencies.

This then goes to a demodulator ( in the case of CB, it's an AM or SSB demodulator) and then the audio amplifier chain and eventually to the speaker.

This type of receiver is called a Superheterodyne and is well explained here:

In other designs an adjustable oscillator is substituted for the crystal so you can change frequencies in a variable fashion. This is called a VFO or Variable Frequency Oscillator. The rest of the stages remain the same however many modern radios do dual conversion and have two (or more) intermediate frequencies. If I recall correctly, the FCC called for crystal control on CB since VFO designs had been around for decades in the 50's.

Here's another web page that describes the issues with a single IF and why it evolved into a double conversion system:
« Last Edit: May 26, 2012, 02:02:36 AM by N4CR » Logged

73 de N4CR, Phil

Never believe an atom. They make up everything.

Posts: 82

« Reply #6 on: May 29, 2012, 07:27:16 AM »

Not just CB radios, but the early 2 meter FM radios also required separate crystals for Tx and Rx, as this was before the days of stable, affordable, VHF PLL's.  And to make it seem more complex, there were multiplication stages after the respective oscillators that required additional math to arrive at the desired frequency for the crystals.  I still remember figuring it out with paper and pencil, as this was in the dark ages before home computers or even simple calculators.


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