Need help understanding crystal overtones

[KC9YTD]

I apologize for the long-windedness, but if anyone around this board understands how this works please bear with me, I think I've got 90% of it understood, but maybe not!

For this particular situation the 4ch scanner says it's VHF Hi only (148 - 174 Mhz).

Are the overtones determined by the individual crystal? Or are they determined
by the scanner? If they are determined by the crystal, would it be possible to
use some 1st Overtone crystals and some 3rd Overtone crystals simultaneously?
(Or are ALL VHF Hi band crystals 3rd overtone crystals?)

Based on the VHF Hi conversion formula (Crystal Frequency Needed = Desired Frequency - 10.7 Mhz, divided by 3), could I use a 50.616 Mhz
crystal in this scanner that only receives 148 - 174 mhz, when it's a 3rd
overtone crystal? (because to get 162.550 Mhz that I need, the formula would go
162.550 - 10.7 / 3 = 50.616 Mhz)
Also, could I get a 146.685 Mhz channel to happen, even though it's outside the
148 - 174 Mhz capability of the scanner by using a 45.328 Mhz 3rd Overtone
crystal?
And at the opposite end of the spectrum from those 2 examples: Could I get a
456.350 Mhz channel to operate, even though it's outside of the 148 - 174 Mhz
VHF Hi spectrum by using a 148.55 Mhz 3rd Overtone crystal? The formula would
say 456.350 Mhz ( Desired Freq) - 10.7 Mhz divided by 3 = 148.55 Mhz Crystal
Frequency, which is within the scanner's operating VHF Hi range.

If I'm not able to use the 3rd overtone crystals to receive beyond the scanner's natural 148 - 174 Mhz band, what is the point of confusing the @#$% out of everyone? I noticed this while reading this link someone else in these forums posted elsewhere:
http://www.n4mw.com/Regency/actr10hlusm.pdf
Reading this again makes it seem like the range the actual Crystal Freq falls in (VHF Lo, VHF Hi, UHF) determines which overtone formula should be used: VHF Lo: Crystal Freq = Desired Freq + 10.7.........UHF: Crystal Freq = Desired Freq - 10.7, divided by 9.....
Sorry again for the novel, I condensed it down a couple times before submitting!

 

[n5ims]

Crystals are cut to work at specific frequencies and crystal based radios require them to be cut as needed for that radio's circuitry. The overtone method isn't designed to make things harder, but to make them more reliable. If you cut your crystal outside of a known range of values, they either don't work, or may break easily, so to get reliable crystals, they keep them in this range and multiply (or divide as appropriate) the crystal's frequency to get the one you need for your radio. If the crystal is too thick, it won't vibrate and if it's too thin it will crack or break apart when it vibrates. When it vibrates correctly, the frequency will be just right and your radio will work.

 

[zz0468]

Are the overtones determined by the individual crystal? Or are they determined
by the scanner?

Yes - meaning, both play a part. The crystal has a fundamental frequency that is the lowest frequency that it will vibrate at. It will also vibrate at select harmonics - these are referred to as overtone modes. The radio that the crystal is intended for will be optimized for the desired overtone mode, and the crystal itself will be tuned such that at the desired overtone mode, the frequency it produces will be at exactly the desired frequency.

If they are determined by the crystal, would it be possible to use some 1st Overtone crystals and some 3rd Overtone crystals simultaneously?

Not likely. There are many factors in the radio itself that determine how the crystal operates, so crystals are usually cut for a specific make and model of radio. Now, there was a great deal of commonality between different makes and models of crystal scanners, but it wasn't universal, by any stretch of the imagination.

(Or are ALL VHF Hi band crystals 3rd overtone crystals?)

Most probably were, but 5th overtones were used in some radios.

Based on the VHF Hi conversion formula (Crystal Frequency Needed = Desired Frequency - 10.7 Mhz, divided by 3), could I use a 50.616 Mhz crystal in this scanner that only receives 148 - 174 mhz, when it's a 3rd overtone crystal? (because to get 162.550 Mhz that I need, the formula would go 162.550 - 10.7 / 3 = 50.616 Mhz)
Also, could I get a 146.685 Mhz channel to happen, even though it's outside the 148 - 174 Mhz capability of the scanner by using a 45.328 Mhz 3rd Overtone crystal?

Just to clarify the overtones a bit, a 50 MHz 3rd overtone crystal is actually a 16.6667 MHz crystal that's being made (by virtue of the oscillator design) to oscillate at 50 MHz. The X3 function in the formula is actually occurring in the local oscillator/multiplier chain, so that 50 MHz oscillator then makes a 150 MHz injection signal for the mixer. Adding the IF frequency of 10.7 MHz would make the receiver operate at 160.7 MHz... all from a crystal that COULD oscillate at 16.6667 MHz.

The reasons for using crystals in their overtone modes was somewhat covered by the previous post, and relates to the physical requirements of making a reliable crystal. A crystal that actually vibrates at 50 MHz in it's fundamental mode could be too thin, and easily broken. So a thicker crystal is made. But a thicker crystal will oscillate at a lower frequency, so it's operated on a harmonic, or overtone.

Does that make any sense at all?

And at the opposite end of the spectrum from those 2 examples: Could I get a 456.350 Mhz channel to operate, even though it's outside of the 148 - 174 Mhz VHF Hi spectrum by using a 148.55 Mhz 3rd Overtone crystal? The formula would say 456.350 Mhz ( Desired Freq) - 10.7 Mhz divided by 3 = 148.55 Mhz Crystal Frequency, which is within the scanner's operating VHF Hi range.

No. The actual operating range of the receiver is determined by components in the receiver itself, not the crystal. A VHF only radio won't operate on UHF just by putting a UHF crystal in it.

That being said, many UHF crystals will cause the VHF receiver to operate just fine on some VHF frequency, but you would have to understand the the parameters of the crystal to make anything other than random use of it.

If I'm not able to use the 3rd overtone crystals to receive beyond the scanner's natural 148 - 174 Mhz band, what is the point of confusing the @#$% out of everyone?

The point isn't to confuse everyone. It's a complex subject that only makese sense when you understand the basic principles of electronics in general, and radio in particular.

I noticed this while reading this link someone else in these forums posted elsewhere:
http://www.n4mw.com/Regency/actr10hlusm.pdf
Reading this again makes it seem like the range the actual Crystal Freq falls in (VHF Lo, VHF Hi, UHF) determines which overtone formula should be used: VHF Lo: Crystal Freq = Desired Freq + 10.7.........UHF: Crystal Freq = Desired Freq - 10.7, divided by 9.....
Sorry again for the novel, I condensed it down a couple times before submitting!

No need to apologize! It's a good set of questions. Just remember, the overtone mode of the crystal is not related to the multiplication factor in the local oscillator chain. It's two separate multiples, and you really only need worry about what the radios multiplier is doing, and not about whether it's a 3rd, or 5th, or 7th overtone crystal. THAT part is built into the specifications for the crystal itself, and the crystal manufacturer would know what to do based on the make and model of the radio.

 

[WA1ATA]

The original poster also asked if he could use this scanner to receive 146.685MHz by using a 45.328 Mhz 3rd Overtone crystal? I started to answer that it would work, but at slightly reduced sensitivity due to the front end 148-174 selectivity.

But the other question is whether he would be better off using that crystal freq, or using the crystal freq calculated for 146.685 + 10.7+10.7 = 168.085MHz ??

I'm also curious about how a single conversion receiver with 10.7MHz IF can handle the whole 148-174 band without additional per-channel tuning?

For example, if the formula the original poster quoted is correct, then the 3rd harmonic of the local oscillator is 10.7MHz below the desired freq. To receive 174MHz, that would be local osc freq of 174-10.7 = 163.3MHz.

But in all of the mixer designs that I've seen, this would also convert 152.6MHz into 10.7MHz (163.3-10.7 = 152.6). The reception of 152.6MHz would be reduced only by if it is rejected by the front end section of the scanner. But in this case, 152.6MHz is in the 148-174MHz passband of the front end, so I would expect this signal to come through at full strength.

What am I missing?

 

[zz0468]

Image rejection is a problem, but it's possible to build image rejecting mixers. I don't know if any specific models of scanners used that technique, but I would imagine so. You can get 25-35 db of image rejection just in the mixer design, and that seems about along the lines of the performance of the old crystal controlled scanners. A strong signal on the image frequency would come blasting in loud and clear, though. And that's just the way it was.

 

[WA1ATA]

Thanks for the tip, zz0468, I had a feeling there was a simple explanation that I was missing.

I just did a bit of web search on image rejection mixers and found a good explanation at Image Rejection Mixers and Image-Rejection Mixer Catalog, Image Reject Mixers, Image-Reject Mixer. And Image Rejection Mixers - Microwave Encyclopedia - Microwaves101.com has some plots of gain and phase errors vs. image rejection.

Image rejection mixers work by doing two rf paths, with one shifted by 90 degrees in a wideband phase shifter. Then both signals are applied to identical mixers. Then the output of the two mixers are added back together. There is another 90 phase shift in the mixers, but of opposite signs for the desired and image freqs. So when the output of the two mixers are added back together the desired signals add, while the image freq is rejected (to the extent of the matching of the two rf and mixer channels).

It sounds complicated at first glance, but the phase shifter can be just a small coil and capacitor network, and the duplicate mixer just means a few more diodes.

 

[KC9YTD]

Thanks for the replies. The only thing I was having trouble with was how exactly the overtones worked, and how they could work in your favor. I've played with these old crystal scanners since I was little, and I had never even heard of overtones and using crystals for frequencies other than what they were originally intended for. I probably had cheaper or average crystals, etc when it sounds like a crystal that could oscillate at 16.67Mhz all the way to 167Mhz would be much higher quality.
I was hoping there would be a way for me to use this old Regency scanner that has been around forever, making it useful again picking up the skywarn repeaters that are just below it's VHF Hi frequency range. I can see on the circuit board where I think those frequencies would be filtered out even if I did find a crystal in VHF Hi using the 3rd overtone calculation. I've probably thrown away a lot of good crystals not realizing they act differently in each scanner.
Thanks again..

 

[jackj]

You might be able to re-tune that Regency scanner to cover at least part of 2 meters. You'll need some test equipment and some info on the receiver so you know which coils to crank on.

You have gotten some good info on crystal oscillators. An overtone is just a harmonic, nothing special about them. There are several ways to cut a crystal for different operating modes and an overtone is just one of the several modes. Also it is possible that the oscillator will be followed by a multiplier stage that will output the 2nd, 3rd, or 5th harmonic from the oscillator. So your 3rd overtone crystal oscillator running at a base frequency of 16.6666 Mhz puts out 50 Mhz and is followed by a multiplier stage that outputs the 3rd harmonic at 150 Mhz.

 

[KC9YTD]

Yah it's always nice to be able to talk with people that know their stuff. That level of knowledge only exists with people that really love this stuff. I'm a member of several different forums, mainly in electronics (an arcade games board, audio equip, computers, etc), and the way people are helpful and willing to share what they know is unique amongst this group of people. It would have taken me a day and a stack of books to absorb what I got from 1 post.
Thanks