Crystal oscillator

hello,

I am trying to understand if there are differences between various crystal sizes (at same parameters), can you recommend an guide ?

I am particularly interested to know if there are differences between HC49/HC49R packages (tall ones) and HC49-S package (short one) and what is the role of the capacitors connected to ground ?

I have an arduino uno r3 and I have noticed that there are two different oscillators on the board, one for 16U2 and another one (smd?) for the 328P, and this one does not seem to have any capacitors, why ?

thanks,
razvan radu

Different sized crystals will have different tolerances, temperature profiles, etc. Bigger ones are often (though not always) more accurate and stable at a wider range of temperatures.

As for the 328P crystal - it isn't.

If you look carefully, it actually has 3 pins, not 2. This is a crystal (or sometimes ceramic) resonator. It's basically a crystal with the capacitors built in - the middle pin connects to ground. It's used to save space, and the internal capacitors can be better matched to the crystal to give more accurate frequencies.

ok, so the one for 16U2 is an design choice ?
can an identical resonator (to the one used for 328P) be used for the 16U2 as well ?

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razvan radu

It could, but as it is so much smaller it is less accurate and has worse tolerances. As the 16U2 is communicating over USB timing is absolutely critical, so a crystal with better tolerance is better. For synchronous, and low speed asynchronous (SPI, I²C, RS232, etc) communications the timing isn't as critical, so a cheaper and less accurate crystal or resonator can be used.

This is a crystal (or sometimes ceramic) resonator. It's basically a crystal with the capacitors built in - the middle pin connects to ground. It's used to save space, and the internal capacitors can be better matched to the crystal to give more accurate frequencies.

Well I would instead say all 3 pin resonators with built-in caps as used on micro-controllers are made from ceramic material and all 2 pin quartz crystals resonators are made from of course quartz crystal material. There are also 2 pin ceramic resonators available. The word resonator describes an electrical function, while the words ceramic and quartz describes the material the specific resonator is made of. Both the ceramic and quartz material used in resonators are in a crystalline lattice form, so there can be a lot of missed assumptions and possible confusion when trying to describe these specific components.

Lefty

Well I would instead say all 3 pin resonators with built-in caps as used on micro-controllers are made from ceramic material

Agreed - when it's a quartz crystal and not ceramic it tends to be termed an "oscillator" not a "resonator".

majenko:

Well I would instead say all 3 pin resonators with built-in caps as used on micro-controllers are made from ceramic material

Agreed - when it's a quartz crystal and not ceramic it tends to be termed an "oscillator" not a "resonator".

That again is subject to misunderstanding and is not proper word usage in my opinion. Quartz crystals and ceramic resonators are just 'passive' components. For a resonator to actually generate a frequency signal requires it to be used in an electrical circuit that includes active amplification and proper signal feedback which then in total will become an oscillator. In the case of a AVR mega chip the active oscillator components, minus the external frequency controlling resonator is built into the chip, thus creating a crystal or ceramic oscillator. And most AVR chips also have an optional internal fixed frequency (often 8Mhz) R/C 'resonator' so that one can eliminate the need for any external resonator if they so desire. Note that there are also available external crystal (or possibly ceramic) 'oscillator modules', which are generally DIP or SM packages of various sizes that include both the resonator and oscillator electronics all in one package. Such modules directly create a specific output signal (both square and sine wave are available) Without any additional components.

I know I might appear to being picky on this topic, but I've found that there is often a lot of misunderstanding and errors made when describing these components and what function they actually perform. I've been fascinated by crystal resonators sense I was exposed to ham radio in the 60s (they really are amazing devices), so I think I have a good grasp on the subject.

PS: Here is a listing showing both crystals and crystal oscillators to clearly show the difference:

Lefty

if I may suggest, maybe you can write a guide about them

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razvan radu

I've been fascinated by crystal resonators sense I was exposed to ham radio in the 60s (they really are amazing devices)..
Why in 60s? - the qrp hams love them even today - they can pull them a lot :slight_smile:

majenko:
Different sized crystals will have different tolerances, temperature profiles, etc. Bigger ones are often (though not always) more accurate and stable at a wider range of temperatures.

As for the 328P crystal - it isn't.

If you look carefully, it actually has 3 pins, not 2. This is a crystal (or sometimes ceramic) resonator. It's basically a crystal with the capacitors built in - the middle pin connects to ground. It's used to save space, and the internal capacitors can be better matched to the crystal to give more accurate frequencies.

Ceramic resonators are not quartz and about 1000 times worse in performance. They are used because they are cheaper and/or because they take less time to start resonating (due to the lower Q-value). Because they don't have to be vacuum-mounted the load-capacitors are usually built-in.

Very low frequency crystals need larger cans, but the larger cans were the only size made in the early days before miniaturization.
Very very low frequency crystals are cut as a tuning fork shape and this allows a much smaller package than otherwise (32768Hz crystals are like this)