How do I select and design a crystal oscillator?

Thread Starter

mcardoso

Joined May 19, 2020
116
Hi All,

I am finalizing a clocked circuit design. I need to actually implement a clock signal and I was hoping to get some guidance on selecting appropriate components for the clock.

Requirements:
Minimum Frequency (3x max input signal frequency): 2MHz
Maximum Frequency (slower than worst case propagation delay path + setup time): 18MHz
Nominal Frequency: 10MHz - seems reasonable but open to discussion
Voltage: 5V CMOS (driving clock inputs of CD74AC175 and CD74AC109), I think?
Fanout: Qty. (11) CD74AC175 + Qty (3) CD74AC109

I see these are crystals (XTAL) and oscillators (OX XO). It seems like everything I have read points me to use an oscillator, but when I go look at Digikey to pick one, they are crazy expensive ($10-200 each). What am I missing here? What is the best design for this part of the circuit?

-Mike
 
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Thread Starter

mcardoso

Joined May 19, 2020
116
OK, more searching is showing me that these are easier to find in 4, 8, & 16MHz flavors. Unsure the best way to determine if a particular crystal can drive all my chips?
 

Thread Starter

mcardoso

Joined May 19, 2020
116
Is there some reason you need a SAW (Surface Acoustic Wave) oscillator?
Newbie picking out his first crystal (well second if you count my wife's engagement ring) ;)

No, but in all honesty I don't know what I need. A week ago I was first reading what clocked circuits were.

Hi there. The oscillator should not be an afterthought when designing.I hope you have a wave
Solder set up.& Why SAW oscillator?
This is hobby project stuff. Hot air rework station and soldering iron only. Is the crystal at risk of damage?
 

Thread Starter

mcardoso

Joined May 19, 2020
116
My guess from your comments is you want an oscillator , not a crystal.
an oscillator is all the parts , including the crystal / resonator, capacitors and buffers, in a package, so it always oscillates, crystals are very dependent upon layout.

try one of these as an example
https://www.digikey.co.uk/product-detail/en/abracon-llc/ACH-16-000MHZ-EK/535-9171-5-ND/675366
I think you are correct. I was hoping for something that will give me a clean clock signal with no fuss about failing to start.

If there are so many elements involved in crystal design, why doesn't everyone use an oscillator?

Also, any reason I shouldn't go for a smaller SMT oscillator as opposed to the 1/2" square radial package you linked? A smaller package would make my design easier.

Finally, any concern with the 14 chip fanout from the oscillator you linked? If there was, how would I know from the datasheet?
 

andrewmm

Joined Feb 25, 2011
395
Why does not every one go for a oscillator over a crystal. cost. In bulk crystals and a buffer are cheaper.

SMT package, perfect, most are. As this is primarily an armature forum, SMT causes some people problems.

driving 14 loads. not enough info there. Are they all the same type of chip, are thay always powered, what frequency, what voltage. generally , if driving more than one or two loads,I'd put a buffer after the oscillator, gives the cleanest clock, such as

https://www.digikey.co.uk/product-detail/en/nexperia-usa-inc/74LVC1G125GW-125/1727-3488-2-ND/1023280

Just check the voltage / current you need ( add up all the current sin at Vin Low and Vin High of your receivers. )
 

MrChips

Joined Oct 2, 2009
21,335
How stable and accurate do you need the clock frequency to be?
You can build a very reliable 2MHz with RC and a Schmitt trigger circuit.
 

Thread Starter

mcardoso

Joined May 19, 2020
116
Why does not every one go for a oscillator over a crystal. cost. In bulk crystals and a buffer are cheaper.
Makes sense. Only making 6 of these so the couple of cents makes no difference.

SMT package, perfect, most are. As this is primarily an armature forum, SMT causes some people problems.
Fair. I've done maybe 10 PCBs with 0603 and SOIC/TSSOP chips with good success. I think as long as I'm not getting anything stupid small I should be OK.

driving 14 loads. not enough info there. Are they all the same type of chip, are thay always powered, what frequency, what voltage. generally , if driving more than one or two loads,I'd put a buffer after the oscillator, gives the cleanest clock, such as
I will be driving Qty. (11) CD74AC175 + Qty (3) CD74AC109. These are CMOS ICs powered at 5V and whatever frequency oscillator I pick (probably 8MHz). The datasheets are linked in the first post. I don't see anything for current draw at the clock input on the datasheet. I'm good with the buffer. The one you linked looks fine for 5V operation I think.
 

Thread Starter

mcardoso

Joined May 19, 2020
116
How stable and accurate do you need the clock frequency to be?
You can build a very reliable 2MHz with RC and a Schmitt trigger circuit.
Honestly not accurate at all. If I understand everything correctly, anything between 2 and 18MHz will be acceptable to sample the input. I picked 8MHz since it is right in the middle and shouldn't risk any issues with drifting off the ends of the acceptable range. It can even drift in that range while it is running for all I care.
 
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