27Mhz Crystal

[patroclus]

I want to get a 27Mhz clock signal.
I got a 27Mhz quartz crystal. I usually used a few caps and resistors along an HC inverter to get the clock signal, but never used more than 4Mhz.

for example :

**broken link removed**

Can I do the same for the 27Mhz crystal?? Do caps and resistors need to change?? I can't find clear information about this.
Thank you.

 

[Optikon]

I want to get a 27Mhz clock signal.
I got a 27Mhz quartz crystal. I usually used a few caps and resistors along an HC inverter to get the clock signal, but never used more than 4Mhz.

for example :

**broken link removed**

Can I do the same for the 27Mhz crystal?? Do caps and resistors need to change?? I can't find clear information about this.
Thank you.

You can usually. Try it with 20 pF capacitors. It will also depend on the characteristics of the buffer and you may have to tweak the 1Meg resistor.

I did this with an AT cut 30MHz crystal not too long ago.. I am of course, assuming you have a similar type.

 

[Nigel Goodwin]

You need to be careful of the type of crystal you get! - by 27MHz, some crystals are overtone crystals, which means their fundamental resonance is a sub-multiple of their marked value. This means you MUST use an LC oscillator design to get the desired frequency.

Assuming you're talking about clock crystals?, a 27MHz one should be a fundamental crystal - but an RF crystal could well be overtone.

 

[patroclus]

I really don't know much about quartz crystals yet.
I only used 4Mhz one with the CMOS inverters, appart from clocking PICs.
My 27Mhz crystal is like this one :
**broken link removed**

but it says "HEC 27.000Mhz".
That's all...

 

[patroclus]

nobody knows how can I make this crystal to give me a clock signal of 27Mhz??

 

[JimB]

The picture of the crystal is not much help, it is impossible to determine the characteristics just by looking.

I suggest that you build the oscillator using the basic inverted circuit and measure the output frequency, if it is 27 Mhz - you have cracked it!
If it is only 9Mhz, you need an overtone circuit.
Read the article in your link, it gives information on how to make an overtone crystal work in that circuit.

JimB

 

[Sebi]

This will work...

 

[patroclus]

if it is only 9Mhz, you need an overtone circuit.

Why could it give only 9Mhz? what does overtone mean in this context??
Isn't it a simple circuit to make any 27Mhz crystal to oscilate?

thanks

 

[stevez]

Just a note of caution - in the overtone mode, the overtone frequency is close to an intergral (odd) multiple but not an exact integral multiple.

I used to use 7 mHz crystals for the 21 mHz band - the oscillator was configured as a tripler, if I am not mistaken. In this case the resulting output was an exact intergral multiple.

 

[JimB]

Why could it give only 9Mhz? what does overtone mean in this context??
Isn't it a simple circuit to make any 27Mhz crystal to oscilate?

thanks

A crystal operates by mechanically vibrating at the required frequency. For low frequencies a larger piece of quartz is used than for higher frequencies.
As the frequency goes up, the piece of quartz gets smaller and thinner, eventually it gets too small to be practical.

So for higher frequencies, the quartz vibrates on an overtone, 3, 5 or sometimes 7 times the fundamental frequency.

Just plugging a crystal into an oscillator will usually result in it oscillating on its fundamental frequency.
If the oscillator is designed for overtone operation, the crystal can vibrate at one of the overtone frequencies.

A 27Mhz crystal I would expect be a 3rd overtone type. So to operate at 27Mhz you need an overtone oscillator circuit. If you put the crystal into an ordinary oscillator you will get the 9 Mhz fundamental.

JimB

 

[Nigel Goodwin]

Incidently, presuming you want a logic level 27MHz for a micro or something?, I happened to notice the other day that Sky Digiboxes (UK digital satellite receivers) have a 27MHz oscillator module in them!.

 

[danielsmusic]

are you sure 27MHz is not too fast, some chips will not much. for example:
a 4024 will not take more than 5MHz.

i don't think you will need caps if you want full speed leave them out

i have tested bothe of these drawings with and without caps.

 

[patroclus]

danielsmusic, I used thouse circuits when using 32KhZ and 4Mhz crystal, but I don't know if it would work with 27Mhz. also, I don't know how to calculate the caps and resistor values for sure, for a certain crystal...

But, how do I know if my 27Mhz crystal is overtoned crystal, and it vibrates as a 9Mhz? How can I make it oscilate? (a simple circuit?). And if it vibrates at 9Mhz, how can I build an overtone circuit??

Thanks!

 

[JimB]

For info on crystals and oscillators, look here:

**broken link removed**

JimB

 

[patroclus]

If I buy a 4 pin oscillator, Will It work stand alone??
I mean, I won't need to use any external circuit to get the 27Mhz signal, will I??

Thanks.

 

[Nigel Goodwin]

If I buy a 4 pin oscillator, Will It work stand alone??
I mean, I won't need to use any external circuit to get the 27Mhz signal, will I??

Thanks.

That should be fine, just power, ground, and the output pin!.

BTW, what do you want the 27MHz clock for?.

 

[patroclus]

To clock a CPU in an old videogame system (NES). It is clocked with a 24Mhz clock, I think, but I want to make it run a bit faster. Just experimentations

 

[Nigel Goodwin]

To clock a CPU in an old videogame system (NES). It is clocked with a 24Mhz clock, I think, but I want to make it run a bit faster. Just experimentations

Depending on how the system is designed, changing the crystal frequency may well prevent it working at all?, if the video generation is run from this crystal.

 

[JimB]

A crystal oscillator module does "exactly what it says on the tin", it gives out a square wave at the stated frequency.
No external components needed.

JimB

 

[Joel Rainville]

To clock a CPU in an old videogame system (NES). It is clocked with a 24Mhz clock, I think, but I want to make it run a bit faster. Just experimentations

Hey, that's interesting. Let us know the results!

I'm guessing vsync problems and weird music, but it all depends on how the clock signal is used in various parts of the system.