crystal oscillator for applacition of 150 MHZ

[BOHAA]

crystal oscillator for application of 150 MHZ

Dear all,
I want to implement a crystal oscillator circuit to generate a signal of 150HHZ and the available XLAT I have is 49.86MHz Can anybody help me to find this kind of circuit ?
And through my search in internet I found this circuit. However, I am not quit sure if this circuit will generate the required frequency(150MHz).
Here is the circuit

Thank you alot
Regards

 

[FvM]

An overtone oscillator usually doesn't work well without a tuned circuit (LC-oscillator) suppressing oscillations at the fundamental.

 

[neddie]

You are only going to get 147Mhz. That good enough? You may be able to pull the xtal frequency a bit to get closer to 150Mhz , but you probably wont be able to pull it enough.
The circuit drawn will AFAIK run at the fundamental frequency of 49.86 MHz , you need some sort of overtone oscillator as mentioned above.

 

[BOHAA]

Dear FvM , Dear neddie

Thank you so much for your helpfull comments , I appreciate all your efforts

I still do not understand How I can make the crystal oscillator works at overtone ? How I can get a signal of 150MHz or around it from a crystal oscillator of 49.86MHZ?

Yes, dear neddie that is good for my application circuit but can you please help me to design its circuit ?

 

[FvM]

Overtone crystals have a special cut that promote oscillations at the overtone and inhibit it at the fundamental. They are also tuned to a specified overtone frequency, which is usually slightly different from an integer multiple of the fundamental.

Nevertheless should it be usually possible to make a regular fundamental crystal work at an overtone, but you should use an oscillator circuit with LC tank, as said.

There is however the possibility that the said 50 Mhz is already an overtone frequency, than it's difficult to use a higher overtone. Another option is to run the oscillator at the fundamental with a succeeding frequency multiplier circuit.

 

[neddie]

**broken link removed**
http://jaunty-electronics.com/blog/2012/08/overtone-crystal-oscillator/
**broken link removed**
http://www.axtal.com/data/publ/ukw1979_e.pdf

 

[BOHAA]

Thank again for all of you

Your comments are deeply helpful for me and I appreciate all your efforts

Now I lowered my frequency range down to 100MHz and I tried the circuit above ( Colpitts XLAT oscillator with XLAT 100MHz) but the maximum output frequency was 59 KHz ONLY

I couldn't find what is the problem.

My question is : Is Colpitts circuit above a good choice for XLAT 100MHZ ??

Thank you guys for your time

 

[kripacharya]

Colpitts can be made to work. However in your posted circuit the feedback caps (C2 C3) are too large for the intended freq. Also a 2n3904 might not have sufficient fT, and a better bjt should be used. Maybe S9018 or similar. Also its not clear from your posted circuit what your Vcc is, and hence what your bias point is.

Proper choice of emitter resistor helps. Choose Re ~ 0.5 / (F x C3)

And finally, as has been said here already, crystals above 20-25MHz are usually overtone crystals. Your 49.86MHz is most likely a 3rd overtone xtal. A 100MHz will be a 5th or 7th overtone crystal. To make it work, you have to place a 'trap' between the desired overtone freq, and the immediate lower overtone mode. It is not really necessary to have a tuned LC.

For example :
a 100MHz 5th overtone xtal (fundamental is approx 20MHz) has the 3rd overtone at approx 60MHz. Hence the trap needs to be designed for ~80Mhz
a 100MHz 7th overtone xtal (fundamental is approx 14.3MHz), has 5th overtone at approx 71MHz. Hence trap should be for ~84MHz.

 

[FvM]

And finally, as has been said here already, crystals above 20-25MHz are usually overtone crystals. Your 49.86MHz is most likely a 3rd overtone xtal. A 100MHz will be a 5th or 7th overtone crystal. To make it work, you have to place a 'trap' between the desired overtone freq, and the immediate lower overtone mode. It is not really necessary to have a tuned LC.

Yes. But is designing a trap really more simple than a tank for the intended output frequency? Both will be essentially LC circuits.

 

[kripacharya]

Yes. But is designing a trap really more simple than a tank for the intended output frequency? Both will be essentially LC circuits.

True. But a trap need not be at the precise frequency required. Just anywhere below that but higher than the lower overtone. So we can use simpler fixed inductors/ caps.
And since the higher overtones are naturally of lower magnitude, the xtal oscillates at the desired overtone only.

 

[FvM]

"Tuned" LC tank neither implies exact tuning, depending on the quality factor.

 

[BradtheRad]

It only works for odd harmonics to tune the LC tank to respond to an incoming clock frequency. The idea is for an upward transition to instigate the tank when the sinewave is going upward...

likewise for a downward transition to sync with the downward-going part of the sine.

It does not work when you try to tune the tank circuit to the 2nd harmonic.

Here is a simulation to illustrate.

 

[kripacharya]

It only works for odd harmonics to tune the LC tank to respond to an incoming clock frequency.

This further illustrates that a square wave's fourier transform consists only of odd harmonics of the fundamental, in a certain ratio. A sawtooth however would contain even harmonics as well. Try your circuits with a sawtooth input and see what happens ?

But this is off topic.

 

[BOHAA]

Colpitts can be made to work. However in your posted circuit the feedback caps (C2 C3) are too large for the intended freq. Also a 2n3904 might not have sufficient fT, and a better bjt should be used. Maybe S9018 or similar. Also its not clear from your posted circuit what your Vcc is, and hence what your bias point is.

Proper choice of emitter resistor helps. Choose Re ~ 0.5 / (F x C3)

And finally, as has been said here already, crystals above 20-25MHz are usually overtone crystals. Your 49.86MHz is most likely a 3rd overtone xtal. A 100MHz will be a 5th or 7th overtone crystal. To make it work, you have to place a 'trap' between the desired overtone freq, and the immediate lower overtone mode. It is not really necessary to have a tuned LC.

For example :
a 100MHz 5th overtone xtal (fundamental is approx 20MHz) has the 3rd overtone at approx 60MHz. Hence the trap needs to be designed for ~80Mhz
a 100MHz 7th overtone xtal (fundamental is approx 14.3MHz), has 5th overtone at approx 71MHz. Hence trap should be for ~84MHz.

Thank you Mr.kripacharya for your reply
In the circuit above Vcc is 12V and XLAT of 100MHz

But I have a little confusion about what do you mean by ''''' To make it work, you have to place a 'trap' between the desired overtone freq, and the immediate lower overtone mode. It is not really necessary to have a tuned LC.''''' I mean what do you mean by "trap" ?

Also I fixed Re to 1K ohm and the Cps C3=C2= 5uF ( using the formula you posted ).

Thank you in advance and all of guys have participated actively it is really helpful discussion.

 

[kripacharya]

Not good imo. C2, C3 is way too large. They should be somewhere in the region of less than 50pF ! (There's a rule-of-thumb formula for arriving at this too, but I somehow just can't remember it right now.)

Then Re would be ~ 0.5/(100e6 * 50e-12) = 100 ohms. A multi-turn here can be useful, in series with a limiting resistor for safety.
This also results in a bias current of around 20mA, which is a bit high. So either lower your +12v to around +8v, OR reduce your 3.3K to 1.5K or 1.2K

An alternate to colpitts is the Butler which seems to be the popular choice for overtone operation. Here's a writeup of one such:
**broken link removed**