How to build Crystal 3H OSC?

[tony_lth]

Hi,
I wan to build a 150MHz OSC with a 50M crystal.
There are several solutions：
1. Amp 50M then filter out 150M (3H) and then amp, too complex.
2. Use a 150M 3H cyrstal, but I don't know hoe to build this circuit?
I knew how to build a fundamental crystal circuit, but no knowledge about 3H osc.
What is the key point of this 3H?
What is the simplest circuit of 3H?
Many thanks.
Tony Liu

 

[KlausST]

Hi,

2. Use a 150M 3H cyrstal, but I don't know hoe to build this circuit?

--> use the datasheet of the 150M 3H crystal

Klaus

 

[tony_lth]

Could anyone recommend a vendor of 3H crystal? So that I can download its datasheet.

 

[Warpspeed]

There are many different circuits for third overtone crystal oscillators that you can easily find on the internet.
But the basic idea, is that the oscillator always has an LC resonant circuit tuned to 150 Mhz, to provide a lot more gain at 150 Mhz than at 50 Mhz.
That forces the crystal to oscillate at its third harmonic.

 

[c_mitra]

The max output will be always at the fundamental but if the driver circuit is tuned to the third harmonic, then the feedback gain at the third harmonic will be sufficient to cause oscillation. It has more to do with the driver circuit than the crystal (it must have sufficient output the 3H though)- just you need to choose a crystal that has decent output at the 3rd harmonic.

 

[tony_lth]

Thanks, Warpspeed. So I can insert a 150M BPF between Crystal and amp, right?

 

[Warpspeed]

The max output will be always at the fundamental

No, an overtone oscillator oscillates at the overtone frequency only.
There will be zero energy at the fundamental.
If it oscillates at the fundamental, then your oscillator circuit is not working properly. That sometimes happens !
If it does you need to tweak the LC circuit so oscillation occurs at the harmonic frequeny.
Third harmonic is easy, fifth harmonic and above it gets progressively more difficult.

Thanks, Warpspeed. So I can insert a 150M BPF between Crystal and amp, right?

You won't need a BPF, if you can get the oscillator to oscillate at 150 Mhz, which it should be able to do.
Then 150 Mhz will be the only frequency present.
The output should be a pretty reasonable sine wave, with very little harmonic content above 150 Mhz.

Put it this way, any oscillator oscillates at the frequency where the feedback is greatest. What you need to do is add a tuned circuit tuned to 150 Mhz so that the feedback gain is very low at 50 Mhz, and high at 150 Mhz.
The crystal then resonates at three times the marked frequency and provides an additional gain peak which closely controls the frequency.

 

[c_mitra]

No, an overtone oscillator oscillates at the overtone frequency only.

Sorry for the poor communication; I meant that the energy output will be less when the crystal is vibrating at the third harmonic than (compared to) when it is vibrating at its fundamental.

Much depends on the crystal cut and voltage drive applied to the crystal.

 

[Warpspeed]

Once it begins to oscillate, the amplitude will keep on rising until something either clips or saturates.
The amplitude is not controlled by the crystal, but by the rest of oscillator circuit.

You can build a low power oscillator that works on a low voltage with low current.
Or you can build a pretty powerful oscillator, that has a high output.

 

[vfone]

Usually standard crystals between 24MHz and 80MHz are 3rd overtone, and crystals between 60MHz and 150MHz are 5th overtone. But this situation might change for special crystals.

 

[c_mitra]

Once it begins to oscillate, the amplitude will keep on rising until something either clips or saturates.
The amplitude is not controlled by the crystal, but by the rest of oscillator circuit.

All the energy fed into the crystal by the driver is dissipated in the crystal as heat. If you put too much drive voltage into the oscillator, the crystal will *****. But you are right that a transistor can easily amplify and buffer this voltage (oscillation).

 

[Warpspeed]

Yes, theoretically crystals can shatter, its like the opera singer shattering a wine glass !
But you need to try very hard indeed to do that.
Its not something to lose any sleep over.

 

[RichardHead]

Very few crystal manufacturers are willing to make fundamental mode crystals above 30Mhz. The crystal gets very thin and is prone to cracking during manufacture.
That means that you'll have to use a 30Mhz fundamental crystal and select the 5th harmonic with a high-Q BPF and amplify. Look up frequency multipliers. A benefit is that the pullability of the oscillator will be much better than an OT oscillator.

 

[Warpspeed]

Its not "how to build a 3H oscillator", but you could just buy a 150 Mhz oscillator module if that solves your problem.
**broken link removed**

 

[FvM]

All the energy fed into the crystal by the driver is dissipated in the crystal as heat.

Crystals are very high Q resonators, thus most of the energy "fed into the crystal" is reflected respectively passed through. Only a very small part is absorbed in the ESR. Crystals have a maximum power specification which can be translated to maximum current and voltage.

 

[c_mitra]

Crystals are very high Q resonators, .

You are right, but the Q depends on the cut and the harmonic used. A third harmonic crystal will have considerably poorer Q. And high Q crystals are lethargic to start...

Conventionally, the crystal is modelled as a LCR and the R contributes to the dissipation.

I should have said that the "net" energy fed into the crystal is dissipated as heat.