136 Mhz oscillator

[jjthefreako]

hi,
Im creating a VHF receiver, i need to build an oscillator that will resonate at 136 Mhz. I'm planning on using a crytal and overtone it but i believe i need to buy a crystal that has already been overtoned, but i cannnot find a 136 Mhz crystal. Is there any way i can create an overtone crystal using a fundamental crystal oscillating at 19.4Mhz and overtone it to 7th overtone. Could you guys give me some ideas.

 

[JimB]

A conventional way of creating a signal at this frequency is to use a 45Mhz* 3rd overtone crystal in the oscillator, and then use a tripler to multiply to 135Mhz.

* The actual crystal frequency will of course depend on the require final frequency.


It may be possible to obtain a 5th or 7th overtone crystal which will give you direct 136Mhz.

Ask a crystal manufacturer.

JimB

 

[ccurtis]

You could use your 19.4 MHz crystal to make a fundamental oscillator, then clip the output into a square wave which is rich in harmonics (or generate a square wave directly from the your crystal) and then use a 136 MHz tank circuit to filter in the 7th harmonic. You would then amplify the 7th harmonic to the level you need. You would have to "pull" the 19.4 MHz fundamental a little to get 136 MHz exactly.

You could also buy a 136 MHz oscillator directly:Crystek Crystals, PECL, SMD VCXO, Clock Oscillators

Crystals are designed to resonate at overtones and their overtone freq is stamped on them, if that is how they are intented to be operated. Fundamental crystals are not designed to be resonated at overtones, though I suppose they might be forced into it with less than optimum stability and results. It depends on how the particular crystal is cut.

 

[stevez]

You might examine amateur radio literature from the past. I recall seeing multiple stages of tripling or doubling of frequencies. I also recall my own mental note on overtone - that strictly speaking it might not be an integral multiple of the fundamental frequency though it will be close to that.

If you do research amateur radio literature you might find that VHF receiver projects, particularly for the 2 meter amateur bands might contain useful information.

 

[Diver300]

Overtone crystals are designed to run like that.

Although all crystals do have some activity at the overtone frequencies, crystals designed for fundamental mode will not work at all well at overtone.

The reasons are:-

Adjustment. The overtone frequencies are not exact multiples of the fundamental frequency. During manufacture, the final frequency is adjusted with the crystal running at the overtone frequency.

Activity. Overtone crystals, especially 5th and higher overtone crystals, are ground with finer abrasive or polished before the electrodes are applied. This improves the activity and makes it much easier for the crystal to oscillate at the overtone frequency. Polishing makes little difference for fundamental crystals so it is very rarely done as it is a slow process.

Temperature stability. The angle that the quartz disk is cut from the crystal has a huge effect on the temperature stability of the final crystal. The optimum angle is different for fundamental crystals and for overtones, by about 0.1 degrees of angle, or about 20 - 30 ppm over 60 degrees C.

All these differences are most pronounced between fundamental and 3rd overtone crystals, getting less and less significant between higher overtones, so running a 5th overtone at 7th will work quite well.

However, high overtones are difficult to design oscillators for. Each overtone has less activity than the lower ones. What that means is that a crystal bought as a 5th overtone 100 MHz will be easier to make oscillate at 60 MHz on its 3rd overtone than it will be at its design frequency. It will be easier still to make it oscillate at 20 MHz at its fundamental frequency. The oscillator has to be tuned to prevent it running at the wrong overtone. For a fundamental oscillator, that is easy as it is the lowest frequency and the oscillators tend to have less gain at higher frequencies. At higher overtones, the ratio between the frequencies is less, so the tuning of the oscillator becomes more difficult.

 

[jjthefreako]

Hmm since a crystal oscillator will oscillate at its overtone frequency. I was thinking of using a 45 Mhz crystal 3rd overtone crystal in collpitts oscillator setup with an oscillator, make the collpits arrangement to oscillate at the fundamental frequency of the crytal and create a tank LC circuit at the collector the required 136 Mhz such that it will force the oscillator to run only at that overtone of the crystal. Do u think it will work. I will post the circuit once i create it in pspice.

 

[Nigel Goodwin]

If it's a third overtone crystal, then it's only a 15MHz crystal, you would probably struggle to get it to oscillate so much higher than designed.

Tune the oscillator to 45MHz, then have a multiplier stage.

You can get overtone crystals as high as 136MHz, but I suspect you may well struggle sourcing one - and such high crystals tend to be very fragile.

 

[Diver300]

You can get overtone crystals as high as 136MHz, but I suspect you may well struggle sourcing one - and such high crystals tend to be very fragile.

A 5th overtone 136 MHz crystal would have a fundamental frequency of 27.2 MHz and won't be too fragile, but it will be fairly tricky to make an oscillator for. The oscillator will have to be tuned so that it can't oscillate at 81.6 MHz or 190.4 MHz.

High frequency fundamental crystals do exist, but they are very expensive and often have very large minimum order quantities. They are made by etching the middle of the crystal really thin, while leaving the edge thick so that they can be mounted and used without breaking.

Using a crystal at 45.3333 MHz on its 3rd overtone, followed by a 3 times multiplier, is a good approach. The oscillator has to be tuned to avoid running at the fundamental or the 5th, (15 or 75 MHz ish) but the 5th overtone has less activity than the 3rd, so it is rare to run at that accidentally.

 

[Nigel Goodwin]

Bear in mind, 136MHz is very close to the 144-146MHz amateur band (2m), back in the days before PLL's it was normal to use 12MHz crystals, and multiply then by 12 in two or three stages.