Will this Circuit fry my Crystals?

Diver300 said:

Short answer, no it won't as crystals are really difficult to overdrive.

Long answer, no it won't and here is why. That circuit will run that crystal at around 50 pF load capacitance so it's equivalent circuit is an inductor that tunes at 8 MHz to the 50 pF, so the impedance is around 400 Ohms.

I would expect an amplitude of around 3 V pk - pk on the crystal and so that is about 1 V rms, so the current is about 2.5 mA rms.

The ESR of the crystal will be something like 50 Ohms, so with 2.5 mA flowing, there will be around 300 μW dissipated in the crystal, which is quite a bit less than the 1 mW which is the typical maximum for an HC49 crystal.

https://portal.iqdfrequencyproducts.com/products/details/hc49.pdf

Small crystals will have lower drive levels but they don't get damaged quickly by overdriving. They can age faster if overdriven.

As for what the circuit is doing, it looks like an unmodulated transmitter. There is no control of the amplitude and the direct connection of the antenna to the oscillator circuit will make the frequency unstable. Two of those in the same room will phase-lock to each other. There will be a lot of harmonics generated.

If that is being used as a transmitter, there is unlikely to be any legal way to use it outside of a screened room.

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Nigel Goodwin said:

I'd like to see a high value resistor from gate to ground, I don't like to see the gate floating.

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Fionaa said:

Will 1 Megaohm work?

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Diver300 said:

Short answer, no it won't as crystals are really difficult to overdrive.

Long answer, no it won't and here is why. That circuit will run that crystal at around 50 pF load capacitance so it's equivalent circuit is an inductor that tunes at 8 MHz to the 50 pF, so the impedance is around 400 Ohms.

I would expect an amplitude of around 3 V pk - pk on the crystal and so that is about 1 V rms, so the current is about 2.5 mA rms.

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Tony Stewart said:

I have never burnt out a crystal and understand that microwatt power levels would never be felt as heat.

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Tony Stewart said:

I have never burnt out a crystal and understand that microwatt power levels would never be felt as heat.

So I ran a simulation now to see if the average motional resistance, Rm would dissipate more than the 500 uW rating in this design, but the instantaneous complex peak power is much higher , although decaying slowly.

A previous simulation of a discrete model of a crystal with a Q of 10k proved to me that this amplifies the internal motional capacitance voltage that could stress the breakdown voltage at the lattice structure and produce a pixel arc and slowly degrade the frequency and ESR.

This gives me reason to doubt if it average power level from thermal effects or over-voltage inside the Xtal lattice on peak reactive power levels.
Anyone?

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Diver300 said:

The frequency has changed a lot. Normally that's something you have to decide early on. One of the main reasons to have a crystal is that it defines the frequency with good accuracy.

The 5 MHz HC49/4 crystal may be more difficult to get to oscillate than the 8 MHz HC49 crystal. The actual quartz in an 8 MHz crystal will be around 0.2 mm thick, while the quartz in a 5 MHz crystal will be around 0.32 mm thick. They oscillate in thickness-shear mode (https://en.wikipedia.org/wiki/Crystal_oscillator) and the oscillation is strongest in the middle of the disk of quartz, and reduces towards zero at the edges.

The 5 MHz crystal will have to be thinner at the edges to make that happen, so that is an additional restriction to the oscillation. The 8 MHz crystal in an HC49 crystal will most likely be a flat disk, possibly with a slight thinning at the edges.

The HC49/4 mm crystal can't be round like it is in the HC49 housing, so the sides of the disk have to be cut off. The crystal is oriented so that the motion is along the crystal, not across, but the motion is still affected.

The maximum ESR of the crystal specified by JLCPCB goes up sharply at low frequencies, showing that the design is compromised at low frequencies.

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