Hi everyone,

I am working on a submersible project, and I'd like to know if using a crystal oscillator can lead to complications due to frequency drift due to pressure differential between the surface, and a dept of 10meters for example.

Any suggestions on the topic would be greatly appreciated. FYI, I'd like to use a 32MHz crystal.

Thanks!

 

The 32 KHz crystal uses the tuning fork principle to operate. A pressure change can't affect the structure's natural resonance.

Water can play hob with a circuit, though. Will yours be in a watertight container?

 

For each 10 meters of depth in water from the surface, the pressure increases by 1 atmosphere (~14.7 lbs/in^2).

If you want an inexpensive watertight housing, consider using PVC pipe, available wherever plumbing supplies are sold.

 

The 32 KHz crystal uses the tuning fork principle to operate. A pressure change can't affect the structure's natural resonance.

Water can play hob with a circuit, though. Will yours be in a watertight container?

You wanted to say 32MHz right?

Also, please see this link - http://en.wikipedia.org/wiki/Crystal_oscillator

Here, a paragraph says,

Environmental changes of temperature, humidity, pressure, and vibration can change the resonant frequency of a quartz crystal, but there are several designs that reduce these environmental effects. These include the TCXO, MCXO, and OCXO (defined below). These designs (particularly the OCXO) often produce devices with excellent short-term stability. The limitations in short-term stability are due mainly to noise from electronic components in the oscillator circuits. Long term stability is limited by aging of the crystal.

So what they are saying in this article is not true?

Thanks

 

32 MHz will be quartz but it should be in a sealed enclosure so I doubt it would be affected by such a minimal pressure change.

 

Sorry - i see 32,786 Hz oscillator references enough that I just put the "K" in place. marshallf3 got it right about any pressure change. Are you concerned about absolute accuracy? What is the application?

 

the application is a submersible with a bunch of sensors that i would communicate with. my concern was regarding clock accuracy so that the PIC-sensor communication doesn't get screwed up when underwater.

thanks for your feedback. i appreciate it!

 

Well how much drift is acceptable, are you sensors communicating using a UART or a USART? If it's a USART then the drift wouldn't effect anything, UART possibly, but I doubt pressure changes would so drastically effect the circuit that it would cause a UART misread. Only sure way to know would be to test it under the extremes you think it will encounter.

 

If you can't isolate the crystal from pressure changes, then it would probably be wise to measure the effect of pressure changes. This would be easy to do with some threaded pipe fittings. Install a Schrader valve to pressurize the container. You'll need to make a feedthrough for the needed wires; this can be done by drilling a small hole in a threaded cap and using epoxy to seal in the wires.

You can make a tap for the external thread of the Schrader valve if you have a lathe and some drill rod (for a good chuckle, you can do a web search and find a UK site that will sell you a tap for 85 pounds or so). Otherwise, just drill a hole and epoxy it in place. Or, you might get lucky and find an adapter at your local plumbing supply. Another approach is to remove the valve core, then solder the threaded outside piece into a brass pipe fitting.

 

I forget the diameter and thread pitch but valve stems are of some sort of SAE fine thread.

I'd still bet that, in the small metal cases a crystal come in, one atmosphere of pressure isn't going to alter a thing.

 

Thanks for all your suggestions. I'll get back to you when I am done with the testing Thanks once again!

 

the application is a submersible with a bunch of sensors that i would communicate with. my concern was regarding clock accuracy so that the PIC-sensor communication doesn't get screwed up when underwater.

Are you sure the circuit will actually see the elevated pressure? The air surrounding the circuit will only compress if your submersible doesn't resist the pressure at all. I think I'd be more concerned about temperature.