Post by thread:OT: LC oscillator and PIC as capacitor AtoD

Hello, I am attempting to make a seismometer using a slightly different approac h. The sensor is a capacitor plate where one plate is fixed to the earth (case) and the other plate is attached to a spring; hence when the earth moves the capacitor spacing and therefore the capacitance changes. Normally the capacitor sensor is placed in a bridge circuit but our idea is to use the sensor capacitor as the capacitor in an LC oscillator. As the earth moves, the capacitance changes and so does the frequency of the LC oscillator. (Here comes the part where the PIC comes into play.) I then wish to use a PIC to count the oscillations. The number of counts for a set period of time (determined by the PIC's clock) will be purportional to the capacitance spacing. This seems to me a simple way to digitize a capacitor meter, yet I've never heard of people using this approach. Is there some fundamental reason that this won't work that I'm missing? Horowitz and Hill state in their book, The Art of Electronics, that an LC oscillator is only stable to 1 part in 10,000. Can anyone tell me why that is? Seems to me that an LC oscillator should be extremely stable (assuming one is using a fixed rather than a variable capacitor). Can anyone enlighten me as to why an LC oscillator is only stable to one part in 10^4? Thanks in advance for your help, Philip Slack spam_OUTpslackTakeThisOuTlinex.com

Hello Phil, please note: For L/C-oscillators the biggest influence is given of the temperature-coefficient of the L (not of the C). So you will definetely have a temperature-drift, which will exceed the influence of the capacitor-plate- movement. You might consider to compensate the tempereature-coefficient of the L by using capacitors with the opposite coeffiecient in parallel to your C. Another approach might be to put your complete setup in a "regulated oven". As the temperature-drift is quite small you might also try to compensate it by other means. But I believe it will be difficult! Good luck, Uwe Koenneker

Slow-pass filtering (in effect) should handle this - he's after the sudden changes in frequency, say 0.5 Hz or faster, for seismology; a slow frequency drift over minutes or hours, shouldn't do anything but change the comparison low-passed frequency at the output of the low-pass filter. (Maybe a 10 second Tau there?) I'd be lots more concerned about getting the thing down deep enough into bedrock to reduce road noise, etc. than worrying about something that the software can handle automatically! The way (for example) to detect windshear for aircraft safety can involve low-pass filtering of barometric pressure, temperature, Angle of Attack, etc. data (which, of course, ALL automatically changes as your aircraft is in the air!) - it's the sudden trends, compared to the low-passed information, all combined, not the drift of the inputs, that are a trigger for that "Dee, Doop! WindShear! Windshear! Windshear!" alarm, that hopefully will save all on board the airplane. Mark, .....mwillisKILLspam@spam@nwlink.com Uwe Koenneker (KNN) wrote: {Quote hidden}> > Hello Phil, > > please note: > > For L/C-oscillators the biggest influence is given of the > temperature-coefficient of the L (not of the C). > So you will definetely have a temperature-drift, which will exceed the > influence of the capacitor-plate- > movement. > > You might consider to compensate the tempereature-coefficient of the L > by using capacitors with the > opposite coeffiecient in parallel to your C. Another approach might be > to put your complete setup in a > "regulated oven". As the temperature-drift is quite small you might also > try to compensate it by other means. > But I believe it will be difficult! > > Good luck, > Uwe Koenneker