Circuit to study watch quartz crystals - Page 1
Forum section: electronics / beginners

author: gus789
author_rank: Contributor
timestamp: January 03, 2014, 12:36:30 am
content: Hello,I have been looking into watch quartz crystals (32.768kHz) recently, and was wondering: does anybody know of a good self-oscillating circuit to drive such a crystal with as little current through the crystal as possible, preferably allowing control of the current through the crystal?My approach so far has relied "forced oscillation" using an attenuated DDS oscillator, as follows: the DDS sine wave generator (AD9850) is followed by a 0.1 gain amplifier and then straight into the quartz crystal.The output of the crystal goes to a 1E6 V/A gain I-V converter and the output of that is further amplified ( by circa 20X) and then goes into a precision rectifier followedby a low pass filter. This gives the current though the crystal as a function of frequency and allows measurement of the "resonance peak". It also allows one to monitor shifts in resonance when the crystal is disturbed, by keeping track of the amplitude changes at fixed drive frequency.Now I'd like to try a self-oscillating circuit and measure frequency/phase shifts instead of amplitude.I have experimented with a few Pierce oscillator designs, but was only successful with one using an op amp, which requires some noise injection to start. Still such circuits seem to offer no way to control current through the crystal, and I suspect they tend to overdrive the crystal.Any ideas greatly appreciated.regards

author: JackOfVA
author_rank: Unknown
timestamp: January 03, 2014, 02:20:58 am
content: Google search shows approximately one bazillion hits for 32 khz oscillator circuit.Cypress Semiconductor has a decent data sheet on the subject athttp://www.cypress.com/?docID=38894

author: gus789
author_rank: Contributor
timestamp: January 03, 2014, 02:43:41 am
content: Sure, but how does that part you suggested (and the other bazillion minus one) allows one to adjust the crystal current? The cypress data sheet only mentions that this chip has an internal "drive-level" gain control that prevents gain from exceeding a threshold. Also the AN refers to a microcontroller family.

author: Unknown
author_rank: Unknown
timestamp: January 03, 2014, 04:29:55 am
content: Practical Analysis of the Pierce OscillatorAs you probably know (or should if you're doing these measurements) the drive current is primarily a function of the motional resistance/capacitance/inductance (inherent to the crystal), the limiting (series) resistance, & the various other reactances around the place (crystal, op-amp, & stray).There's really only one of those you can adjust - the limiting resistance. The linked datasheet goes into how to measure & calculate the various factors & determine the actual drive current.(edit: fixed link; damn silly editor...)

author: gus789
author_rank: Contributor
timestamp: January 03, 2014, 05:57:20 am
content: Thanks, that is useful, any idea what the subscript "e" stands for in that sheet? As in Re and Xe?

author: gus789
author_rank: Contributor
timestamp: January 03, 2014, 06:00:08 am
content: Never mind, found it:http://www.statek.com/pdf/tn30.pdf...