mauifan wrote:it worked -- my oscope showed a perfect sine wave at 32k using a watch crystal! Amplitude was on the order of 1-2v.
mauifan wrote:At best, the circuit may oscillate... but the output is clipped. And sometimes I see some harmonics on my scope trace.
mauifan wrote:couldn't get it to work until I added a resistor between the op amp (-) and ground.
mauifan wrote:Theory says that you need a gain of slightly higher than 1 to compensate for circuit losses.
mauifan wrote:but how do I know where that sweet spot is located?
1 place a large value (10-M) resistor across the opamp from output to + input.
This will provide some dc coupling.
mauifan wrote:I can get the watch crystal to oscillate, but the output is clipped -- it looks like a sine wave that has been fed through a diode.
mauifan wrote:My original goal was just to try to get the crystal to vibrate at its indicated frequency
mauifan wrote:the TS922 is a more "forgiving" op amp that has "rail to rail input and output
mauifan wrote:What do I need to adjust to make the circuit work at the new frequency?
mauifan wrote:There is also a fixed 10k resistor between op amp (-) and ground
mauifan wrote:but I am not certain what you mean by provide some DC coupling.
mauifan wrote:Sigh... I have read several tutorials
john444 wrote:The crystal sets the frequency of operation, not the resistors. The resistors are used to set the gain and DC bias the opamp so that it is within it proper operating range. The capacitors are used to stabilize the crystal and the AC signal component.
john444 wrote:mauifan wrote:There is also a fixed 10k resistor between op amp (-) and ground
Are you using split supplies? In other words, do you have 2 power sources?
mauifan wrote:Let me rephrase: Why doesn't the "black box" approach work? For example, why can't I take the original 150kHz crystal that this circuit was designed for and replace it with a 32kHz crystal with no other mods? or a 4MHz crystal?
mauifan wrote:Why doesn't the "black box" approach work?
mauifan wrote:I have noticed that my circuit is indeed quite sensitive to small pot movements.
mauifan wrote:Are you using split supplies?
email@example.com wrote:Take a look at this datasheet: http://cfm.citizen.co.jp/english/product/pdf/CM309S.pdf specifically the "motional (series) resistance" table at the bottom of the page. A crystal running at 3.5-4MHz has a series resistance of out 200 ohms. One running at 10-27MHz has a series resistance of 50 ohms.
Now check this datasheet for a 32kHz crystal: http://www.mouser.com/catalog/specsheet ... -ab26t.pdf . The ESR is about 30 kilohms.
In light of those facts, your question becomes "why can't I replace a 200 ohm resistor with a 30 kilohm resistor and expect the circuit to work?"
john444 wrote:I hope you noticed that most of the example circuits use a single transistor or at most only a few. I do not remember any examples where a opamp is used to drive a crystal. In this type of application, there can be lots of variation - from mfg to mfg & model to model.
mauifan wrote:I am not sure, but I think you just provided a small "aha" moment.
mauifan wrote:If I understand you correctly:
ESR in a crystal decreases with increased frequency. Therefore, the resistor values external to the crystal must be adjusted (i.e. increased when going to a higher frequency) in order to maintain a gain=1.
mauifan wrote:Still not sure about how to calculate the losses so I know how much gain to add from the op amp, but hey... if the above statement is true, I may be one step closer to understanding this subject.
mauifan wrote:That said, I am guessing that the loss I need to compensate for has everything to do with the crystal's ESR and that losses through the crystal's capacitors and inductor (reference the equivalent model circuit for a crystal) can be ignored?.
mauifan wrote:Sidebar: What do you do when you need a spec like the ones you provided, but no data sheet exists?
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