Doesn't it feel great when a circuit finally works the way you intended.
john444 wrote:Now, adding the capacitors across the crystal should bring it into a more stable mode.
The final tweaks will be to set the drive and gain resistors.
But, now that it oscillates, they will be easy to implement.
mauifan wrote:Varying the gain and the pot connected to the crystal varied the frequency,
mauifan wrote:Shouldn't I be able to achieve the same results with a voltage divider using the midpoint as virtual ground?
If only it were so.mauifan wrote:Perhaps there are some miniscule differences between breadboard rows B-E and rows M-P, but otherwise, everything is IDENTICAL. Same wires. Same crystal. Same pots. The only significant difference between the two is the op amps.
mauifan wrote:if the LM324N and the TS922 were truly ideal op amps
Don't kid yourself. There are no shortcuts putting in the time, pouring over datasheets and making every mistake possible to wire up circuits. It is only the mistakes you make and correct that improve your education. When you finality get it to work, is when you have learned what you can from it.mauifan wrote:My last post was a lament about how you experts are able to see the internals of each chip and know how to adjust.
I have played with op-amps before. Some op-amps are better behaved than others. However, self-oscillation is very common. Especially for high-gain, high-speed op-amps like the '922. (And it is in the datasheet.)mauifan wrote:Why are you saying that I need to add the 0.1uF and 10uF capacitors in the first place?
No. 22-pF is small in comparison to the crystal's equivalent capacitance so the phase-shift due to them is small (insignificant). However, they do establish a stable amount of capacitance to ground, so that say - 'waiving your hand over the circuit' no longer has an impact on it. I am sure there are engineering-design justifiable reasons but most of my education has been by experience.mauifan wrote:Why connect the 22pF caps on each side of the crystal? Won't these result in a roughly 180 degree phase shift of the signal?
They have nothing to do with filtering ripple. When you use long, skinny wires to power a high-gain, high-frequency amp then, the inductance and resistance of those wires cause a voltage drop at the chip when it quickly switches the output, drawing a pulse of current. That voltage drop in the supply gets amplified and your amp is now an oscillator. The 0.1-uF especially, provides a low resistance source of current, long enough to power the amp when it draws that current pulse. That is also why they are needed at the chip, not at the battery. They will do no good at the battery. In high-speed circuits, lead-length is kept as short as possible. These are referred to as 'bypass caps'.mauifan wrote:I have seen the recommendation to use 0.1uF and 10uF caps in conjunction with powering an Arduino. Are you saying that the op amp needs these as well? If so, I didn't think that the caps were all that critical when running off batteries.
mauifan wrote:My last post was a lament about how you experts are able to see the internals of each chip and know how to adjust.
john444 wrote:No. 22-pF is small in comparison to the crystal's equivalent capacitance so the phase-shift due to them is small (insignificant). However, they do establish a stable amount of capacitance to ground, so that say - 'waiving your hand over the circuit' no longer has an impact on it. I am sure there are engineering-design justifiable reasons but most of my education has been by experience.
mauifan wrote:Also... my confidence is shaken, but... well... is this normal?
mauifan wrote:Y'all have stated a couple of times that this is a bit like walking on a tightrope, but... well... I would like to be able to design circuits someday without having to ask for help all the time, and today doesn't seem to be that day.
mauifan wrote:This isn't about crystal oscillators, but it does talk about op amps in a somewhat intuitive way. In particular, Pete mentions that the input impedance of real op amps is on the order of 100-1000K.
mauifan wrote:That suggests to me that if the impedance feeding into the inputs was on the order of 10k or less, the op amp would behave more or less like the ideal op amp.
mauifan wrote:So if the input impedance of the amplifier is about 100K, it would make sense to keep the impedance of the filter/feedback network on the order of 10k or less, no?
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