The key issue is that you are going to end up with multiple buffers/inverters with a routing delay between, and high enough gains that you probably can not get the system to bias both buffers, and the interconnect in the pseudo-linear region.
The same thing happens if you link up multiple inverter chips together and then put a crystal in, at some point, actually fairly fast, you switch from a crystal oscillator behavior, where the frequency is controlled largely by the resonate frequency of the crystal, to a ring oscilator, largely controlled by the total propagation time of the loop.
It is more of an issue of propagation time vs transition time. When you stack multiple stages together so that the propagation time is enough bigger than transition time, you go into a different oscillatory behavior than when the propagation time is smaller.
It is a matter where the phase lag from the delay occurs compared to the system gain. If it cause 180 degrees of shift + the 180 degrees from the inversion when the system still have net gain, you will get a natural oscillation (a ring oscillator). Since a crystal has a very high Q, as long as the phase lag from delay doesn't get near 180 degrees, it doesn't affect the frequency that much, but the amplifier does need to be stable to work, which a network that forms a ring oscillator isn't
ts hysteresis on input and this seems to hamper the oscillations.
ne.
h side.
e
erting function in a more complex device that is relevant in this situation ?
n what DC level you might expect to see at this input pin that would not be very close to the input threshold voltage?
or to see just what it does with different delays in the path. Then add a few different crystals to see what happens.
There is nothing about oscillations that requires a "linear region" of oper ation. All oscillators have wide fluctuations on the output essentially li ke the amp has infinite gain. Most of the time it is desired that the outp ut have rapid transitions. The DC biasing comes from having appropriately scaled high and low times so the DC average is at the threshold.
You have not addressed the timing issues I've pointed out by providing data . There is not a huge difference in propagation delays in the two devices. This is a path that can be highly optimized simply by specifying a tight timing spec to get a number close to 10 ns while the CMOS buffer is also in that same range. Waving your hands and talking about excessive routing de lays is not a persuasive argument.
--
Rick C.
--- Get 1,000 miles of free Supercharging
--- Tesla referral code - https://ts.la/richard11209
ch delay is acceptable? I don't know any inverters that don't have measura ble delay.
Sorry, I'm not following how propagation time should be compared to transit ion times. In a 32kHz crystal oscillator will the propagation time matter so much???
--
Rick C.
--+ Get 1,000 miles of free Supercharging
--+ Tesla referral code - https://ts.la/richard11209
Not of the crystal, but of the 'gate' that is acting like the amplifier. If the 'amplifier' has enough 'delay' compared to it 'gain'/'transition time' then it will spontaneously oscillate at a frequency based on its delay as a ring oscillator. At these frequency the crystal will basically act like a capacitor and just couple the signal through.
Note, I said pseudo-linear, a region where a small change in the input will make a somewhat related change to the output. If the output actually gets to the point of saturating, you lose gain, so if you didn't have enough gain prior to getting there, you didn't have enough gain to oscillate at the resonate frequency of the crystal.
When you are exciting the crystal at its resonate frequency, as the output is reaching its positive pseudo linear excursion, the input is being driven to its negative most point and the crystal then starts to pull it positive, causing the output to start to drive negative. If you
The Gate Array even if it has the same propagation delay, may have a significantly higher analog gain if you try to bias it into the linear region, due to the fact that it isn't a single stage but multiple stage. This higher gain may allow that same propagation delay to now have sufficent loop gain to self oscillate. as a ring oscillator, especially when you add some additional phase angle from the R-C loading of the output to the load capacitor for the crystal.
There simple gate, has a much lower gain, because it will be a single stage amplifier, and thus doesn't have enough gain at the frequency where this loop gets a total of 360 degrees of phase (180 from the inversion, plus 180 from delays).
much delay is acceptable? I don't know any inverters that don't have measu rable delay.
u
he
he
nsition times. In a 32kHz crystal oscillator will the propagation time mat ter so much???
The crystal is a capacitor in series with an inductor in a series resonant circuit, low impedance at resonance, high impedance elsewhere. The delays you are describing would oscillate on the order of 100 MHz. A 32 kHz cryst al isn't going to pass that very well. Neither does a 1 Mohm resistor and the 20 pF loading caps with a corner frequency of 8kHz.
--
Rick C.
-+- Get 1,000 miles of free Supercharging
-+- Tesla referral code - https://ts.la/richard11209
operation. All oscillators have wide fluctuations on the output essentiall y like the amp has infinite gain. Most of the time it is desired that the output have rapid transitions. The DC biasing comes from having appropriat ely scaled high and low times so the DC average is at the threshold.
The gain required to sustain oscillations is 1. The crystal has a very low impedance at resonance so a lot of gain is not required. In fact the circ uit will have to saturate because that is how the gain stabilizes at 1.
???
data. There is not a huge difference in propagation delays in the two devi ces. This is a path that can be highly optimized simply by specifying a ti ght timing spec to get a number close to 10 ns while the CMOS buffer is als o in that same range. Waving your hands and talking about excessive routin g delays is not a persuasive argument.
You keep talking about the oscillator operating at very high frequencies de fined by the delay. The crystal that links the input and output is a serie s resonant circuit with a large impedance everywhere other than at the reso nant frequency. It will also operate at overtones, but the load capacitors help to prevent that.
At high levels of oscillation the gain of the overall circuit is 1. That's true for every stable oscillator.
--
Rick C.
-++ Get 1,000 miles of free Supercharging
-++ Tesla referral code - https://ts.la/richard11209
On Thursday, July 2, 2020 at 9:10:18 PM UTC-7, Rick C wrote: ...
t circuit, low impedance at resonance, high impedance elsewhere. The delay s you are describing would oscillate on the order of 100 MHz. A 32 kHz cry stal isn't going to pass that very well. Neither does a 1 Mohm resistor an d the 20 pF loading caps with a corner frequency of 8kHz. ...
Crystals also have a parallel resonance mode at a frequency slightly above that of the series resonance mode.
Most crystals are calibrated for this parallel mode - I have never seen a 3
2kHz crystal that wasn't.
For a crystal in parallel resonance mode, the amplifier should have ~180 de g phase shift and high input impedance and a reasonably high output impedan ce. The effective capacitance across the crystal is required to be the same as that for which it was calibrated to be on frequency. Typically this is
20pF for high-frequency crystals and as low as 6pF for 32kHz ones.
Most oscillators are variants of the Pierce oscillator.
The standard electrical model for a capacitor is a series R-L-C network in parallel with another C (the 'bulk capacitance' of the device limits the impedance at very high frequency). It doesn't really matter that this network has a lot of attenuation at this frequency, if the multi-stage amplifier has more gain than that.
Yes, if there is enough attenuation, you can stop the ring oscillator, but you need to KNOW the gain to be able to make sure you have enough attenuation.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.