Crystal oscillator unbalanced external capacitors

[hioyo]

The below lines are from AVR® Microcontroller Hardware Design Considerations (AN2519).

"In noisy environments the oscillator can be crucially affected. If the noise is strong enough, the oscillator can “lock up” and stop oscillating. To reduce the sensitivity of the oscillator to noise, the size of the capacitor at the high-impedance input of the oscillator circuit, XTAL1, can be slightly increased. Increasing only one of the capacitors does not affect the total capacitive load much, but unbalanced capacitors can affect the resonant frequency to a higher degree than the change of the total capacitive load. However, unbalanced capacitive loads will affect the duty cycle of the oscillation and should not be used. This is especially critical if the AVR device is utilized close to its maximum speed limit."

I have some questions regarding this.

1. How do unbalanced capacitors reduce the sensitivity to noise?
2. They also say "However, unbalanced capacitive loads will affect the duty cycle of the oscillation and should not be used". Does that mean: never use unbalanced load capacitors?
3. Which one is better, balanced or unbalanced load capacitors?

 

[danadakk]

The below lines are from AVR® Microcontroller Hardware Design Considerations (AN2519).

"In noisy environments the oscillator can be crucially affected. If the noise is strong enough, the oscillator can “lock up” and stop oscillating. To reduce the sensitivity of the oscillator to noise, the size of the capacitor at the high-impedance input of the oscillator circuit, XTAL1, can be slightly increased. Increasing only one of the capacitors does not affect the total capacitive load much, but unbalanced capacitors can affect the resonant frequency to a higher degree than the change of the total capacitive load. However, unbalanced capacitive loads will affect the duty cycle of the oscillation and should not be used. This is especially critical if the AVR device is utilized close to its maximum speed limit."

I have some questions regarding this.

1. How do unbalanced capacitors reduce the sensitivity to noise?
2. They also say "However, unbalanced capacitive loads will affect the duty cycle of the oscillation and should not be used". Does that mean: never use unbalanced load capacitors?
3. Which one is better, balanced or unbalanced load capacitors?

This will help I think -

https://www.ti.com/lit/an/szza043/szza043.pdf

I had heard their ratio can potentially give an equivalent G to the circuit, this seems to
imply that (page 5 as well) :

Regards, Dana.


Regards, Dana.

 

[FvM]

Some statements in the AN like affected duty cycle seem to be specific to AVR hardware and don't apply generally.

 

[KlausST]

Hi,

mind that just using C1 and C2 is not very accurate. For more precise calculations you have to enter IO_capacitances and PCB trace capacitances.

Klaus

 

[hioyo]

Some statements in the AN like affected duty cycle seem to be specific to AVR hardware and don't apply generally.

1. How do unbalanced capacitors reduce the sensitivity to noise?

--- Updated Aug 13, 2022 ---

1. How do unbalanced capacitors reduce the sensitivity to noise?

2. Hi,
   
   mind that just using C1 and C2 is not very accurate. For more precise calculations you have to enter IO_capacitances and PCB trace capacitances.
   
   Klaus