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Measurements >> Phase Noise and Jitter Measurements >> Basic questions about phase noise measurement of oscillator in free-running ope
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Message started by Colombo on Jun 2^nd, 2013, 2:40pm

Title: Basic questions about phase noise measurement of oscillator in free-running ope
Post by Colombo on Jun 2^nd, 2013, 2:40pm

Hello guys,
Please, any input regarding these basic questions are very welcome.

1) "Oscillator in free-running operation has its frequency continually changing with time (e.g. figure 1) . It is the frequency drift problem caused by micro-changes in the temperature or VDD."

Why does it change indefinitely in a specific direction?

I thought that it would achieve a steady-state when the temperature stabilizes.

2)" It may be impossible to measure close-in phase noise of Oscillator in a free-running. However, the far-out phase noise can be measure". Why?

3) Oscillator in a free-running operation and inside a PLL (figure 2).

Why (conceptually) the phase noise of the VCO output is different for both cases? I'm not talking about the output of PLL.

Thank you very much. Best regards.

Title: Re: Basic questions about phase noise measurement of oscillator in free-running ope
Post by MikeL on Jun 12^th, 2013, 8:41am

Colombo wrote on Jun 2^nd, 2013, 2:40pm:

1) "Oscillator in free-running operation has its frequency continually changing with time (e.g. figure 1) . It is the frequency drift problem caused by micro-changes in the temperature or VDD."

Why does it change indefinitely in a specific direction?

I thought that it would achieve a steady-state when the temperature stabilizes.

It does not increase indefinitely in a specific direction - but the time until the direction changes is very large. So effectively, there is a 'constant' drift. I have observed oscillators where the drift occurs over several 10s of seconds.

The temperature does not always stabilise!! I have also observed variations in remote devices that align with the time of day/night - over periods of hours.

Colombo wrote on Jun 2^nd, 2013, 2:40pm:

2)" It may be impossible to measure close-in phase noise of Oscillator in a free-running. However, the far-out phase noise can be measure". Why?

Because the close-in phase noise is produced by these slow-variations in phase and frequency. It takes a long time to make the measurement and the 'value' is not constant during that time.

Colombo wrote on Jun 2^nd, 2013, 2:40pm:

Why (conceptually) the phase noise of the VCO output is different for both cases? I'm not talking about the output of PLL.

What do you mean by "not talking about the output of PLL"? The PLL is the reason for the difference. The PLL corrects the slow variations in frequency and phase of the VCO. It does this by comparing the VCO to another device with a 'better' performance. Once the close in phase noise is 'removed' the higher frequency variations are more visible.
I do not know for sure why the PLL noise is higher at frequencies far from the carrier. I think it is the contribution of the other device used in the PLL - but I am not certain.

Title: Re: Basic questions about phase noise measurement of oscillator in free-running ope
Post by Colombo on Jun 2^nd, 2014, 12:38pm

thanks very much, MikeL. Best regards.