How does the phase noise of the LO effect IF accuracy?

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Discussion Overview

The discussion revolves around the impact of phase noise and jitter of a local oscillator (LO) on the accuracy of an intermediate frequency (IF) signal during downconversion. Participants explore the relationship between LO performance specifications and the resulting frequency errors in the output signal, with a focus on calculations related to phase noise.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant questions whether a jitter of 100 fs rms at 1 GHz results in a frequency error of 100 kHz after mixing, seeking clarification on how to improve LO performance.
  • Another participant suggests that jitter is not the appropriate specification for the scenario and recommends using phase noise instead, noting that the drift amount is poorly constrained by jitter.
  • A participant mentions that the phase noise curve's crossing point at 0 dB indicates the expected frequency deviation of the oscillator.
  • One participant expresses a need for assistance in calculating total rms phase error based on given phase noise values at various offsets from the carrier frequency, indicating confusion over existing resources.
  • Another participant estimates a linewidth of about 0.1 Hz for the specific case and suggests that the drift over one day could be at least 10 Hz under stable conditions.
  • A participant highlights the need for clarification on the frequency of the jitter occurrence, noting that jitter has both amplitude and frequency components.
  • One participant states that the phase noise at the output of the mixer will be the same as the LO phase noise, questioning if this aligns with the original inquiry.

Areas of Agreement / Disagreement

Participants express differing views on the relevance of jitter versus phase noise, with no consensus on the best approach to calculate the impact of these factors on the output signal accuracy. The discussion remains unresolved regarding the specific calculations needed for phase noise analysis.

Contextual Notes

Participants note limitations in available resources for calculating phase noise and its effects, indicating that existing tutorials may be incomplete or erroneous. There is also uncertainty regarding the frequency of jitter occurrences and its implications for the analysis.

csopi
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Hi,
I have a roughly 1.1 GHz signal to be downconverted to 100 MHz by mixing it with a 1 GHz local oscillator. I am not sure how to choose the performance of the LO.

In particular: let's assume the LO has a jitter of 100 fs rms. At 1 GHz this corresponds to a frequency error of 100 kHz. Does this mean that after mixing I will have an error of 100 kHz? If yes, how to improve the performance?
 
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csopi said:
Hi,
I have a roughly 1.1 GHz signal to be downconverted to 100 MHz by mixing it with a 1 GHz local oscillator. I am not sure how to choose the performance of the LO.

In particular: let's assume the LO has a jitter of 100 fs rms. At 1 GHz this corresponds to a frequency error of 100 kHz. Does this mean that after mixing I will have an error of 100 kHz? If yes, how to improve the performance?
Jitter is the specification usable for very high-bandwidth (comparable to LO frequency) signals which is likely not you case.
In case of imperfect oscillator you are going to observe your output frequency randomly drifting as you described above, but the drift amount is poorly constrained by "jitter" specification. You should use "phase noise" specification instead. Usually, IF frequency drift is reduced by PLL circuit which reduce low-frequency components of noise by stabilizing local oscillator with the help of crystal or atomic oscillator operating at lower frequency.

Very simplistically, point of phase noise curve crossing the 0dB line indicates your oscillator expected frequency deviation.
 
Last edited:
trurle said:
Jitter is the specification usable for very high-bandwidth (comparable to LO frequency) signals which is likely not you case.
In case of imperfect oscillator you are going to observe your output frequency randomly drifting as you described above, but the drift amount is poorly constrained by "jitter" specification. You should use "phase noise" specification instead. Usually, IF frequency drift is reduced by PLL circuit which reduce low-frequency components of noise by stabilizing local oscillator with the help of crystal or atomic oscillator operating at lower frequency.

Very simplistically, point of phase noise curve crossing the 0dB line indicates your oscillator expected frequency deviation.

Many thanks! I have looked into phase noise, and now I understand that I should calculate the total rms phase error, which describes the "average" deviation of the system. And here comes the problem: how to calculate this? I have found various tutorials on manufacturers' website, most of them is either incomplete or upright erroneous. Could you please help me performing this calculation for the following oscillator? Let's say we have a noise of -120, -150, -165 dBc/Hz at 100, 1k, and 10k Hz away from carrier. Manufacturers do not seem to further elaborate on the issue, but I am not sure how to derive a meaningful value out of this...
 
In your particular case, you likely will see linewidth about 0.1 Hz (assuming worst case random-walk slope 40 dB/decade below 100 Hz). Your drift for 1 day will be at least 10 Hz even in case of perfectly stable temperature.
 
csopi said:
... let's assume the LO has a jitter of 100 fs rms. At 1 GHz this corresponds to a frequency error of 100 kHz.

If we assume the LO has jitter of 100fs rms, this means that periodically your LO will be 100fs rms ahead or behind, in time, an ideal reference oscillator. But you have not specified how often this will occur. Once a day? Once per millisecond? Jitter has both an amplitude (fs, ps, UI etc), and a frequency, and you are not specifying the freq (and I'm not sure where you are getting the 100KHz from).

csopi said:
Could you please help me performing this calculation for the following oscillator? Let's say we have a noise of -120, -150, -165 dBc/Hz at 100, 1k, and 10k Hz away from carrier. Manufacturers do not seem to further elaborate on the issue, but I am not sure how to derive a meaningful value out of this...

The phase noise at the output of the mixer (dBc/Hz) will be the same as the LO phase noise (dBc/Hz). Is this what you were looking for?
 
Last edited:

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