# Calculation of Phase noise at the mixer output

1. ## Calculation of Phase noise at the mixer output

Hello All,

What is the difference between correlated and uncorrelated phase noise? How to calculate phase noise at the output of a mixer when the phase noise of the input signals are specified for the following cases:

Case (A) Mixer is operating as downconverter:(Output frequency = RF frequency - LO frequency)
(1) the input signals are coherent (i.e. phase noise are related to each other because they are phase locked to a single OCXO)
(2) the input signals are non-coherent.(i.e. the two signals are independent and not phase locked)

Case (B) Mixer is operating as upconverter: (Output frequency = IF frequency + LO frequency)
(1) the input signals are coherent (i.e. phase noise are related to each other because they are phase locked to a single OCXO)
(2) the input signals are non-coherent.(i.e. the two signals are independent and not phase locked)

Regards,
akrcal

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2. ## Re: Calculation of Phase noise at the mixer output

-- http://www.ab4oj.com/test/docs/5990-7529EN.pdf

-- http://www.ti.com/lit/an/scaa113/scaa113.pdf

Phase noise is essentially the probability of your signal being in that specific frequency away from the carrier given by a pdf function.

If it says @100 Khz -90dbc, you can interpret it as probability of being 100 KHz away from the carrier at a given time instance is 1e-9.
Above links are Keysight and TI phase noise documents about a PLL and Crystal Oscillator systems. Phase noise is created at VCO or Crystal Osc, then carried all along the way, and it depends on multiple factors such as Loop BW, PLL being fractional or integer etc.

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3. ## Re: Calculation of Phase noise at the mixer output

Usually the uncorrelated noise sources (with similar phase noise) at the input of the mixer, gives lower phase noise at the output of the mixer than correlated sources.

http://micro.apitech.com/pdf/whitepa...lculations.pdf

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4. ## Re: Calculation of Phase noise at the mixer output

Originally Posted by vfone
Usually the uncorrelated noise sources (with similar phase noise) at the input of the mixer, gives lower phase noise at the output of the mixer than correlated sources.

http://micro.apitech.com/pdf/whitepa...lculations.pdf
HI vfone,

Generally in RADAR systems, various LO signals are locked with a single OCXO i.e. their phase noises are correlated. So how the statement "uncorrelated noise sources (with similar phase noise) at the input of the mixer, gives lower phase noise at the output of the mixer than correlated sources" is justified.

akrcal

5. ## Re: Calculation of Phase noise at the mixer output

The reason that many RF systems use a single common OCXO reference is NOT to reduce the phase noise at the output of their mixers.

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6. ## Re: Calculation of Phase noise at the mixer output

Originally Posted by vfone
The reason that many RF systems use a single common OCXO reference is NOT to reduce the phase noise at the output of their mixers.
Hi vfone,

In the document you have attached there are the following figures:

Fig 1.

Fig 2.

In the fig. 1, the phase noise of multipliers D2 and D3 are also taken into account in calculation of overall phase noise but in fig. 2 they are not incorporated. Can you elaborate at this point?

to op, 3 db

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