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General Phase Noise Question

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youngguns21

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What effect does an amplifier have on the phase noise? and What effect does a mixer have on phase noise?
This is a general question assuming that the amp and mixer are ideal.

ie: if my PN is -80 dBc/Hz before a 20 dB amp, does the PN get amplified to -60 dBc/Hz?
ie: if my IF and LO both have -80 dBc/Hz before the mixer, what would you expect at the RF port?

Thanks
 

biff44

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you need to understand the units dbc/hz. DB means "decibels". C means "with respect to the carrier"

there is also an important thing you left off, the "offset frequency" where that phase noise is measured at.

For instance, you might say you have a signal, that is 0 dBm power, and has -80 dBc/hz phase noise at 10 KHz offset.

If you looked at a spectrum analyzer of the signal, you would see it has a peak at the center frequency of 0 dBm. And IF you set the spectrum analyzer "resolution bandwidth" to 1 Hz, then 10 KHz offset from the carrier frequency, the power displayed would be -80 dBm,

So the phase noise is a difference measurement, between the power at 0 Hz offset (0 dBm in this case) and the noise level at the offset frequency (-80 dBm in this case), So the phase noise is 0 dBm-80 dBm, or -80 dBc/hz at 10 kHz offset. or another way to say it, the noise power is 80 db below the CARRIER power.

So let that sink in.

Now if you add a 20 dB gain amplifier, your signal at 0 hz offset will now be +20 dBm. And the noise at 10 KHz offset at the amplifier output will now be -60 dBm. But the difference between the two is still 80 dB. So the phase noise has not changed.

(this assumes you have a pretty good amplifier, some can add to phase noise, but usually only being a problem with very low noise sources)
 

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