mixer phase noise

hi freinds,

i have a question about the relationship between the phase noise noise of the local oscillator and its impact on the output of the mixer in a radio receiver chain.
in fact, do we take in account the phase noise of the LO when calculating the noise figure ?
what are, if they exist, the requirements of the mixer that have to be fulfilled to avoid the impacts of phase noise on the mixer output?
lets take an example in which a local oscillator has a BAD phase noise factor ( without conscidering any numerical values) , so when we qualify it as BAD, what is the direct impact on the output of the mixer
thanks for any kind of help

regards

imar

Well....yes. Phase noise on the LO will directly add to the phase noise of the entire receiver system.

But figuring out the noise figure impact is not so straightforward. Noise figure, in dB is an indication of the sum total of noise of a reciever. This includes AM noise and PM noise, integrated over a wide frequency range. PM noise on an LO will mostly affect the received signal only near the carrier frequency, so when you integrate over a wide frequency band, its effect is smaller.

So you would have to figure out the thermal noise equivalent input to the receiver (usually half AM and half PM noise), figure out your frequency integration bandwidth, and figure out if the LO phase noise is adding to it. In my experience, unless we are talking about very specialized receivers, it is not an issue.

Do not ignore AM noise on the LO source, as the AM noise will get compressed in the mixer LO and turn into PM noise.

Rich

www.maguffinmicrowave.com

Thanks for replay


So, it is useless and unworthy to take care of the direct impact of the phase noise of the oscillator on the output mixer especially with wide bandwidth?

and then, we should take in consideration only the impact of the total noise figure on the hole receiver ?


regards
imar

If "noise figure" is all you care about, then yes. If your modulation is some sort of angle modulation, then noise figure is not as meaningful, and a phase noise specification would be more useful.