Hi all,
My aim is to measure NF of a downconverting mixer, IFout is around 200MHz. I have an agilent PSA for NF measurement and my LO source choices for the mixer are an agilent sinal gen. without ultra low phase noise option and a PLLVCO IC from Hittite. Hittite part has better far out phase noise performance, ie phase noise at 10M offset is 9 dB better than agilent, and it goes flat through higher offset freqs(let me remind my if is around 200M). This effects my NF.
Agilent has better PN at offset frequencies upto 500kHz, as expected.
Previously I used a cavity PBF from KnL around LO, of course that helped. But after finding more about mixer I will desing for a decent LO.
Do you have any comments and/or suggestions on this topic, (biasing and matching improvements are already done at mixer side)? I am especially asking about LO effects on NF.
Thanks in advance...
I guess I could not explain myself clearly. I am aware that close in phase noise does not (much)effect mixer NF but far out PN, which may be the same as AM noise of LO you are referring to (i dont know are they the same), has an effect on NF since it adds to noise floor. My concern is that when measuring IF at 200M, LO+200M noise floor level should clearly effect mixer NF, am i right?Noise figure of a mixer is the effect of THERMAL noise. PHASE noise of the local oscillator CANNOT affect the NF as it is exactly orthogonal to the thermal noise. In other words, the phase noise is similar to jitter- the PN spectrum shows how fast the carrier is moved due to vibrations and thermal noise pushing the carrier frequency.
Due to this orthogonality, mixer or receiver NF cannot be affected. BUT any oscillator also has the AMPLITUDE noise, directly the result of the thermal noise on carrier AMPLITUDE. A noisy oscillator does have an AMPLITUDE as well as PHASE noise. To counter the effect of the first (amplitude) noise on mixer NF, we prefer to use BALANCED mixers; in such device, the LO AM noise is reduced by up to 20 dB by the balanced structure.
I agree on Thermal noise. the point is PN also has impact.
Amplitude noise in LO is something i am new and interesting to me. if AM noise is important why to use balanced structure why not some clipper circuits?. Also Many questions i am raising here. we see double and triple balanced mixers each for IF, LO and RF.
Does this mean we are handling Amplitude noise of RF and IF signals apart from LO? i guess not.
I believe balance is the method to improve RL on port that intern improve the isolation.
Sorry if i mistake and request jiripolivka to give more explanation.
Regards.
Hi, the root cause of improvement of NF, in my view, is that you used filter to filter other harmonics or spurs of the LO signal, then there are less spurs in mixer in the working band, and that can sharply decrease the NF.
BTW, too bad PN of LO might mean that there are some spurs in the LO signal spectrum.
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