How to interpret these phase noise plots? - Page 1
Forum section: electronics / projects

author: erikka
author_rank: Regular Contributor
timestamp: July 12, 2025, 02:45:27 pm
content: I'm trying to do phase noise measurements using a FSEM30 and the PN.EXE program from the GPIB libraryThe plots are shown belowThe pink trace is a measurement of the REF OUT of the FSEM30. Would I be correct to assume that this plot reflects the phase noise of the LO PLL of the FSEM30?The blue and red traces are both a HP8672A measured at 3 GHz. The red trace is done with the FSEM externally locked to the HP8672A. The difference is very small so I think I may conclude the references of the HP8672A and the FSEM30 have (much) lower phase noise and I'm measuring the phase noise of the HP8672A output as I'm assuming the FSEM30 has much lower phase noise (see pink trace)The green trace is the tracking generator output of a SSA3032X+ also at 3 GHz.Does this all seem correct?

author: WideBandwidth
author_rank: Contributor
timestamp: July 12, 2025, 03:28:26 pm
content: Pink trace: Your conclusion seems correct to me. You would be measuring any phase noise not correlated with the reference of the FSEM30, which would be the pll-generated phase noise (e.g. the phase detector noise) and the noise of the LO vco. There may also be multiple LOs used in the signal path. However, since they should all be synthesized from the same reference, they do not all contribute phase noise within the pll bandwidths in a additive fashion since they are correlated. Outside the pll bandwidths, the noise of the VCOs would add however, since they should be uncorrelated at these large offset frequencies.Red and Blue: By locking the two together, their phase noises becomes correlated at low frequency offsets. The lock typically has a small bandwidth (on the order of 10 or 100 Hz), and you may be able to configure this bandwidth depending on how much you paid for your equipment (the snakes lock these things behind paid options...). The red and Blue traces appear to converge at around 50 Hz, which seems to indicate their reference locking bandwidth is around 50 Hz. Any offset larger than 50 Hz would yield the same phase noise measurement between red and blue. The difference below 50 Hz is the uncorrelated noise coming from the two instruments.You never mentioned the reference frequency of the FSEM30, but I'd assume it's either 10Hz or 100 MHz? So you would expect the phase noise floor of the FSEM30 to be lower at the reference frequency than at 3 GHz, since generating a higher frequency from a lower frequency reference incurs an unavoidable phase noise penalty of 20log(N), where N is the frequency ratio. So assuming a 100 MHz internal reference, that means the measurement at 3 GHz would have around 29 dB higher noise floor compared to a 100 MHz measurement within the internal PLL bandwidth. (You can check this in the SA manual, they will list phase noise at certain center frequencies). This seems to account for the difference between the red and blue/red curves, and indicates that at least some frequency offset range of your measurement is limited by the phase noise floor of the spectrum analyzer.

author: radiolistener
author_rank: Super Contributor
timestamp: July 12, 2025, 03:35:56 pm
content: can you share data sample file?

author: erikka
author_rank: Regular Contributor
timestamp: July 12, 2025, 06:35:20 pm
content: Quote from: WideBandwidth on July 12, 2025, 03:28:26 pmThis I do not understand.The first IF of the FSEM30 is at 7.9 GHz. Any input below 7 GHz is mixed with the first LO between 7.9 GHz and 14.9 GHz.The difference in contribution of the phase noise of the first LO of the FSEM is maximum 3 dB for any input frequency between 0 and 7 GHz due to this high first LOThe second LO down converts to 740 MHz.The internal reference is at 120 MHz and can be locked to "any" external frequency, probably using a slow lock. This could indeed explain the phase noise difference below 50 Hz

author: RFDx
author_rank: Regular Contributor
timestamp: July 13, 2025, 05:55:58 am
content: Quote from: erikka on July 12, 2025, 06:35:20 pmYes, the pink trace is basically the PN of the 1st LO. The 2nd and 3rd LO are obtained from the internal, very clean 120 MHz VHF reference by direct multiplication and are, in terms of PN, better than the 1st LO. All FSEx30 models are equipped as standard with the low phase noise module, which decreases the PN of the 1st LO by at least 20dB. This explains the very good result compared to that of the HP signal generator.

author: erikka
author_rank: Regular Contributor
timestamp: July 13, 2025, 06:36:54 am
content: As requested some data files