In a message dated 3/2/2007 03:11:25 Pacific Standard Time, [EMAIL PROTECTED] writes:
Phase noise of -115dBc/Hz @ 10Hz for a 100MHz carrier sounds a bit steep when compared to one of Wenzel's ultra low noise ULN series which achieves -125dBc/Hz @ 100Hz. (these oscillators are probably the best you can buy). I guess it will probably climb to about -112dBc/Hz @ 10Hz. Your prediction postulate that the close-in phase noise of two devices, the digitally down divided 100MHz and the state-of-the-art 100MHz low-noise oscillator, will be comparable. I don't mean to contradict you, since I am really not an expert, but this spec sounds a little suspect. Maybe there is something I am missing? If this spec is correct, I am strongly considering it. Hi Stephan, you are probably right, I did make some aggressive assumptions about the phase noise of the Fox oscillator and DDS, that's why I said 'theoretically'. But as Bruce described, when dividing a frequency by 10, the phase noise offset also moves closer to the carrier by 10x. Also, a DDS is not strictly 'a divider'. It's a bit more complex, especially since Analog Devices didn't publish all of their specs yet :) Will let you know what kind of phase noise Fox will claim for their Xpresso parts. BTW: since you are using the signal as a 'digital' clock reference for a DDS, I think you really need to also look at the jitter spec (RMS, Total, Deterministic versus Random, etc) rather than just close-in Phase Noise. Wenzel has some Excel spread sheets that calculate ADEV from Phase noise, and I am not convinced that it is that easy to go between Phase Noise, ADEV, and Jitter by such simple calculations. For example, Wenzel's calculations don't take into account any spurs, which will affect your ADEV, and show up as deterministic noise on the Jitter measurement spreading out the Bell curve jitter histogramm, but you will never see them on a close-in Phase Noise plot if they are outside of the Sideband noise spectrum being shown (for example if they are inside the other sideband bandwidth not being measured). Also, phase noise measurements show all noise, including amplitude, phase, and frequency deviations of the analog signal within the measurement bandwidth. Jitter measurements only show the noise present on the zero-crossing of the signal - but that with a huge measurement bandwidth, so these will likely measure slightly differently. Of course the zero crossings are what affect the digital DDS you are driving. Take a look at the Wavecrest instruments (SIA-4000, DTS-2070 etc) these measure the signal essentially with a huge measurement bandwidth (2GHz - >10GHz BW) so they capture all relevant noise that will cause jitter. This compares to close-in phase noise which only has a measurement BW of 0.1 to 1MHz typically and disregards any noise sources present outside this BW. Not sure what will affect your system more: jitter due to e.g. deterministic noise, or close-in phase noise. You may have to do some experiments to see how well theory matches reality. bye, Said <BR><BR><BR>**************************************<BR> AOL now offers free email to everyone. Find out more about what's free from AOL at http://www.aol.com. _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts