Electronics > Beginners
Measuring an OCXO - or not
KE5FX:
--- Quote from: imo on October 11, 2018, 07:53:07 am ---@KE5FX: do you mean a software DDS? Or an external DDS chip?
--- End quote ---
You can think of an external DDS chip as the last stage of a software DDS. The software knows exactly how much tuning error exists at any given moment, and (with some effort) it can keep track of how long that error has existed. Each time it reprograms the tuning word to steer the GPSDO output, it can add or subtract a few parts in 10^-10 (or whatever) to compensate. As long as the error is so small that it's impossible or impractical for an external user to notice it before it's corrected, it's all good. :)
jpb:
Leapsecond has an analysis + measurements on the FE405 which uses this approach
http://www.leapsecond.com/pages/fe405/
But the FE405 uses the DDS ouptut in a PLL with a further output crystal to smooth out most of the DDS noise, there are still some spurs visible though.
iMo:
While reading the first patent - this is something I need to understand:
--- Quote ---The essentially zero-beat feed-back signal from the second mixer 30 closes the phase-locked loop and locks the vcxo 38 to the desired output frequency.
--- End quote ---
metrologist:
So, it appears I am not measuring phase noise of the oscillator with my equipment. There is so much signal and I have a 10dB pad on the RF Input, the noise floor of the spa is ~-100dBm. The largest spur was ~-70dBc, but I was thinking of using the noise marker feature. I'll look again tonight.
metrologist:
Posting another set of weekly data. I adjusted the plots so the traces should line up... and also the trendlines on the top plot are now linear and lower is a power, both projecting 10 samples. I think I should change the lower plot to MADEV.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version