J. Freed
I followed Step 1 of the double mixer test plan in T2400327 under "1. Characterize frequency references."
PNSPIDACTestStand.pdf Shows the results in dBc/Hz. And PNSPIDACTestStandrad.pdf shows the result in rad/sqrt(Hz).
Besides the 80MHz standard reference there were 2 devices under test, the IFR 2023A and the SRS SG382. These two devices went under 3 tests involving different combinations of
1. Timing to a 10MHz frequency standard produced by the 80MHz Standard. (Time Standard)
2. PLL locking to a different 80MHz frequency ref through the tune in port. (Lock)
During this process it was assumed that the 80MHz standard and the added 80MHz ref would have a simmilar noise profile as they come from a simmialar OCXO.
A 4th test was added to the Time Standard/No Lock test which used the SR785s internal high pass filter (-3dB at 0.16Hz) to remove some of the DC components from the test. This is because the results of the initial test were inconclusive as the noise floor of the SR785 was too high. The noise floor was too high because there was a DC signal that caused the input range of the SR785 to be about 20dB. The high pass filter, removed the DC signal, lowering the noise floor at the cost of signals below about 0.2Hz not being accurate. We believe this is caused by the fact that while the PLL locking can be controled (tries to lock signals to destructivly interfere so to lower output power for Phase noise tests), there was no control by the Time Standard on phase differences between oscilators. As Time Standard/ No lock, showed the best results above 0.06Hz and is closest to the SPI set up design, this fact may be important when doing futher tests.
Between IFR and SRS, the SRS showed better phase noise performance below 7kHz. While IFR showed better performance above 7kHz. As such, SRS shows more promise in SPIs phase noise range of interest 0-4096Hz.
After a talk with Jeff what is really important to show is the TIme Standard/Lock which is the 80 MHz OCXO, IFR and SRS which we believe is most represntative of phase noise
These are, one at a time, locked to another 80 MHz REF OCXO. In this it was assumed that the 80 MHz OCXO and the 80 MHz REF OCXO would produce the same phase noise, as such contribute equally to the output phase noise. As such the output was manually halved to represent the only the 80 MHz OCXO. As stated before these measurments also had a Time Standard (sometimes called time sync) where the 80 MHz OCXO is synced to the site's 1PPS. And the IFR and SRS have a 10MHz signal time standard signal that was produced by the 80 MHz OCXO divided by 8 and sent into the back of the devices.
PNSPIDACTestStandrad(sync_lockOnly).pdf Shows only the results of the Time Standard/Lock, which we believe to be the most represntative of the phasenoise of these devices.
Since it was assumed that the two OCXOs would have the same phase noise, this also makes our noise limit equal the OCXO. As such, it can be seen from this graph that we are at the noise limit in measuring the phase noise of the IFR and SRS between 0.1Hz-10Hz.