Hi Erik!
Great progress! Sure interesting to look at them phase-noise plots,
right? It's a really good tool in addition to the stability of ADEV and
friends.
As I recall it, the ADE-1 is not documented to be isolated, but it is
very obvious when you look down the backside of it. However, it has
capacitive coupling and one should consider both common mode rejection
and common mode loading it down for these to work well.
Word of caution when it comes to levels, as the windowing filter used
causes shifts in noise-levels, so estimation of noise-levels becomes a
little bit tricky as you try to get the nitty gritty right, but getting
the overall shape view you already gained a lot with the things you
achieved.
A technique used to push further down into lower noise-levels is the
cross-correlation technique, where you split the signal into two
channels, each being exactly what you have now, and then rather than
squaring the output of the FFT from each channel, you multiply one with
the completment of the other, then average on those. This allows you to
supress the noise of each reference oscillator. You do not have to go
there from start, as you already make very useful measurements, but I'm
just suggesting what may lie up ahead.
Compared to some of the other sources, the Rigol SG does fairly well,
but then again, things can be even more quiet. For the XO you can see
the 15 dB/Oct slope as expected for flicker frequency. Try to locate the
source of the peaks you see and see if you can clean it up. The XO seems
to be a fairly good DUT for doing that.
Cheers,
Magnus
On 2022-06-25 20:07, Erik Kaashoek via time-nuts wrote:
Thanks to all the great help from people on this list I was able to
make some progress in doing close-in phase noise measurements.
The setup consists of a VC-OCXO going into the LO port of an ADE-1
mixer, The DUT into the RF port, the IF port is low pass filtered and
used to steer the VC-OCXO and is send to a high quality 24 bit USB
audio capture unit connected to a PC running ARTA.
The ADE-1 mixer was selected because that all ports are completely
isolated from each other, there is no common ground, which helps to
reduce ground loop problems a bit.
The log plots from ARTA confirm a 130dB dynamic range and the
resolution bandwidth is supposed to be about 1Hz. Each plot was
averaged over 10 measurements
All input signals where normalized by attenuators to have their
carrier at 0dB in the plot.
Using a generator at variable frequency offset it was confirmed the
audio input is flat down to 1Hz.
Using a generator with phase modulation down to 0.001 degree the
sensitivity of the measurement chain was checked. (20dB level
reduction with every factor 10 reduction in phase modulation depth)
It is expected to have at least +/-5dB level inaccuracy.
The DUTS measured where:
- a fairly clean XO (PN_XO.JPG)
- a rather bad GPSDO output (PN_GPSDO.JPG)
- The not so famous cheap Chinese TCXO (PN_TCXO.KPG)
- The output of a Rigol SG (PN_Rigol.JPG)
The XO is the cleanest
The TCXO shows odd spurs between 10 and 40 Hz and the PN does not drop
down as it should (spec states: -135dBc/Hz at 1kHz offset)
The GPSDO is terrible, this demonstrates you can have a 1e-10 ADEV at
1s tau from a bad oscillator.
The Rigol is not so clean and a PLL shoulder seems to be present just
above 1kHz.
Next step is to add low noise gain close to the mixer LPF output to
get more dynamic range and a better VC-OCXO (Morion MV170 (PN
-100dBc/Hz at 1Hz offset) to lower the impact of the reference VC-OCXO
Erik.
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