Magnus, Bob,
When the mixer is operating in the linear region for the DUT input (0dBm
or lower), would it be possible to use a calibrated noise sources to do
an extra verification of the noise level measurement?
Of course with a noise source you get 3dB as both sidebands fold.
Verification steps:
Verify the DUT output level is correctly brought to 0dB (using
attenuators) using a calibrated spectrum analyzer
Connect the DUT to the phase measurement setup and set the reference to
a 500Hz offset to get a beat note and verify the beat note is registered
at 0dB, change the DUT level some dB up and down to confirm its in a
linear region.
Measure the per Hz output power of a noise source using a calibrated
spectrum analyzer and a noise marker set to 10MHz.
Connect the noise source to the phase measurement setup and check if the
noise level is measured at level measured by the spectrum analyzer + 3dB
This should work if the RBW of the phase measurement is indeed set to 1Hz.
Another verification option may be to use the phase modulation of a
signal generator. This can not check the effective noise bandwidth of
the FFT but it can check linearity over the whole range.
The output of the mixer is terminated with 50ohm so a factor of 10 in
voltage should give a 20dB power step.
When operation in the linear range the phase noise measurement setup
should measure 20dB less with every factor 10 reduction in phase
modulation depth where 90 degrees is equal to 100% modulation depth so
equal to the signal you get when measuring a beat note.
When measuring with modulation depth of 90,9,0.9,0.09 and 0.009 degrees
the measured level should step from 0,-20,-40, -60 to -80dB
Any feedback?
Erik.
On 26-6-2022 20:52, Magnus Danielson via time-nuts wrote:
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
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