In article ,
says...
Thanks for all the help.
The phase noise results seem unachievable, but I hardly think Minicircuits
are lying, and that it should be pretty easy to achieve -75dBc at 1,000Hz
away. Even if it was -60dBc I'd be happy. But all I know is I'm getting the
same results on all POS-900Ws I have.

I'm using an Anritsu MS2661C, and basically just hoping to see the
traditional 'needle spike' at Fc. (which is what I see from other VCOs with
the Spectrum Analyzer on the same settings). But, instead, it's more like a
triangle than a needle, and nothing seems to change that.

You're subtracting 30 dB (i.e., 10*log10(RBW)) to account for the 1-khz
RBW, right?

Also, to look at noise at a 1 kHz offset, you need a much narrower RBW
filter. Looking at 1 kHz from the carrier through a 1 kHz filter, you
will be only -6 dB down (or -3, depending on how your analyzer specifies
its RBW) with a perfect signal! Take a look at the parameters in the
graphs at http://www.qsl.net/ke5fx/synth.html -- I use a 100 Hz RBW
filter to make measurements at 1 kHz from the carrier.

Also, what other VCOs are you comparing to? A rule of thumb is that
equivalent-quality VCOs tend to get about 6 dB noisier per octave of
frequency. A very good 900-MHz VCO will have about the same noise
profile as a crappy 90-MHz VCO.... and that's without even getting into
the tuning-sensitivity question that others have raised.

Finally, simple temperature-related drift (both short term and long
term) will make it tough to measure anything 1 kHz from the carrier of a
900 MHz VCO. You're expecting almost one part per million stability
from an unlocked VCO, and that isn't reasonable. To characterize phase
noise at 1 kHz from the carrier, you should:

1) Lock the VCO with a PLL whose loop bandwidth is at least an order of
magnitude lower (100 Hz or less)

2) Use an analyzer RBW at least an order of magnitude lower (100 Hz or
less)

3) Perform this test only with a stabilized analyzer (otherwise, its
internal LO's noise performance may be even worse than the VCO under
test... something that's especially likely with lower-frequency VCOs)

4) Don't forget the 10*log10(RBW) adjustment factor.

Those four steps will get you close, but to do the job right, you need
two mo

5) Distinguish between AM and PM noise by using the VCO under test to
drive a mixer's LO port to saturation, with a significantly-cleaner
reference driving the other port of the mixer

6) Compensate for the noise-equivalent bandwidth of your analyzer's RBW
filters. Typically this will make the true noise values 2-3 dB worse
than the observed values.

All 6 of these steps may be necessary to reproduce Mini-Circuits'
catalog specification for the VCO.

-- john

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