[USRP-users] Curious About Phase Noise of N210

Ben Hilburn ben.hilburn at ettus.com
Tue Feb 14 15:31:10 EST 2012

On Tue, Feb 14, 2012 at 12:12 PM, Robert Watson <
robertwatsonbath at googlemail.com> wrote:

> On 14 February 2012 18:04, Ben Hilburn <ben.hilburn at ettus.com> wrote:
> <snip>
> > If you decide to do this yourself, the quality of the external reference
> > really shouldn't affect it to a large degree, so I wouldn't fuss over it
> too
> > much.
> <snip>
> Is it really true that the reference isn't that important?

As you go on to describe, it really depends on the bandwidth you are
looking at.  More below...

> If I understand the N210 schematics correctly the 10MHz reference
> (internal or external) is applied to the AD9510 to phaselock a 100MHz
> VCXO. Within the PLL bandwidth (3kHz?) the phase noise will be around
> 20*log10(10) (ie 20dB) worse than the reference. Outside the loop
> bandwidth off the PLL the phase noise will be that of the untamed
> VCXO. The 100MHz clock from the VCXO makes its way to the
> daughterboard interface and, in the case of the WBX board, is used a
> reference clock for the ADF4350 wideband VCO/PLL. So, depending on the
> output frequency and  PLL loop bandwidth the final output phase noise
> will be further degraded depending on the effective frequency
> multiplication factor. Basically shouldn't the phase noise of the
> output will be 20*log10(N) times worse than the reference source phase
> noise?

Yes, everything you just said is true.  If you are very interested in the
phase noise within the bandwidth of the PLL, it is absolutely true that the
quality of your external reference will be a critical component.  As you
stated, outside of the PLL bandwidth, the phase noise will be dominated by
the VCXO performance.

I didn't realize that you were so interested in very narrow-band
measurements!  My previous comment certainly doesn't apply, in this case.
 Most of our customers are interested in wider-band applications, and I was
operating under the same assumption for this conversation.

> For our work the performance of the N210 is fine, the limitations lie
> elsewhere, so for me at least it's a bit academic - nonetheless be
> nice to know.


> PS. Can't find the schematics on the new Ettus site?

Yeah, they aren't up yet.  The new site is still undergoing quite a bit of
work, and the support / resources section isn't complete yet.  If you need
something, contact me off-list =)

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