At 03:49 PM 10/6/2007, Bill Tippett wrote:
> As an example of this, take a look at Phase Noise:
>
>Rig 1kHz 2 10 20 50 100 1M
>
>K3 -110 -119 -136 -140 -143 -144 -150
>5000 -123(flat)*
>
>*(Sherwood) "Phase noise does not fall off at 6 dB per octave as expected.
>Flex believes the present phase noise limitations are caused by A/D
>clock jitter". ("Clock jitter" is another word for phase noise).
>
>In this case the K3 has the advantage, since its phase noise
>integrated over the entire spectrum will be lower. The 5000's relatively
>higher phase noise will likely be an issue, especially for VHF users.
Perhaps what Rob is measuring isn't actually phase noise? What's the
measurement methodology? I note the K2 also doesn't show the 1/f^2
characteristic (going from 10 to 100 kHz goes from -136 to -144, not
the 20 dB one might expect)
The effects of clock jitter are basically the same as with any other
mixing process, it's just stated in different units (you figure it
out by integrating the noise power spectrum), but there's no
particular reason that the A/D clock noise would have a flat
spectrum. (The sampling of an A/D is just like mixing). Most A/D
interfaces derive their clock by dividing down from a crystal
oscillator of some sort, so unless the divider is really crummy, the
noise power distribution of the A/D clock looks just like that of the
crystal (reduced by 20log10(N) as expected).
Certainly, when I was measuring such things with the ratty on-mobo
sound interface and a SDR1000, the noise contribution from the sound
card clock wasn't flat.
I suspect that:
a) Flex hasn't analyzed the problem in sufficient detail, and is
throwing out a glib assertion of A/D clock jitter as a possible explanation.
b) you're seeing a measurement artifact from Rob's measurement process.
It could well be that what Rob is measuring does reflect the actual
performance, but that the observed noise floor isn't phase noise.
If anyone has more information, I'll be happy to look at it, and do
the analysis. The effects of non-ideal oscillators in radios is
something I spend a fair amount of time looking at, as is measuring
the performance of insanely quiet oscillators. (When you're trying to
receive a 7.15 GHz carrier at -155 dBm in deep space, the receiver LO
has to be pretty quiet so you can use a very narrow filter, and we
have a typical "coherent turn around" spec that the Allan deviation
has to be <1e-15 over 1000 seconds)
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