On 9/28/12 8:31 PM, Chris Albertson wrote:
On Fri, Sep 28, 2012 at 5:29 PM, Bob Camp <li...@rtty.us> wrote:
HI
Sort of an open ended question, but there is a fairly simple couple answers:
SInce it's close in phase noise and not far removed, things like PLL's are
going to transfer it directly from the reference to the output. It will of
course scale by 20 log N where N is the amount you multiplied or divided the
reference frequency by. Double the frequency and the phase noise goes up by 6
db.
So in my example case of scaling the 10Mhz t-bolt to 14.5Mhz Assuming
a perfect DDS chip the T-Bolt's phase noise would be scaled up by 20
Log(1.45) I'm assuming this works, that I can go from 10MHz to
120Mhz and then to 14.5MHZ and the total effect is the same as going
directly from 10 to 14.5, except for the noise the equipment
introduces as added.
You can guess the real question here: "how good does the 10MHz
reference need to be to test real-world receivers?
It has to be quieter than the oscillator in the real world receiver. If
your real-wold receiver is a cryogenic ruby maser with a downconveter
driven by a hydrogen maser reference, then the answer is "really, really
good"..
If the real world receiver uses a run of the mill TCXO, then not nearly
as good.
The 20log10(N) thing does work pretty well. In a PLL synthesizer,
you'll pick up a little extra noise from the phase detector and other
circuitry, but for back of the envelope to see if your idea is going to
work, the 20log10(N) is just fine.
This gets into a whole interesting area of microwave source design,
because "inside the loop" the phase noise is the reference oscillator
multiplied up (20log10(N) noise), and outside the loop, it's the
microwave oscillator. So you have an interesting optimization problem,
particularly if you want tuning over a wide range. Wide range VCOs
implies that the MHz/volt gain is quite high, so noise on the tuning
signal shows up on the output. The resonator is often lower Q, so that
it can be moved around by the control signal (usually some sort of
varactor scheme), and that means the "medium distance away" phase noise
suffers. High performance DROs for instance, have a tough time
tuning the entire 50 MHz deep space comm bands at 7 or 8 GHz
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