At 05:29 PM 11/22/2005, [EMAIL PROTECTED] wrote:
Hello Flexers,
I did a freq cal four days ago and have checked it
everyday since. Here is what I see on my SDR, -1Hz to +2Hz.
Also a comment about the Rubidium, Cesium and Crystal oscillators.
Oscillators are characterized by short term(phase noise) and long
term(drift) freq stability. The phase noise it what concerns us
most when it comes to receiving. We want it low enough so that it
doesn't swamp weak signals. Ideally it should be >10db below the
noise floor of the receiver so as to not reduce sensitivity and
hence dynamic range. It is the 'Q' of the resonant component(s)
that determine the phase noise. The quartz crystal is still the
leader when it comes phase noise. Rubidium and Cesium are superior
in time keeping (long term stability), but have awful phase noise.
These are the 'real' atomic clocks. The navy is the largest user
of these. No GPS under water! Regarding GPS, its timing is
based on the Cesium clocks.
There ARE actually sources with better close in phase noise than a quartz
crystal, just in case you see one at a hamfest or surplus place (or, you're
wealthy enough). A hydrogen maser, for instance (that's what we use at
work, JPL, when we're concerned about such things.. but then we have an
infrastructure to distribute the maser signal around, and a budget for the
support staff). A more portable high Q resonator is a sapphire resonator
(which is used in some high performance phase noise test sets). Another
one is a superconducting cavity resonator (which isn't as impractical as it
might seem, with high temperature superconductors(liquid nitrogen temperature).
As far as long term stable sources go, there's also things like Mercury Ion
traps, which I believe can provide Cs quality long term, but also are high
Q so they're good phase noise.
However, as John says.. there's lots of really good quality 10 MHz crystal
oscillators out there fairly cheap.
