Posted by
Leigh L. Klotz Jr WA5ZNU on
Apr 05, 2011; 5:35pm
URL: http://elecraft.85.s1.nabble.com/K3EXREF-and-Trimble-Thunderbolts-tp6242158p6243171.html
Julian, G4ILO wrote
What are the advantages / disadvantages of this type of frequency standard over the Efratom LPRO-101 which is a rubidium standard?
Julian, G4ILO
I'm a duffer at this time/frequency stuff, but I've scored well enough at the ARRL FMT to get my call in QST, and I gave a presentation on it at the Bay Area Maker Faire last year, and a NIST scientist told me I didn't say anything egregiously wrong, so I'll try to stick to what I understand here. If you want to know more, go to Febo.com or time-nuts and read a bit (before you ask questions) and you'll find people with literally orders of magnitude more experience (and interest) than you ever thought possible.
Both an Rb oscillator and a GPS-DO (and for that matter, your own Cesium standard) can provide you with a frequency output, but they're different types of things. A GPS-DO is a managed device with an output, like WWV or the grid 60 Hz (mains 50Hz) frequency. A Rubidium oscillator is like a quartz crystal, but more stable over the long term (modulo a well-defined linear drift), so once it's set, it stays set exceptionally well, though usually the phase noise isn't as good as a temperature-controlled quartz crystal. A Cesium standard is called a standard because it's how we define time; when properly adjusted, it will remain in calibration quite well.
A GPS-DO usually contains a crystal oscillator which is controlled (disciplined) to match the data coming in from GSP satellites, which themselves contains Cesium standards which themselves are actively managed by the US government. So it's like listening to WWV or one of the other time signals on HF, but it uses the 1.4 GHz range and averages multiple signals, so you get better results with smaller propagation-induced errors. There are some Rubidium GPS-DO devices as well, some commercial and some done by smart hams, but even those, as far as I know, contain a quartz crystal oscillator which itself is disciplined by the Rubidium bulb microwave resonance.
There's arguments about different ways of disciplining all of these oscillators and if you're interested, look for the time-nuts mailing list or read on febo.com, or read about Allan Deviation. But briefly: there are multiple sources of noise, and if you measure the noise on different time scales, you get different answers about how much noise there is. So, roughly, phase noise is at one end of the chart and how far your wristwatch is off at the end of the month is at the other end. What time scale you care about determines what type of oscillator you use and how you measure and take care of it.
A Rubidium oscillator does quite well on many of these time scales, better than Cesium in some of them, but worse than a plain quartz crystal in others.
Unfortunately, my guess is that buying an Rb standard and using it instead of the quartz TCXO in the K3 would result in worse phase noise which translates into a noisier band RX and noisier TX. Quartz is pretty much the way to go for transceivers given the areas of concern that hams have. (I.e., would you rather have 20dB less band noise or be able to get within 0.1 millihertz of the band edge instead of just 1 millihertz of the band edge?)
More philosophically, we define time in terms of what Cesium does. Both Cs and Rb physics packages have knobs to adjust for external magnetic field ("C" Field), so both can be set wrong, but Rubidium bulbs "age" and change frequency (though at an fairly well characterized rate), so you can't substitute a Rubidium oscillator for a Cesium one. According to NIST, gravity is a big effect as well, so if you ever move one of your oscillators, its calibration field is toast. And finally, temperature control is of the utmost importance, and those who are serious use multiple nested containers for temperature isolation.
For ham use as a frequency standard, in terms of phase noise performance, the algorithm used and the oscillator that's being disciplined (and how often, as part of the algorithm) has the strongest influence.
For example, a GPS-DO is additionally affected by what satellites are in view (the "constellation"), and the Trimble units are affected by the changes more so than more expensive units which are designed as frequency references, because they make abrupt changes when the constellation changes, which is pretty much every few minutes as sats pass over.
The eBay Trimble units in particular have trouble with this. They were designed for E-911 installations in cell sites, where they were used to calculate distance via speed of light to a mobile unit. My (unfounded, personal) belief is that it was more important that nearby units all had the same idea of the time than that they have reduced noise on the shorter-term. So they make more frequent adjustments to their internal quartz oscillators that are comfortable for a frequency standard, though a number of smart folks have figured out algorithm tweaks to mitigate this effect, making the temperature the dominant factor, for time scales of 1hr - 24 hrs.
The K3XREF product that Elecraft is developing is, to my understanding, an external disciplining interface for the TCXO in the K3. It's probably a frequency counter / microprocessor which reads the internal oscillator and the external 10 MHz reference, and when the internal oscillator doesn't produce the right number of cycles in 10 million of the external oscillator's cycles, it adjusts the voltage on the TCXO to bring it back into spec. But if you do this too often, you'll introduce phase noise into the K3 (think of it as FM-ing). Wayne N6KR has said it does this a few times a second an has achieved a trade off between accuracy and phase noise. (I presume he doesn't adjust during TX, for example.)
I leave my K3 on most of the time, and I've found that it is seldom off more than +/- 3 Hz. But the K3XREF would let it off +/- 1Hz as soon as you turn it on, provided it's hooked up to your external reference. The actual received frequency is only valid for one mode and one filter, once you calibrate it, since when you shift modes or filters the offets of the various internal IF stages varies. (Keep that in mind if you use the K3 for the ARRL FMT.)
Leigh/WA5ZNU