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From: | Robert McGwier |
Subject: | Re: [Discuss-gnuradio] Multiple USRPs |
Date: | Thu, 08 Dec 2005 11:28:05 -0500 |
User-agent: | Mozilla Thunderbird 1.0.2 (Windows/20050317) |
Most of us here do not need this level of complexity. We can do with much less. For example, I do coherent signal processing at HF, VHF, UHF to do timed difference of arrival geolocation with receivers covering a few hundred square kilometers by doing the same function with GPS receivers taming an oscillator. The GPS tamed oscillator is used to cohere the systems and to generate the pulse train that aids in calibration of the transfer function in front of the digitization process. I can geolocate a reasonable SNR signal at (say) 10 MHz using this system to a few meters and carrier phase is definitely in the calculation.
So one of the first lessons I learned in my travels was "Who is on first and What is on second". These players include:
NEVER use a PLL'd thing as the oscillator driving a system when you need high phase stability. It is okay to use a temperature compensated voltage controlled high quality crystal oscillator. You want the phase stability of any system guaranteed by a high quality oscillator (crystal, etc.).
If you also need ACCURACY, you use your Hy maser or you GPS tamed reference oscillator to tune the high quality VCO to get on frequency. Your design problem following this realization is to optimize which of the two different systems dominates the Allan Variance at what kind of time offsets. Short term phase stability is determined by the high stability (crystal) oscillator. Long term stability is determined by the Maser or GPS tamed beast used in a Tom Van Baak style mechanism to keep the VCO pulled in. The latter is provided by (say) the Reflock II boards available from TAPR.
Cheers, Bob Matt Ettus wrote:
This raises a question I've always had about phase-coherence of PLL oscillators that use a common clock source. Will they be coherent "enough" for things like astronomical interferometry?Yes.The individual PLLs will still have "close in" phase noise components that are unrelated to on another. I suppose that given that you typically integrate over several seconds, such artifacts get cancelled out. But at very fine timescales, I imagine that PLLs locked to a common clock are *not* a good way to get decent coherency?The close in noise is from the COMMON reference, so there is nothing to cancel, it is perfect. It is the further out phase noise components which are not the same between the 2 PLLs, since those components are from the individual VCOs. These integrate out fairly quickly, and are much smaller anyway. Matt
-- Laziness is the number one inspiration for ingenuity. Guilty as charged!
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