On Aug 17, 2010, at 2:24 PM, Gregory Maxwell wrote: > I'd also like to echo the 10MHz comment. GPSDOs Clocks with excellent > long term stability show up at fairly low prices on ebay all the time > (excess from cell site deployments, I assume). I have a couple of > them. What they don't usually have is the very low phase noise that > I'd want for a clock which is eventually going to multiplied up to GHz > levels. So a USRP1 clock board which was primarily a 10->64MHz > up-converter with very low phase noise would be exactly what I would > want.
For my bench frequency reference, I selected a surplus Trimble Thunderbolt with the newer version of OCXO that's supposed to be nearly as good as the HP double-oven OCXOs. I'll power it from an HP bench supply, since the switchers that are often supplied with the surplus Thunderbolts are said to add a lot of phase noise to the oscillator output. If the statistics from the monitoring program that I used during my first test run of the oscillator are to be trusted, then it should be able to provide a reference accurate to within tens of parts per trillion. I haven't finished installation of my GPSDO yet; I still need to finish repairing the (cheap, broken) HP bench supply that I got from eBay and assemble a power cable. I may get this done tonight, as the parts I needed just arrived today. The outdoor antenna (a surplus Lucent +26dB antenna as used at cell sites) is already installed and cabled into my house. My own idea for a USRP clocking replacement was to use a common and fairly inexpensive VCTCXO rated for temperature stability of around +/-2.5ppm (available at Digi-Key in frequencies commonly used in cell phones, GPS receivers, etc.), drive a PLL+VCO with that to generate 64 MHz (or 52 MHz, or any other frequency that the TCXO+PLL+VCO can generate), with an additional PLL to lock the VCTCXO to an external 10 MHz input. I'd also include a microcontroller which could measure the VCTCXO control voltage while locked to an external reference, and then drive that same voltage with a DAC when the external reference isn't present. Thus, while attached to the GPSDO the internal reference would be slaved to a very good reference, and then when that reference isn't available (such as when operating "in the field") the on-board VCTCXO would at least be trimmed to compensate for aging. The memorization of the nominal VCTCXO tuning voltage might be triggered automatically, or by a push button, or by a GPIO controlled by the USRP hardware. The VCTCXO might be replaced with a voltage-controllable OCXO if desired for better holdover stability, but I figured that VCTCXO should be adequate for my needs. Does that architecture sound reasonable? If so, would anybody like to save me the trouble of designing and building it myself? ;) -- Mark J. Blair, NF6X <[email protected]> Web page: http://www.nf6x.net/ GnuPG public key available from my web page. _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
