); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY Hi Everybody,
Thanks a lot for your help concerning the Ethernet question. It seems we will stick to a custom made timing network for the moment. In any case I would like to describe you the principles of the PLL we use in our timing system. **First of all our timing signal nowadays consists of a data stream Manchester encoded at 500kB/s (1MHz carrier). Data are always sent in packets of 4 bytes, with a preamble and codes violations at the beginning and the end to delimit the frames. Between frames we send a 500kHz clock to feed the pll. This the classic CERN timing signal that has been around since beginning of the 80s. **The phase detector is fully implemented in the FPGA. The PHD waits for at least 3 ticks of the 1MHz carrier to perform a measurement. This way I remove the intersymbol interference in more or less reasonable way. To do this measurement I use a counter clocked by an external 44.736MHz free running oscillator. Then I do an average of 2048 samples and pass it to a PI. I have put some logic in the PHD to saturate in the extreme values, this way I can have very small bandwidths without any lock-in problem. In theory if the 44.736MHz XO is uncorrelated with the 1MHZ then the noise introduced by it will be of NoisePHD~= (1/12)^.5*Txo/(Nsamples)^.5 ~22ns/3.4/(2048)^.5 ~140ps If you measure for one second then you get: NoisePHD1sec~22ns/12/(1e6)^.5~ 7ps (this is ~7e-12 in only one second, which is not too bad) This always relies on the "uncorrelation" of the 44.736MHz with the oscillators under test. I don't know if one can really assume this as a fact... In any case, I guess this can be used as cheap way to compare frequency standards. **The control is a typical PI. The output is connected to 16 bit DAC which drives a TCXO (cft-125 by CMAC). The bandwidth of the loop is ~2Hz. When the PLL reference signal is disconnected I pass to the DAC an averaged value of the TCXO frequency over ~16 sec The jitter I measured with a good scope is ~120ps between two modules for several minutes. The hold over I have obtained with the cft-125 is better than 5us for 30min (normally much better) Pablo ALVAREZ SANCHEZ CERN - AB Department CH-1211 Geneva 23 GSM : +41 (0)76 48 72191 Phone : +41 (0)22 76 78431 Fax : +41 (0)22 76 69318 Building : 864-1-A30 _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
