As far as I can see the PLL loop filter (SR620) has a corner frequency of about 33Hz. My HP counter 5385A has simply a switch to toggle between the external source and the internal osc.
Am 18.11.2012 22:45, schrieb Bob Camp:
Hi Most modern counters have an internal oscillator that they lock up to the external reference. The PLL bandwidth is (hopefully) chosen to optimize the noise contribution of the internal oscillator (quiet broad band) and the external reference (quiet close in). It will reject anything MHz away from the reference. Even if the reference signal is used directly, it would only be a problem if divided by an odd number. Even number division will wipe out the jitter. The most likely gate times are all a power of 10, so the division should always be even in a counter. Bottom line - you are unlikely to run into trouble using 10 MHz out of a Z3805 as reference signal. Crazy stuff…. Bob On Nov 18, 2012, at 4:35 PM, Volker Esper<[email protected]> wrote:...A PLL multiplier will (likely) also ignore the sub-harmonic, the jitter will not show up if you use the 10 MHz as a counter timebase in a HP counter...Yes, I wondered what the histogram would show when using the 2-maxima-Z3805 signal as a reference for the SR620 counter. Would the PLL filter it or would I get a mixer-like result with two maxima in the histogram? I applied the Z3805 to the reference input and the SMX signal generator to the counter input. The histogram looked equal to that I got when using the internal oscillator of the counter for reference. That obviously confirms Bob's statement. Volker Am 18.11.2012 21:23, schrieb Bob Camp:Hi This is one of those interesting areas where a broadband measurement (like a counter) and a narrowband measurement (like a DMTD) will give very different results. The counter will see the sub-harmonic, and call it jitter. It can show up in an ADEV calculation. A DMTD will ignore the sub-harmonic, and show no added jitter. With a DMTD, the ADEV will look fine. A PLL multiplier will (likely) also ignore the sub-harmonic, the jitter will not show up if you use the 10 MHz as a counter timebase in a HP counter. Which one's right / does it matter - as always, that depends on what you are trying to do with the signal. Bob On Nov 18, 2012, at 1:26 PM, Volker Esper<[email protected]> wrote:@Azelio: Yes, (like Bob told) it's the phase lock, that locks the two maxima. Actually the counter is able to look between the 60ps spaced towers, have a look at the pictures in the first mail of this thread. However, there is a smallest time value the counter can determine, but it is much smaller than 60ps. Volker Am 18.11.2012 17:11, schrieb Bob Camp:Hi The 5 MHz and 10 MHz are "locked together" by the multiplier. Their relative phase is fixed. The 5 MHz component is not free to wander independently relative to the 10 MHz. Since it's a doubler, you get a two peak plot. If it was a X3 you would get three peaks. It really only makes sense if you look at it on a scope. Of course at 60 ps, it would have to be a mighty good scope. Much easier to see when the sub-harmonic is a bit stronger... Bob On Nov 18, 2012, at 9:53 AM, Azelio Boriani<[email protected]> wrote:Interesting this, but why exactly 60pS? I would expect also something in-between. That is, to have the 5MHz energy 64dB below, in my opinion nothing prevents to have the edge to wander not only at 60pS but continuously between 0 and 60pS. Maybe the SR620, being a 25pS resolution counter, simply doesn't see what's going on in-between and the resulting histogram looks that way. On Sun, Nov 18, 2012 at 3:51 AM, Bob Camp<[email protected]> wrote:Hi A little more detail: What's going on is more clear if you trigger a scope on the positive edge of the 5 MHz and look at the 10 MHz. With an ideal multiplier, both the positive and negative edges of the 5 MHz should line up exactly with a positive edge of the 10 MHz. In reality, negative edge (the one not triggered) does not quite line up. It's a bit ahead (or behind) the ideal location. Since it's a small angle, the delta in phase and delta in amplitude both follow the same basic law. Bob On Nov 17, 2012, at 9:36 PM, Bob Camp<[email protected]> wrote:Hi Just good old Fourier series. Bob On Nov 17, 2012, at 9:12 PM, Volker Esper<[email protected]> wrote:I'm impressed - but what law is behind this? Am 17.11.2012 21:26, schrieb Bob Camp:Hi 60 db isn't to bad a number. More or less: 100 ns -> 100 ps is 1000:1. 20 log of that is 60 db. 100 ps to 60 psis about 4.4 db. That would sum up to -64.4 dbc. The main gotcha is that you *might* also have some 15 MHz (and higher) energy in the signal as well. Also phase gets into the calculation. Still, pretty close.Bob On Nov 17, 2012, at 12:50 PM, Volker Esper<[email protected]> wrote:So let's have a look into the machine... and what do we see? There'sa nice little Symmetrcom oven, with the sign reading "5.000 MHz" - bingo!May be there's a time saving way to determine the energie of the subharmonic: using my spectrum analyzer. It tells me, that there's a 5 MHz subharmonic at the level of -62dBc.How would you have calculated the energy? What would be your ansatz? Thanks so far Volker Am 17.11.2012 17:55, schrieb Bob Camp:Hi That's what you get if you have "sub harmonic" energy in the outputof your OCXO. I'd bet you a warm glass of beer that you have a 5 MHz / doubled to 10 MHz MTI OCXO in your Z3805. If you have a lot of time on your hands, you can calculate the likely level of the energy from the amount of jitter (spacing between the two peaks) you get.Bob On Nov 17, 2012, at 11:41 AM, Volker Esper<[email protected]>wrote:Hi, while playing with my recently aquired TIC (SR620) and measuringthe period time of some oscillators I discovered something I hadn't expect at all:The output of my GPSDO (Z3805) writes two maxima in the periodhistogram (at a spacing of 60ps).I didn't believe that result and assumed an inherent error in mymeasuring setup or the counter itself.So I plugged another oscillator, the reference TCXO of my signalgenerator (R&S SMX), and that result made me happy and uneasy at once: The TCXO hat only one maximum.I havn't calculated the ADEV curve, yet. See pictures. Why does my GPSDO produce such a weird result? Cheers Volker - DF9PL<DSCF1437_bbb.jpg><DSCF1439_bbb.jpg>_______________________________________________time-nuts mailing list -- [email protected] To unsubscribe, go tohttps://www.febo.com/cgi-bin/mailman/listinfo/time-nutsand follow the instructions there._______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go tohttps://www.febo.com/cgi-bin/mailman/listinfo/time-nutsand follow the instructions there._______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go tohttps://www.febo.com/cgi-bin/mailman/listinfo/time-nutsand follow the instructions there._______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go tohttps://www.febo.com/cgi-bin/mailman/listinfo/time-nutsand follow the instructions there._______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go tohttps://www.febo.com/cgi-bin/mailman/listinfo/time-nutsand follow the instructions there._______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go tohttps://www.febo.com/cgi-bin/mailman/listinfo/time-nutsand follow the instructions there._______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there._______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there._______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there._______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there._______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there._______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there._______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
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