Re: [time-nuts] Rubidium Performance: DDS noise effect on 10 MHz
On 2/4/2012 12:17 AM, John Miles wrote: In the current 5680A units, the 10 MHz output comes from the 60 MHz VCXO (divided by 6 in CPLD) and not direct from a DDS. If my architecture understanding is right, the DDS signal output is mixed with the VCXO output only at the 114th harmonic of 60 MHz, and it's the PLL (looking at the optical signal from the Rb) that drives the VCXO to keep it lined up. As I understand it, DDS phase noise should be divided by a factor of 6*114 by the time it appears at the 10 MHz output, and at larger frequency offsets the amplitude should also be (significantly) reduced by the PLL loop filter. I don't have any phase noise measurement tools myself, so this is just an academic argument, but if there is significantly more noise on the 10 MHz than expected for a 60 MHz VCXO, I wonder if it's just inadequate filtering of an internal power rail. Is the unit under test being driven by a linear, or switching supply? What made me suspect the AD9832 is that the PN/spur behavior is so close to what I'd expect from it. Analog Devices' own plots contain a lot of strong spurs, and the chip's actual SFDR spec is in the -70 dBc range that I'm seeing (blue trace attached). But yes, you're right, it would make a lot more sense to use the DDS as an offset generator for the control loop, rather than as an output device. I tried it with a couple of different supplies, linear and otherwise, and the noise didn't change meaningfully. Since it's so far out of line with FEI's specs, I have to believe that I either got a couple of bad examples -- although they lock very quickly and look essentially unused inside -- or I don't have the pinout right, and am using the wrong ground or something like that. Seems pretty unlikely that this is normal operation. Let's see what JohnA comes up with. -- john John, From your comments, it sounds like you may be measuring one of the earlier 5680A's that could be tuned over a large range with a DDS output. These earlier ones had a 50.25 MHz internal osc which was locked after multiplication to the rubidium frequency. Then there was a DDS on the output that could be programmed over a wide frequency range. The newer ones have a 60 MHz internal oscillator. This is multiplied up to 6840 MHz and subtractively mixed with ~5.3125 MHz from a DDS to get to the rubidium frequency. The feedback from the rubidium cell locks the 60 Hz which is divided by 6 for the 10 MHz output. No DDS on the output generation. The DDS in the loop can digitally adjust the rubidium lock frequency that tunes the 60 MHz, to fine tune the output only around 10 MHz. Do you know which type of 5680A you are measuring? ___ 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.
Re: [time-nuts] Rubidium Performance: DDS noise effect on 10 MHz
From your comments, it sounds like you may be measuring one of the earlier 5680A's that could be tuned over a large range with a DDS output. These earlier ones had a 50.25 MHz internal osc which was locked after multiplication to the rubidium frequency. Then there was a DDS on the output that could be programmed over a wide frequency range. That sounds plausible. I haven't taken the time to bring myself 100% up to speed with the many different variations and options for these little boxes, but I could swear I'm looking at a low-resolution, unfiltered, uncleaned-up DDS. The newer ones have a 60 MHz internal oscillator. This is multiplied up to 6840 MHz and subtractively mixed with ~5.3125 MHz from a DDS to get to the rubidium frequency. The feedback from the rubidium cell locks the 60 Hz which is divided by 6 for the 10 MHz output. No DDS on the output generation. The DDS in the loop can digitally adjust the rubidium lock frequency that tunes the 60 MHz, to fine tune the output only around 10 MHz. Do you know which type of 5680A you are measuring? This one is marked S/N 0339-65969, purchased from http://www.ebay.com/itm/260930018124 . It seems to have the pinout documented at http://vk2xv.djirra.com/tech_rubidium.htm for serial 0127-96634 (pin 1=V+, pin 2=GND, pin 3=+5V, pin 7=RF out.) However, VK2XV claims that s/n 96634 was non-programmable. I haven't tried hooking up a serial terminal to mine, but I did notice that there was some negative voltage on one of the other pins (9?) that might correspond to an RS-232 signal level. Can't complain too much at $40/each, anyway I believe the seller's claim that these are new or nearly so, even though they look like they've been removed from an installation. The internal foam insulation is usually discolored from heat, while it looks great in the two units that I bought from this seller. -- john ___ 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.
Re: [time-nuts] Rubidium Performance: DDS noise effect on 10 MHz
Hello, John, Your phase noise plot seems quite similar to that sent by Said some time ago (4 Jan), that looks like a lot of spurs quite evenly separated. It is likely from the price and p/n that your unit is one of the newer ones with 60MHz xtal and the DDS inside the loop - and I would think that the pn from the DDS would be removed by the loop. But since the division from 60MHz is done by a CPLD, perhaps the signal is corrupted by this CPLD, or perhaps it is corrupted by a switching regulator inside the unit. The 60MHz signal is available inside the unit, in a small coaxial connector. Perhaps it would be interesting to also analyze it. Regards, Javier El 04/02/2012 10:43, John Miles escribió: From your comments, it sounds like you may be measuring one of the earlier 5680A's that could be tuned over a large range with a DDS output. These earlier ones had a 50.25 MHz internal osc which was locked after multiplication to the rubidium frequency. Then there was a DDS on the output that could be programmed over a wide frequency range. That sounds plausible. I haven't taken the time to bring myself 100% up to speed with the many different variations and options for these little boxes, but I could swear I'm looking at a low-resolution, unfiltered, uncleaned-up DDS. The newer ones have a 60 MHz internal oscillator. This is multiplied up to 6840 MHz and subtractively mixed with ~5.3125 MHz from a DDS to get to the rubidium frequency. The feedback from the rubidium cell locks the 60 Hz which is divided by 6 for the 10 MHz output. No DDS on the output generation. The DDS in the loop can digitally adjust the rubidium lock frequency that tunes the 60 MHz, to fine tune the output only around 10 MHz. Do you know which type of 5680A you are measuring? This one is marked S/N 0339-65969, purchased from http://www.ebay.com/itm/260930018124 . It seems to have the pinout documented at http://vk2xv.djirra.com/tech_rubidium.htm for serial 0127-96634 (pin 1=V+, pin 2=GND, pin 3=+5V, pin 7=RF out.) However, VK2XV claims that s/n 96634 was non-programmable. I haven't tried hooking up a serial terminal to mine, but I did notice that there was some negative voltage on one of the other pins (9?) that might correspond to an RS-232 signal level. Can't complain too much at $40/each, anyway I believe the seller's claim that these are new or nearly so, even though they look like they've been removed from an installation. The internal foam insulation is usually discolored from heat, while it looks great in the two units that I bought from this seller. -- john ___ 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. ___ 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.
Re: [time-nuts] Rubidium Performance: DDS noise effect on 10 MHz
Maybe his are new. But I bought one from ggg*fitting that was listed as new turned out very used, and in conversation with them they had to admit they do not know what they have. If it still has its original factory setting, measuring the frequency will tell you if it had extensive running time or not. Bert Kehren In a message dated 2/4/2012 4:44:20 A.M. Eastern Standard Time, jmi...@pop.net writes: From your comments, it sounds like you may be measuring one of the earlier 5680A's that could be tuned over a large range with a DDS output. These earlier ones had a 50.25 MHz internal osc which was locked after multiplication to the rubidium frequency. Then there was a DDS on the output that could be programmed over a wide frequency range. That sounds plausible. I haven't taken the time to bring myself 100% up to speed with the many different variations and options for these little boxes, but I could swear I'm looking at a low-resolution, unfiltered, uncleaned-up DDS. The newer ones have a 60 MHz internal oscillator. This is multiplied up to 6840 MHz and subtractively mixed with ~5.3125 MHz from a DDS to get to the rubidium frequency. The feedback from the rubidium cell locks the 60 Hz which is divided by 6 for the 10 MHz output. No DDS on the output generation. The DDS in the loop can digitally adjust the rubidium lock frequency that tunes the 60 MHz, to fine tune the output only around 10 MHz. Do you know which type of 5680A you are measuring? This one is marked S/N 0339-65969, purchased from http://www.ebay.com/itm/260930018124 . It seems to have the pinout documented at http://vk2xv.djirra.com/tech_rubidium.htm for serial 0127-96634 (pin 1=V+, pin 2=GND, pin 3=+5V, pin 7=RF out.) However, VK2XV claims that s/n 96634 was non-programmable. I haven't tried hooking up a serial terminal to mine, but I did notice that there was some negative voltage on one of the other pins (9?) that might correspond to an RS-232 signal level. Can't complain too much at $40/each, anyway I believe the seller's claim that these are new or nearly so, even though they look like they've been removed from an installation. The internal foam insulation is usually discolored from heat, while it looks great in the two units that I bought from this seller. -- john ___ 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. ___ 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.
Re: [time-nuts] Rubidium Performance: DDS noise effect on 10 MHz
From: John Miles jmi...@pop.net So far, the FE-5680A is doing well in the ADEV department, but its AD9832 DDS chip adds some substantial noise and spurs to the 10 MHz output. [...] The overall SSB C/N ratio I'm seeing between 1 Hz - 100 kHz is pretty much in line with Analog Devices' data sheet, as are the spur levels. In the current 5680A units, the 10 MHz output comes from the 60 MHz VCXO (divided by 6 in CPLD) and not direct from a DDS. If my architecture understanding is right, the DDS signal output is mixed with the VCXO output only at the 114th harmonic of 60 MHz, and it's the PLL (looking at the optical signal from the Rb) that drives the VCXO to keep it lined up. As I understand it, DDS phase noise should be divided by a factor of 6*114 by the time it appears at the 10 MHz output, and at larger frequency offsets the amplitude should also be (significantly) reduced by the PLL loop filter. I don't have any phase noise measurement tools myself, so this is just an academic argument, but if there is significantly more noise on the 10 MHz than expected for a 60 MHz VCXO, I wonder if it's just inadequate filtering of an internal power rail. Is the unit under test being driven by a linear, or switching supply? ___ 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.
