Re: [time-nuts] How Rubidiums make their frequency
On Thu, April 20, 2006 7:25, Poul-Henning Kamp said: And also, something else i don't understand: Why do the newer GPS satellites rely on Rb standards rather than Cs standards? They last longer. And they are less noisy and more stable, up to a day or so. -- Björn ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] How Rubidiums make their frequency
[Clocks in GPS satellites] I said drift-free Rubidium :-) They last longer. I've seen comments about Rubidium having better short term stability than Cesium. What's short in that context? How often do the ground stations talk to the satellites? Is it reasonable to correct for the drift? If the drift is slow enough the communication channel is already in place. It shouldn't be too much work for the ground stations to track the frequency as well as position. There is probably a good writeup on this someplace. Anybody seen one? -- The suespammers.org mail server is located in California. So are all my other mailboxes. Please do not send unsolicited bulk e-mail or unsolicited commercial e-mail to my suespammers.org address or any of my other addresses. These are my opinions, not necessarily my employer's. I hate spam. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] How Rubidiums make their frequency
In message [EMAIL PROTECTED], Hal Murr ay writes: Is it reasonable to correct for the drift? If the drift is slow enough the communication channel is already in place. It shouldn't be too much work for the ground stations to track the frequency as well as position. They have very complex models for the drift and they give very good match to reality. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 [EMAIL PROTECTED] | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] How Rubidiums make their frequency (was RE:time-nuts Digest, Vol 21, Issue 22)
Most rubidiums have two tuning adjustments, one for the cavity frequency and one for the lamp RF frequency. I'd start by tuning those properly and see how your frequency offset changes. Somewhere on the servo you'll find a test point for observing the second harmonic of the modulation. Clip that to your oscilloscope and adjust the cavity tuning for maximum response. Somewhere else, you should find a DC light level monitor, tune up the lamp frequency for maximum light. One or the other (or both) of these adjustments may move the frequency back towards the midpoint of the C-field adjustment. If you get them optimized it'll also improve the temperature sensitivity and long-term drift. -RL Please note that I am now automatically deleting all Email originating from comcast.net due to the large amount of spam originating from their servers. Please register your complaints with ComCast, since they have been ignoring mine. Robert Lutwak, Senior Scientist Symmetricom - Technology Realization Center 34 Tozer Rd. Beverly, MA 01915 (978) 232-1461 Voice [EMAIL PROTECTED] (Business) (978) 927-4099 FAX [EMAIL PROTECTED] (Personal) (339) 927-7896 Mobile - Original Message - From: Christopher Hoover [EMAIL PROTECTED] To: time-nuts@febo.com; [EMAIL PROTECTED] Sent: Thursday, April 20, 2006 5:31 AM Subject: Re: [time-nuts] How Rubidiums make their frequency (was RE:time-nuts Digest, Vol 21, Issue 22) Tom Clark wrote: Christopher Hoover asked: one issue remains: i have to crank the magnetic field setting almost to its high limit (9.91/10.00) to get 5 MHz out; lower settings give a frequency that is too low. i presume this is unusual. i have a rudimentary understanding of the rubidium oscillator physics, but i do not understand what would cause this. can i buy a clue? I don't know the Tracor, but I imagine it is like most of the other Rubidiums in it's innards. Hi Tom, That's a good explanation, thank you. I got that much already. (While it took some time for my phyiscs, chemistry and semiconductor physics studies to kick back in, it all clicked last week.) The Tracor, indeed, seems fairly ordinary compared to the other Rb's I have, going by the test points and the various labels inside. There is the typical synthesizer section, audio demodulation section for the photocell, etc. So what I'm really wondering is this: is there a wear mechanism or failure mode in the physics package that might explain why I have to crank the mag field almost to its high limit to get 5 MHz? (This question presumes the mag field control is working properly, and I cannot say for sure that it is.) Thanks, -- Christopher. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] How Rubidiums make their frequency
Good point on the crystal frequency. It gets multiplied about 680 times. IIRC, iron can be magnetized by jarring (striking) it while in a magnetic field. Since you don't know the transportation history of the device, degaussing is in order if the crystal is within lock range. If degaussing is difficult, make sure that the C field changes smoothly with the adjustment. A milliammeter in the line to the coils will work. You won't see the change with a compass if the mu-metal shielding is working (true?). Ah, can you damage mu-metal by striking it? Bill Hawkins Twenty years ago, my company installed CRT monitors in a titanium plant. As luck would have it, the monitors were not far from a DC buss carrying 50,000 amps to the magnesium chloride electrolysis cells. When the DC was turned on, the monitors showed the lower left quadrant of the screen in the upper right quadrant. The customer was not amused. The problem was fixed with mu-metal shields fit to the CRTs. Seems to me the mu-metal had to be handled with care. -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of Rex Sent: Thursday, April 20, 2006 4:56 AM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] How Rubidiums make their frequency (was RE:time-nuts Digest, Vol 21, Issue 22) On Thu, 20 Apr 2006 02:31:08 -0700, Christopher Hoover [EMAIL PROTECTED] wrote: Hi Tom, That's a good explanation, thank you. I got that much already. (While it took some time for my phyiscs, chemistry and semiconductor physics studies to kick back in, it all clicked last week.) The Tracor, indeed, seems fairly ordinary compared to the other Rb's I have, going by the test points and the various labels inside. There is the typical synthesizer section, audio demodulation section for the photocell, etc. So what I'm really wondering is this: is there a wear mechanism or failure mode in the physics package that might explain why I have to crank the mag field almost to its high limit to get 5 MHz? (This question presumes the mag field control is working properly, and I cannot say for sure that it is.) Thanks, -- Christopher. I could be way off, but are you sure the xtal osc is centered on the desired range? Having the adjustment procedure for your unit would be a good advantage. I got an Efretom rubidium a few years back that would not lock. I went through all the proceedures and found that the crystal had drifted out of the lock range. I was able to get it working but at the limit of the xtal trimmer. Had to change a fixed capacitor to get it back near the mid of the xtal trimmer. So, I'm thinking I might have got it locked by messing with C field if I had a problem like this. Would have locked but off correct freq. Not sure if this is a reasonable discussion, you may have this stuff all understood. Do you have the full service documentation? ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
[time-nuts] How Rubidiums make their frequency
Christopher Hoover asked: one issue remains: i have to crank the magnetic field setting almost to its high limit (9.91/10.00) to get 5 MHz out; lower settings give a frequency that is too low. i presume this is unusual. i have a rudimentary understanding of the rubidium oscillator physics, but i do not understand what would cause this. can i buy a clue? I don't know the Tracor, but I imagine it is like most of the other Rubidiums in it's innards. Inside the physics package of a Rb, a cell with some Rubidium is heated to (that's why Rb's run not!) enough so that it is turned into a gas. Both light and microwaves illuminate the cell. If no magnetic field is present on the cell, the Rb gas has a hyperfine resonance (the difference in frequency between two infrared transitions of the Rb gas) at 6.8346826128 Mhz. When a magnetic field is imposed, the energy difference between the two hyperfine states changes. In the RF part of the signal path (here, the block digram of a typical Rb standard helps. See Page 3 of [1]this Symmetricom White Paper .) Let's start with some convenient oscillator at, let's say 10 MHz. Multiply it up to 60 MHz and then hit a Step Recovery Diode to get the 114th harmonic at 6840 MHz. Then difference between the 6834.. and 6840 MHz is 5.31738+ MHz. In the standard Rb configuration, we apply a magnetic C-field to bring the difference frequency upwards by 4.89 kHz to 5.3125 MHz which happens to be 5MHz + 5/16MHz. Back in the early days, we didn't have nice programmable DDS chips, but simple dividers/multipliers could make the 5/16 MHz adder. So what you are doing by tweaking the magnetic field to shift the RF resonance of the Rb cell so that it matches the arithmetic quirk that the 6834 MHz is almost contains the neat 5/16 MHz in the tail-end digits. Hope that helped -- 73, Tom References 1. http://www.symmttm.com/pdf/Precision_Frequency_References/wp_mmrfs.pdf ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] How Rubidiums make their frequency
Since we can now make DDS's with arbitrary frequency resolution, could you make an Rb oscillator without the magnetic field adjustment? Wouldn't that reduce a source of error in frequency? Then we'd be left with the ideal resonance frequency, right? Are there any other influences on the resonance frequency? I assume temperature and density don't matter. Matt On 4/19/06, Tom Clark, K3IO (ex W3IWI) [EMAIL PROTECTED] wrote: Christopher Hoover asked: one issue remains: i have to crank the magnetic field setting almost to its high limit (9.91/10.00) to get 5 MHz out; lower settings give a frequency that is too low. i presume this is unusual. i have a rudimentary understanding of the rubidium oscillator physics, but i do not understand what would cause this. can i buy a clue? I don't know the Tracor, but I imagine it is like most of the other Rubidiums in it's innards. Inside the physics package of a Rb, a cell with some Rubidium is heated to (that's why Rb's run not!) enough so that it is turned into a gas. Both light and microwaves illuminate the cell. If no magnetic field is present on the cell, the Rb gas has a hyperfine resonance (the difference in frequency between two infrared transitions of the Rb gas) at 6.8346826128 Mhz. When a magnetic field is imposed, the energy difference between the two hyperfine states changes. In the RF part of the signal path (here, the block digram of a typical Rb standard helps. See Page 3 of [1]this Symmetricom White Paper .) Let's start with some convenient oscillator at, let's say 10 MHz. Multiply it up to 60 MHz and then hit a Step Recovery Diode to get the 114th harmonic at 6840 MHz. Then difference between the 6834.. and 6840 MHz is 5.31738+ MHz. In the standard Rb configuration, we apply a magnetic C-field to bring the difference frequency upwards by 4.89 kHz to 5.3125 MHz which happens to be 5MHz + 5/16MHz. Back in the early days, we didn't have nice programmable DDS chips, but simple dividers/multipliers could make the 5/16 MHz adder. So what you are doing by tweaking the magnetic field to shift the RF resonance of the Rb cell so that it matches the arithmetic quirk that the 6834 MHz is almost contains the neat 5/16 MHz in the tail-end digits. Hope that helped -- 73, Tom References 1. http://www.symmttm.com/pdf/Precision_Frequency_References/wp_mmrfs.pdf ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] How Rubidiums make their frequency
In message [EMAIL PROTECTED], Matt Ettus writes: Since we can now make DDS's with arbitrary frequency resolution, could you make an Rb oscillator without the magnetic field adjustment? Wouldn't that reduce a source of error in frequency? Then we'd be left with the ideal resonance frequency, right? Are there any other influences on the resonance frequency? I assume temperature and density don't matter. In fact density/pressure does matter and is one of the major reasons why rubidiums drift: Rubidium is absorbed into the glas container and as the pressure drops the frequency pulls. The absorption also makes the glas darker and darker, being a major wear-out mechanism for Rb units. As far as I know, this is why Rb is never classified as a primary standard: A drift-free unit has yet to be constructed. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 [EMAIL PROTECTED] | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] How Rubidiums make their frequency
From: Dave Brown [EMAIL PROTECTED] Subject: Re: [time-nuts] How Rubidiums make their frequency Date: Thu, 20 Apr 2006 09:13:08 +1200 Message-ID: [EMAIL PROTECTED] - Original Message - From: Matt Ettus [EMAIL PROTECTED] To: [EMAIL PROTECTED]; Discussion of precise time and frequency measurement time-nuts@febo.com Sent: Thursday, April 20, 2006 7:55 AM Subject: Re: [time-nuts] How Rubidiums make their frequency Since we can now make DDS's with arbitrary frequency resolution, could you make an Rb oscillator without the magnetic field adjustment? snip--- I suspect the FEI 5650 series does just that- a 50.255+ MHz VCXO is 'locked' at 1/136 of the 6.8GHz Rb resonance and there is no 'user' adjustment provided for the physics package. I suspect the magnetic field IS preset in the factory but thereafter intended to be left well alone. Well, first of all they state that it is 1/136 and yes it is preset put available as a controlable thing as an option. See their sales-material and you find it (I did): http://www.freqelec.com/ The 50.255+ VCXO drives an AD9830-all fine tuning of the output frequency takes place by setting the DDS. Can anyone comment further on this? On a related issue- the physics package in these units draws a significant current at startup- I'm considering current limiting the suppy until the current drops back to a lower value-I guess this delays the warmup somewhat but it would reduce the current required from the supply to the steady state value, for example, thus allowing a smaller power supply rating. I would not do that. However, the heater supply isn't as critical so it would allow itself for a very simple supply anyway. Cheers, Magnus ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] How Rubidiums make their frequency
Hi.. In his message, Paul-Henning Kamp writes that a drift-free standard has not been yet designed... But Isn't Cesium drift-free? Since the SI second is standardized as de duration of 9192631770 oscillation of the hyperfine transition of the atom 133Cs? If Cesium drifts, theren should be a more formal definition of the second (Such as density, maximum C-field or level of purity). Does anyone here has it? And also, something else i don't understand: Why do the newer GPS satellites rely on Rb standards rather than Cs standards? Since Rubidium is known as less precise than cesium? Is there a reliability issue there (Rb clocks are more reliable / longer MTBF tha Cesium clocks). I don't know... 73 de Normand Martel VE2UM (A PLL based 9192631770 synthesizer i once imagined...) 10 MHz ---(/250)--4PPS-, OCXO | | ,' | `-(/)---[Filter]---(VCO)--*--829.08 MHz--, | | | `---(/20727)---' | | ,41454 PPS(/2)' | `--(/)---[Filter]---(VCO)--*--1838.526354 MHz--, | | | `---(/44351)---' | | ,---' | `-(*5 SRD)--9192631770 Hz (to physics) __ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
Re: [time-nuts] How Rubidiums make their frequency
In message [EMAIL PROTECTED], Normand Martel writes: Hi.. In his message, Paul-Henning Kamp writes that a drift-free standard has not been yet designed... I said drift-free Rubidium :-) And also, something else i don't understand: Why do the newer GPS satellites rely on Rb standards rather than Cs standards? They last longer. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 [EMAIL PROTECTED] | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
