John Green said: > mine got a lot worse with TC set to 500 sec. Depends if that was Short term Osc Freq noise or long term Phase drift? Short answer: A TC of 500 can causes a LOT more (x10 +) long term Phase shift error if the temp is changing, especially when the Dac_Gain and Damping are not also set correctly.
Here comes those darn Tbolt setting trade offs again. First point is: If you are a battery backed cell site or the average Ham that just wants to know that the in house freq reference is within 1e-9 (1ns drift per second) the default Tbolts settings are fine and make an OK plug and play unit. On the other hand if you are a NUT & would like 10 ns per day (1e-13 freq) then the factory defaults are not so good (Said). Another major trade off is if you want the best 1 sec ADEV numbers OR the lowest long term Phase errors. Takes different settings. Using the TC and Damping settings, different trade offs or compromises can be made. BUT until the Dac_Gain is set correctly (which the default setting is NOT), their proper setting And interaction is just a shot in the dark. Dac_gain is the sensitivity of the OXCO EFC input in Hz per Volt. Basic way to find the correct value is to disable the tracking, then output a + and - 0.1 volt Dac difference from its nominal tracking value using Tboltmon S/W, Average the measured + - HZ freq change of the 10 MHZ, multiply by 10, and update the Dac_Gain AND SAVE (sign is negative) If you don't have a external counter with enough resolution, You can find the Dac_Gain using the Tbolt 'turnover test'. > Even putting them in a box to shield them from room temperature variations, > they seem to wander around a lot. My answer depends on what you mean by 'a lot' , but some general comments. It would seem that there are many different OXCO used in Tbolts, from no oven to double oven. Even with my very small sampling, I've seen units where case temp change does not matter much at all to where the temp should be held to under 0.1 deg C change per hr. You may have one of the poor Temperature performing units, BUT more likely what is causing it to wonder around a lot is the GPS. We know the GPS wonders around a lot short term, much more than Temperature can, over short periods of time like 10 seconds. It is safe to say that there is something 'broken' somewhere, if a TC setting of 10 secs works better than a setting of 100 + secs. Just a guess on what is broken is "the Data or setup". BUT if you are using a common PS on both units, check it, especially the +12 volts, It should be stable to better than 1mv. Yours may be changing by a volt, to cause the type of errors you are seeing. Also be sure the units are not jumping in and out of holdover due to a high AMU setting and low GPS signal level or indoor antenna. If you can post or send me a plot of what your errors look like, I could make a better guess of what is broken. For those that do not want to fiddle, or customize their settings, but want a better performing Tbolt than the Default settings, For an average NON cell site unit that is in a nice environment, using an outdoor antenna, inside with stable temp, not under the heater outlet, and not being bounced or moved around, A compromise set of values better than the Defaults are: 1) Dac_Gain -3.5 Hz/Volt (default is -5) BUT best to set it to the correct measured value. (If off, the values below will not be correct) Range, unknown, 2) TC of 500 nominal, (default is 100), Range 250 to 750 seconds, Lower numbers give better results if the temp is changing and not controlled, Higher numbers give better weak signal answers. 3) Damping 0.7 nominal (Default is 1.2) Range 0.5 to 1.5 Lower numbers give less long term phase error, Higher numbers lower short term Osc ADEV noise. 4) AMU 2.5 (default is 4), Range 1 to 4, Lower numbers if weak GPS signal. 5) Elevation mask to 15 deg (default is 5) A band-aid compromise setting so that the unit does not lock onto low level signals. ( better to use elevation than AMU because of the behavior of Tbolts control loop) Also important, be sure the unit is in fixed position and not 3D, position tracking or survey mode, AND that it has the antenna's correct location loaded and saved. ws ********************** ----- Original Message ----- From: "John Green" <wpxs...@gmail.com> To: <time-nuts@febo.com> Sent: Tuesday, November 24, 2009 7:45 AM Subject: Re: [time-nuts] Thunderbolt reception problems >I have been following the discussion regarding Tbolt performance closely. I > have 2 of them and have not seen performance even close to what others > report. I don't seem to have sensitivity related issues that others have > reported. I am feeding Tbolt and Z3801 from a common antenna through a > purpose made GPS splitter. The Tbolt seems to see more sats quicker than the > Z3801. The Tbolts I have are extremely temperature sensitive. Even putting > them in a box to shield them from room temperature variations, they seem to > wander around a lot. This sounds crazy and is not in line with what others > have reported but when I changed the TC to 500 seconds, mine got a lot > worse. I found that a TC of around 10 seconds with a damping of 1 made for > the least amount of phase slip compared to a Z3801. > > _______________________________________________ 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.
