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Re: Super low noise PSU for the OXCOs
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As it happens I got an LT3045 module a few days ago, but had not yet opened the bag. I've been working on a leaky roof. And discovered I'm in *really* poor shape. 90 minutes and I'm beat. I just calibrated a Mastech GPS-1850D 0-20 V 0-5 A linear supply so it reads to within 0.05%. Though manual, that should serve very well for testing the LT3045. I can put a Peltier device on the fins of the LT3045 module and then fins on the opposite side and measure temperature variation. Should be fun. I've got a pair of 34401As to measure the input and output voltages. I couldn't find any information about long term aging in the LT3045 datasheet. Do you know of any? I'm modifying a 3497A to multiplex 100 sources to a 3457A for long term monitoring. I'm going to buy 50 new ones and compare them side by side. Is there an improvement in stability with aging? What is the limit of stability? Can I predict it? I'll have two loops of air flow on either side of a Peltier device so I can vary the ambient temperature in a voltage controlled fashion when I compare the new and used devices. Half on each side. I'll actually have to cut the number of devices because of the need to measure thermocouples interspersed with the LM317s. My expectation is I'll be able select pairs of LM317s such that the output voltage is especially stable over temperature and at the same time reduce the voltage from the transformer to the 20 V input limit of the LT3045. I just checked the fit of a Peltier module on the LT3045 and I just need longer screws. Have fun! Reg On Sunday, September 10, 2023 at 02:01:42 PM CDT, Gerhard_DK4XP via groups.io <dk4xp@...> wrote: On 10.09.23 18:46, Reginald Beardsley via groups.io wrote:
> I'm familiar with the LM317 and happened to find used ones. They have > a 1.25 V zener reference > which should be very stable due to aging. I'm hoping that I'll be able > to pair tempcos to be zero under normal lab conditions. Ignoring the progress in electronics of the last 40 years does not help. No, they have a band gap reference and it shows in the horrible noise plots. Band gap references live on amplifying the difference between tiny voltages generated by small currents and different current densities. The LM317 / 350 is 40 dB worse in noise than LT3042 / 3045 or the negative LT3094.? The file names of the trace files show the part values used in their resp. standard data sheet circuits. A 1.25V Zener would have next to no noise at all, but a very soft knee. If the LT* are too weak, an external power transistor can take the heat. The noise behavior stays essentially the same. (data sheet of 3043/45 page 17 or so) I have described that in 2022 Dubus, and soon on my web site, when I have re-established ftp access to the maintainer interface. I wonder how LT designed the reference current source. With 0.5% Susumu thin film ref resistors, the output voltage is usually on spot within a milliVolt. Thick film resistors can have worse 1/f noise than carbon composition when there is a DC voltage across them. The noise plots have been made with averaging over 10 * 2 ADA-4898 op amps. The preamp in front of the Agilent 89441A FFT analyzer features a noise density of 220 pV/rt Hz. At very low frequencies, it shows that the input capacitor is too small (noise rise steeper then 1/f ). There are noise plots of Zeners, LEDs abused as voltage reference and other things on my Flickr account < ?? > 73, Gerhard?? DK4XP |
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