Are the mains from grid not important? TOPAZ? - Page 1
Forum section: electronics / general

author: Zucca
author_rank: Unknown
timestamp: July 27, 2018, 09:29:17 am
content: Sorry to pull down the quality with maybe a stupid questions.It seems to me that the DUT Device mains voltage and frequency are never logged in your ppm graphs.Is there a particular reason for that?I have some ideas but don't know exactely what's the reason behind.On top of that, do you guys maybe use some TOPAZ Ultra Isolation Transformer in your Lab?http://www.surplussales.com/Transformers/Pdfs/tp-91092-31_N.pdfThanks in advance.

author: Echo88
author_rank: Frequent Contributor
timestamp: July 27, 2018, 09:57:43 am
content: I never knew which search terms i need to use to find good transformers with shielding. Thanks for the info!I once thought maybe one could elevate performance of 3458A by constantly checking the mains frequency and use the actual frequency to control the ADC, instead of how the 3458A does: checking the mains frequency only at startup. But since the datasheet claims the following i didnt continue the thought:"For line frequency ± 1%, ACNMR is40 dB for NPLC ≥ 1, or 55 dB for NPLC≥ 100. For line frequency ± 5%,ACNMR is 30 dB for NPLC ≥ 100."I dont know if heavy mains voltage changes affect measurements.

author: dl1640
author_rank: Regular Contributor
timestamp: July 27, 2018, 10:08:39 am
content: Most metrology instruments have built in noise cut countermeasure (filters, trans...)But i dont know if it would affect the measurement while your mains voltage flunctuate rapidly.通过我的 PRA-AL00 上的 Tapatalk发言

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 27, 2018, 08:09:35 pm
content: I used to use a nice Topaz on the test bench at work, but I've never seen them actually cure a problem. I've got at least one stashed out in the garage, just in case, but have never felt a need to hook it up. I'd think they could cure ground loop problems if you have 'em.

author: floobydust
author_rank: Super Contributor
timestamp: July 27, 2018, 08:50:51 pm
content: An isolation transformer useless to get rid of powerline noise and hash, unless you make a clean "separate" ground- as the Topaz doc states.Electrical codes require earth ground and neutral to be bonded in the electrical panel.This means your local (building) ground is contaminated with powerline noise from everyone else on the same distribution line.A really quiet lab would use an isolation transformer with dedicated (building) ground-rods to completely (isolate) break the connection to the utility distribution network.I do this in EMC labs, recording studios and medical screened-rooms.

author: pigrew
author_rank: Frequent Contributor
timestamp: July 27, 2018, 10:18:25 pm
content: I've been experimenting with my Solartron 7081, which has a clock locked to the mains frequency, using a PLL. It doesn't work too well.In my case, the mains frequency oscillates by perhaps a tenth of a hertz with a period of about one minute. This was evident on the measured voltages, as the meter's clock was not stable. In addition, the autozero value is dependant on the clock frequency due to amplifier bias currents. I think the clock stability is more important than the power line noise, in most situations.In my case, I made the PLL loop filter slower, and things improved, but this means that the clock isn't as precisely locked to the power line, so lower mains rejection. Maybe if I did it again, I've have it adjust the clock with a step change only right before the autozero happens, every fifteen minutes or so.