Test equipment for measuring Wideband gap devices (GaN) (Floating) - Page 1
Forum section: electronics / testgear

author: Glenn0010
author_rank: Regular Contributor
timestamp: April 08, 2022, 10:28:14 am
content: Hi All,I've recently started my PhD and am trying to figure out what equipment I need, specifically scopes and probes.I've calculated that to measure switching events of around 9 ns I'd need at minimum about a 500 MHz scope and 500 MHz probe.This is not taking into account the HF ringing at the end of a turn off edge for example as this will require even higher bandwidth.Most scopes at 500 MHz BW will do then job for low side measurements however, I need isolation for high side (floating) measurements.With that in mind I can start looking at scopes. At work we have a R&S RTH1004. 500 MHz bandwidth scope with channel to channel isolation..This scope should do the job for low side measurements but I have a few concerns for floating measurements. It has a CMRR rating of 100 dB at up 100 kHz. This rating is not specified at higher frequencies I could manually measure this at higher frequencies. Furthermore, it has channel to channel isolation of 40 dB (100:1) for channel as the same sensitivity where the input freq is < than the analog bandwidth. How would this affect my measurements say if I had a floating measurment on 1 channel and a ground referenced measurement on another?I've been trying to look into the channel to channel isolation but can't find much information.Could anyone shed some light on this?Thanks Glenn

author: PartialDischarge
author_rank: Super Contributor
timestamp: April 08, 2022, 11:13:17 am
content: With common modes as fast as GaNs create the best option is to use isolated probes for high-side measurements. Differential probes have 2 problems above 1MHz, not only is the CMRR low which will present a large distortion at the output but also they have a long tailed ringing after the common mode switching, which usually prevents from even interpreting signals after the common mode voltage is stable. Isolated probes do have much better CMRR at higher frequencies.About the isolated channel scope from RS, write to their tech support and ask them if they can provide data for the CMRR at other frequencies. The channel to channel isolation should also be specified with frequency, for a 400V common mode you’d see 4 volts of distortion at other channels

author: Glenn0010
author_rank: Regular Contributor
timestamp: April 08, 2022, 12:15:19 pm
content: Yes, I've been doing research on diff probes and their CMRR is terrible at HF, their bandwidth is not great either. GaN systems on one of their AN recommend the BumbleBee Differential HV-Probe. It has a BW of 400 MHz which is just about usable and 35 dB CMRR at 400 MHz which isn't too bad. Though I'm less than impressed with the long input leads it has. Those aren't really great for good HF measurements. It is relatively "cheap" at £3.8k. Seeing as I'd need two of them really to probe Vgs and Vds.The other option is the TIVP05/1 with 500 MHz or 1 GHz BW. CMRR of 80 dB at 1 GHz. These are great probes but cost £16k and £24k each. It will take some convincing to get these bought.In the mean time I contacted R&S support about CMRR at HF

author: PartialDischarge
author_rank: Super Contributor
timestamp: April 08, 2022, 12:23:07 pm
content: Where have you seen the CMRR specified as 35db @400MhZ?

author: Glenn0010
author_rank: Regular Contributor
timestamp: April 08, 2022, 12:36:00 pm
content: Datasheet, bottom of page 2.

author: PartialDischarge
author_rank: Super Contributor
timestamp: April 08, 2022, 12:45:54 pm
content: I personally find that CMRR hard to believe, for one reason that it does not decrease from 100k to 1M, and that it greatly exceeds the cmmr of good Tektronix probes , also it decreases 5dB from 50 to 100MHz, and also 5dB from 100MHz to 400, fishy because in this HF region the currents are less controllable in the attenuators legs and CMRR decreases much fasterAs you can see in the RS diff probe CMRR, and these are good probes, at HF it goes down fast

author: Glenn0010
author_rank: Regular Contributor
timestamp: April 08, 2022, 02:09:25 pm
content: Fair enough, I can see what you mean. At the moment I'm trying to figure out what the university does and does not have. It's proving to be impossible to get an answer from anyone on what's available

author: PartialDischarge
author_rank: Super Contributor
timestamp: April 08, 2022, 02:29:21 pm
content: Anyway, I find your initial assessment that you need a 500MHz differential bandwidth probe way off. For 9ns anything with a “real” bandwidth of more than 100MHz will do. For the scope of course, 500MHz nowadays is fine.Of course this assumes that probe loading is not going to be a limiting factor.

author: 2N3055
author_rank: Super Contributor
timestamp: April 08, 2022, 02:39:49 pm
content: Quote from: PartialDischarge on April 08, 2022, 02:29:21 pmI agree.. 9ns is not really fast, 100-200 Mhz scope would be more than enough..Take a look at Cleverscope CS448:https://cleverscope.com/news/cs448/That is a instrument purposely built for that kind of work. It is also amazing 14 bits, so you get amazing dynamic range..

author: Glenn0010
author_rank: Regular Contributor
timestamp: April 09, 2022, 08:05:22 am
content: So these are my calcultions on how I came up with 500 MHz. I followed an applcation note.First off - looks like the rise time I was actually looking at is 4 ns not 9. This is the swtiching wave form for the GaN device at rated load. Don't know why I wrote 9 on here.System Bw =(approx) 0.35/tr = 87.5 MHzI then double the system Bw to account for the device getting faster when not running at full load and operating at different conditions. so Sys Bw = 200 MHzSys Bw = 1/SQRT(1/(BWscope^2)+1/(BWprobe^2)) = using both probe and scope BW of 500 MHz gives us a system BW of 353 MHz which is above our requirement.Obviously there is a decent ammount of rounding but I don't think the numbers are too unreasonableAny thoughts?With regerads to the clever scope, I was looking for that vid but couldnt remember the name as I've seen it years ago, cheers for pointing it out. Looks like that scope is now end of life, looks like they have now the CS548 coming in april but I haven't found specs for it yet. I'm not sure wheter its BW will be improved over the rpevious model.Cheers