RF package for USA Cal Club Round 3 - Page 1
Forum section: electronics / rf-microwave

author: rhb
author_rank: Super Contributor
timestamp: May 04, 2020, 11:01:51 pm
content: I'm beginning to focus attention on what to put in an initial RF calibration kit which can circulate among participants via a medium flat rate box. You can pick up current thinking here:https://www.eevblog.com/forum/metrology/usa-cal-club-round-3-suggestions/msg3049090/#msg3049090I just bought HP 110 dB/10 dB and 11 dB/1 dB attenuators which I plan to have commercially calibrated to use to calibrate a traveling reference. I'm a single 08512-20019 port extension cable short of a full 8510C going up to 26 GHz. So I'm hoping that I can assemble a pretty well characterized kit. I've also got a 4284A, 4285A, Tek 11801/SD-24 TDR system, 8970B & 346B. As is the case for most hobbyists, I can't justify a full annual commercial cal of my gear. But I can justify getting a small number of items calibrated which will let me check the other gear.If you are interested in an RF cal kit, I invite you to add your voice. What is your biggest calibration need?Have Fun!Reg

author: hendorog
author_rank: Super Contributor
timestamp: May 04, 2020, 11:22:18 pm
content: I'd suggest a shared VNA OSLT cal kit made from decent quality off the shelf parts. There wasn't interest in this in the past, but now with the NanoVNA becoming popular, more people have VNA experience and knowledge.These standards could be swept by those of us with properly characterised VNA cal kits as they do the circuit - and thereby can become calibrated children, which are monitored over time because they are being measured by the same people on the same instruments regularly.Also I have just converted an HP 8515A test set for use with my 8753 VNA, and so can donate a couple of the high quality fixed 3.5mm attenuators I recovered (6dB and 13dB from memory).

author: rhb
author_rank: Super Contributor
timestamp: May 05, 2020, 01:11:06 am
content: Thanks. That's a good idea. I shall pursue it.Because of my background in seismic, I am having a hard time grasping why one needs a perfect cal set. I've not gotten serious enough to spend a few days on the math, but it surely seems to me that I could calculate the deviation of a physical cal kit from ideal without a lot of difficulty and then apply a correction factor.It's a very close analogue to a routine task in reprocessing old seismic data. It's common that the field data is available, but the instrument cal data is not. So you derive the instrument cal data from the dataset and first principles. It may not be quite as good, but damn close. I've never seen a good comparison to reality. But I certainly could not find fault with the results in a practical sense.Have Fun!Reg

author: hendorog
author_rank: Super Contributor
timestamp: May 05, 2020, 01:22:48 am
content: Yes you are absolutely correct. As long as the kit is characterised then it does not need to be perfect.Characterisation is as simple as sweeping the standard on a calibrated instrument.There is however a caveat!Due to the limitations in older gear (i.e. the stuff we all have) the characterisation data for those instruments is based on a simple model we all have seen (C0, C1, delay impedance etc). This model will not fit poorly manufactured standards very well over a wide bandwidth. So errors are introduced fitting the actual measurements to the model.It is possible to work around this in two ways:Do the correction calcs on a PC instead of on the instrument - i.e. if you give me any set of parts, along with the S1P or S2P data from a calibrated sweep, then I can use that to make calibrated measurements. I can also use it to 'calibrate' my own parts - with a loss of accuracy.Use the instrument built in calcs as normal, but limit the bandwidth of the model.So if the parts are not good enough for a 0 to 6GHz model, then we could instead create two models, one for 0- 3GHz and one for 3 to 6GHz. Or even finer to reduce the error introduced by model fitting.Edit: Also working from first principles is possible using an RF modelling tool. I haven't tried this and don't have the software to do it, I believe Dr Kirkby has done work in this area as part of his cal kit development.

author: rhb
author_rank: Super Contributor
timestamp: May 05, 2020, 04:29:13 am
content: Agreed.I'd like to see something better than the continued use of an approximation that dates from when most engineering calculations were still done with a slide rule.Reg