Electronics > Projects, Designs, and Technical Stuff
5uH Aerospace LISN: How dumb would I be to "throw one together"?
TimNJ:
Thank you.My ignorance to most things RF is showing. Indeed, VNA and an SA+TG are not intended to measure the same things. Not sure how I got that in my head. Maybe I’ll get a $50 nanoVNA but I haven’t done much research to check if it’s a suitable device.
Regarding the 10uF caps, I guess I was just looking for a sanity check if putting 10uF between line/neutral and earth (at 400Hz) really made sense. Obviously the standard calls for it, but sheesh...that’s 40 ohms at 400Hz...and there’s two of them. I’m going to research more in the morning, but just thinking about putting 20uF to earth in a public work area just sounds bad, personally. Interestingly, the DO-160 conducted EMI measurement actually doesn’t use a direct measurement at the LISN. Rather, it uses a current probe. So, in this case, the spectrum analyzer can be isolated from the measurement setup. I may need to move the transient limiter to a small dongle outside the LISN in this case, presuming it still applies for current probe measurements.
In CISPR measurements, the SA earth and LISN earth are one. In this case, I wonder if the LISN chassis can just be referenced to a local plane, instead of a plane referenced back to the building’s electricity. Just spitballing, will update with my findings.
I’ll work on a KiCad project this week with some beefier inductors. Thanks!
Jay_Diddy_B:
Hi,
Don't worry about the transient limiters being included in the LISN. They are 10dB 50 \$\Omega\$ attenuators in the measurement range. This means that if you forget to terminate the LISN in 50 \$\Omega\$ the maximum return loss is 20dB. It is 20dB because the signal goes through the attenuator, getting attenuated by 10dB and then is reflected going back through the attenuator. You simply add a 50 \$\Omega\$ termination on the BNC connector.
The transient limiters are a good things.
The LISNs I have designed so far were designed using OrCAD. I am moving to KiCad. I have started putting the parts into my KiCad library:
I just threw them on a board so I could see what they look like.
I make the 3D models in Fusion 360.
Coilcraft make an alternative inductor to the Wurth 744364 series that is cheaper and has a higher SRF and is footprint compatible.
For the DC testing the 10uF capacitors don't matter.
What voltage is the AC testing at 400Hz?
Regards,
Jay_Diddy_B
TimNJ:
Thanks. I mostly understand the function of the 10dB pad + HPF. The CISPR LISN we use (I believe) just has a very low capacitance diode across each BNC. Perhaps I'll just do that to provide some level of protection. For the current probe (https://www.tekbox.com/product/TBCP1_200_Manual.pdf), I think the bandwidth is probably low enough at low frequency to prevent front-end overdrive, although I suppose transients are still possible if something is left unterminated (??) Once again talking out the wazoo.
Thanks for the Coilcraft suggestion. I also found Bourns PQ2614 series inductors. (PQ2614BLA-1R5K). 3 of these in series. Typical resonant frequency is 60MHz so that might push the upper-end bandwidth out a little, might help for radiated emissions pre-compliance testing on the input power cable.
https://www.digikey.com/product-detail/en/bourns-inc/PQ2614BLA-1R5K/PQ2614BLA-1R5K-ND/7695895
The voltage at 400Hz is 115Vac nominal. So, that's on the order of 6A through the earth conductors of the building with 2x10uF. Just seems crazy.
10uF box-style film cap presumably has a fair amount of inductance anyway, so I wonder how effective is in the MHz range anyway. The recommended type per the standard is a feed-thru type with metal shell, with the metal shell bonded to earth/ground plane. Anyway, one of the commercial LISNs says this:
--- Quote ---The mounting plate of the LI-325C is left unpainted in order to facilitate connection to earth ground in its installation, which is essential due to high leakage current.
--- End quote ---
...so I guess it is what it is. We'll make sure the earth connection is very solid, if that's what keeps it safe. Otherwise, I imagine reasonable voltage could even build up on the enclosure.
Pitrsek:
Depending on your voltages, the textbox lisn might work for you (different box/connectors, bigger wires). I would not be afraid too much if you skirt the impedance limit lines for pre-compliance testing. fan might be needed to cool the inductors.
You can measure impedance by shunt s21 method. Basically you calibrate/normalize for through connection and connect dut between hot and gnd. Google for "2-Port Shunt-Thru". Check your setup against a known impedance to asses accuracy.
TimNJ:
Thanks! My company doesn't have a VNA, but as I mentioned, I might consider NanoVNA, assuming it's good enough. (I assume the $5000 Keysight VNAs are expensive for a reason? Thanks for the pointers, as my RF knowledge is at the "don't even know the right things to Google" stage.
Regarding the design, I've decided on 5x Coilcraft SER1590 1uH parts.
https://www.coilcraft.com/getmedia/81a5b014-f8b3-44cc-abe2-de459d23320f/ser1590.pdf
I simulated the design with the provided LTSPICE frequency domain model. The theory is very good...I'm not sure if the upper end frequency range would really behave that nicely in practice, but who knows.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version