A Handful of Questions on Pre-compliance - Page 1
Forum section: electronics / projects

author: Greece2000
author_rank: Contributor
timestamp: November 07, 2023, 11:40:33 pm
content: Hi,I have a few questions that I don't know the answer to or want other opinions on related to radiated emissions pre-compliance. I'm currently doing pre-compliance to CISPR25 and my setup seems not to be able to measure any emissions from my EUT. Even with an ambient environment of 0dBuV from 0-1GHz, turning the EUT on doesn't show an increase in signal.I've decided to use the Tekbox TBMA1 and the TinySA Ultra and am beginning to wonder if the TSA lacks in sensitivity. To sort out my problems I need to better understand some concepts.1. Antenna FactorWhat is the relation between antenna factor and the spectrum analyzers noise floor? Is the antenna factor describing an absolute limit to the minimum signal it can output or is it simply a conversion between the real signal value and the output signal for a given frequency.If it's the latter then any antenna should be theoretically be able to measure any signal given appropriate low noise amplification can be used to read it.2. EarthWhat is the purpose of connecting earth to a reference ground plane? Is it only for safety or is it also for standardization? Will radiated emissions results be different for CISPR25 if one does not connect the reference ground plane to the earth, and if so by how much?

author: thm_w
author_rank: Super Contributor
timestamp: November 08, 2023, 12:45:46 am
content: Antenna factor:https://www.antenna-theory.com/definitions/antennafactor.phphttps://www.ahsystems.com/notes/How-to-use-antenna-factors.phpTinySA ultra noise floor should be around -90dB. I would expect to see a bunch of ambient crap on the sweep right. Is there some frequency you expect your device to transmit at?Maybe post a screenshot of the TinySA sweep.TBMA1:https://www.tekbox.com/product/tbma1-biconical-antenna/

author: Greece2000
author_rank: Contributor
timestamp: November 08, 2023, 01:20:49 am
content: Sorry I should have specified I can see all the FM/AM bands perfectly fine when doing OAT or in my office. However there is no way to determine sensitivity problems by looking at those ambient signals.The major issue here is it's difficult to remove this ambient and it can sometimes be easier to just know the relative peaks of your EUT signals by testing in a quiet environment that isn't fully calibrated. Tekbox recommends the use of a TEM cell for this but I have access to an empty underground carpark which removes the ambient problem better by serving as a 0dBuV environment.In the carpark the signal is 0dBuV both with the EUT on and off. I can't help but feel something is wrong because I expect some kind of emissions. For instance I have a 150kHz switching converter designed without considerations for EMI with 1.5m power leads, I would think that I should see some kind of common mode noise between 30MHz and 300MHz.

author: thm_w
author_rank: Super Contributor
timestamp: November 08, 2023, 01:43:32 am
content: Thats the carpark sweep or an office sweep?The device is loaded with maximum expected load?You'd see the 150kHz and harmonics on the conducted emissions. With radiated, if its a shielded metal enclosure you may or may not see much. Depends on how hard its switching.https://incompliancemag.com/article/radiated-emi-from-a-buck-converter/I guess you could bring the antenna right up to the wires and see if anything jumps out at that point?

author: Greece2000
author_rank: Contributor
timestamp: November 08, 2023, 02:04:01 am
content: That's a sweep from an open field. The carpark sweep I haven't shown because it's more or less a straight line at 0dBuV the entire way from 30MHz to 1GHz.Device is maximally loaded yes although for switching converters I'm pretty sure EMI is worse in discontinuous mode.I see emissions from near field probes. I'm just going to work on seeing if I can amplify the signal, the antenna theory link shows that the latter understanding of antenna factor was correct.Do you have any thoughts on the second question?

author: jonpaul
author_rank: Super Contributor
timestamp: November 08, 2023, 07:26:10 am
content: You cant get valid compliance or precompliance for conducted or radiated EMI data without calibrated SA, correct antennas and LISN,See Setup for screen room/Anechoic chamber or free field needed.Any local EMI lab can do it.A few moments of research on DuckDuckGo (NOT GOOGLE) should guide you.Jon

author: erikka
author_rank: Regular Contributor
timestamp: November 08, 2023, 02:44:34 pm
content: Quote from: Greece2000 on November 07, 2023, 11:40:33 pmDid you enable the built-in LNA? This will give you about 20dB lower noise floorDid you reduce the RBW to get lower noise floor? Yes, the sweep time increases a lot with lower RBW but its there it you need it.

author: bson
author_rank: Unknown
timestamp: November 08, 2023, 08:50:20 pm
content: Quote from: Greece2000 on November 07, 2023, 11:40:33 pmThink of it as closing a circuit to other earth-referenced emitters. Their RF energy reaches your plane and turns into a minuscule current to earth. This makes your plane act as a shield; if there's no common reference it acts as a waveguide, so will end up scattering or lensing fields rather than absorbing them like a shield. The converse is true as well, in that by earth referencing a power plane your emissions become easier to block.

author: Greece2000
author_rank: Contributor
timestamp: November 08, 2023, 11:31:33 pm
content: Quote from: erikka on November 08, 2023, 02:44:34 pmReduced RBW yes. Thank god the Ultra has an internal LNA, I was under the impression an external LNA was required because of this page:https://tinysa.org/wiki/pmwiki.php?n=Main.LNAFor the curious: watch?v=iOOeNR_GN88

author: Greece2000
author_rank: Contributor
timestamp: November 08, 2023, 11:38:41 pm
content: Quote from: bson on November 08, 2023, 08:50:20 pmThis makes sense.I had once wondered why a LISN is used for radiated emissions. After some thinking it became clear but I wasn't able to determine the magnitude of uncertainty related to not using a LISN until I saw some papers on it. Here is one:https://ieeexplore.ieee.org/document/6038345If anyone knows of any studies that quantify the uncertainty related to the use and not use of earth during radiated emissions testing please let me know.

author: nctnico
author_rank: Super Contributor
timestamp: November 08, 2023, 11:46:29 pm
content: Unfortunately the Tekbox TBMA1 is the wrong choice for use below -ballpark- 150MHz; it just isn't sensitive enough. Especially when used with a spectrum analyser which has a low sensitivity to start with.A LISN or ISN can be usefull to have an idea what frequency band a cable may be radiating at if there is too much ambient noise.

author: Greece2000
author_rank: Contributor
timestamp: November 14, 2023, 01:07:44 pm
content: With the LNA turned on and an RBW of 3kHz and below, in very quiet environments, using the TBMA1 I can see signals from my EUT reliably from 110MHz up to 600MHz. All of the interesting things are happening at -120dBm and below.The tinySA Ultra (not the regular version) claims a DANL of -169dBm/Hz, this is at a RBW of 1Hz which the tinySA cannot actually do (minimum is 200Hz) but if true is equivalent to the SSA3000X-R.For the regions where I see no signal the noise floor is -127dBm, adding 30dB for the antenna factor and 1dB for the cabling this is equivalent to a noise floor of 4dBuV which has a 36dB margin for my limit line. The minimum margin required for sensible estimation is 6dB according to UN ECE R10 so I'm satisfied w/e signals I'm not seeing from my EUT are not going to cause problems.