Hi folks!
I recently had the opportunity to do pre-compliance testing for conducted emissions (CISPR 25).
I'd like to share some feedback.
Initially, I have used a purely home-made LISN (courtesy of Idpromnut).
Then, I could use a Tekbox TBOH1 LISN. Results were similar to the home-made LISN.
I'm using a Rigol DSA832-TG (well the TG is irrelevant here), with the EMI option (I use that for the CISPR 9kHw RBW and I can also do quasi-peak measurements).
However I only had one of each of those LISNs and I wanted to do the measurements with two matched LISNs. I believe that's the proper way of doing things and it allows to differentiate between common-mode noise and differential-mode noise (by comparing the noise on the + and - power supply wires).
So I have used the design available here (thanks Jay_Diddy_B
) to build two identical LISNs :
https://www.eevblog.com/forum/projects/5uh-lisn-for-spectrum-analyzer-emcemi-work/msg404662/#msg404662Here is my test setup. The LISNs are located at the top-left of the picture (you need to be logged-in to view the pictures)
Screenshots :
Here's another one (measurements are not as good, because of different input settings and lower sweep time - but I wanted to show the pass/fail limits feature) :
And here are the measurements from the certification lab :
As you can see the results are actually pretty close. It looks like the home-made setup underestimates the noise by 2dB but I wouldn't submit the design to the certification lab again with a 2dB margin only, so it's still pretty useful (plus, I know the bias now).
NB: I have enabled the input corrections to compensate for the 10dB attenuator built into the LISN, so that what's displayed on the spectrum analyzer matches the supposed readings. I like this embedded 10dB attenuator, it really makes a difference, previously (with other LISNs) I was getting warning from the spectrum analyzer when I was turning on the UUT ("IF signal out of range"), now it's silent, which is good.
Also there is a DC-block at the input of the spectrum analyzer as an additional protection (Minicircuits BLK-89S+), I could probably safely remove it, but it doesn't hurt, its insertion loss is negligible.
I have used a peak-detector for fast measurements, with a relatively long sweep time (I have chosen to use 50s to make sure I get all the noise). I could go much faster but by nature the noise of my device is transient so I need to make sure the spectrum analyzer spends some time on each frequency to get a good reading.
The span I used was 10kHz-15MHz because I know (from the pre-compliance measurements at the certification lab) that there is nothing interesting between 15MHz and 30MHz.
If anyone has suggestions for improving the setup, or if you have any question, just let me know !
PS: The black cable running from the bottom right of the picture is a USB UART for configuring the UUT. Don't forget to disconnect it before starting testing however it will screw up the grounding scheme (computer ground is safety ground, just like the copper plate and the LISN "-", which is different from the UUT "-")
uski