EMI-Measurements of a Volt-Nut

[MiDi]

Andreas, I do not think it is relevant, the reports are updated.

Material will be separated by date code for conversion to the new die. ADA4522-2 cutover date was 1918.

[Andreas]

Hello,

I did some measurements with the ferrite that Frank used in his latest LTZ6+LTZ7 references.
https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg3263778/#msg3263778

The advantage of such a ring ferrite is that one could use a force sense terminal by using 4 wires on the common mode choke. Against a ready made CAN choke with only 2 wires like used here:
https://www.eevblog.com/forum/metrology/emi-measurements-of-a-volt-nut/msg3002670/#msg3002670

Since I feared that one winding and ~2 uH would not be enough for a useful filtering (the CAN common mode choke has 51 uH) I first tried the maximum possible with the 0.5 mm wire that I had on hand giving 20.5 windings (20 outside 21 inside the core).

The diagram 20201023_EPCOS_B64290L38X830_20_5_wdg.PNG shows a good filtering of -46 dB but a rather low resonant frequency of 5.5 MHz which might give insufficient filtering at higher frequencies. (And of course I would have to use a thinner wire to get a common mode choke). At 1 MHz the measured inductivity gives ~560 uH.

So I tried 2x10.5 windings as common mode choke hoping that the resonant frequency would double.
20201024_EPCOS_B64290L38X830_10_5_wdg.PNG shows the result. Resonant frequency shifted to 7 MHz and of course a reduced maximum filtering (-34 dB).  Measured inductivity @1 MHz ~ 140 uH.

Next try with 2x5.5 windings so having ~50uH measured inductance like the CAN choke.
Again 7 MHz self resonant frequency with maximum filtering -24 dB.
The CAN choke had -34 dB @ 20 MHz and showed no resonance up to 20 MHz.

Finally 2*1.5 windings with maximum filtering of -8.25 dB at 6.5 MHz self resonant frequency.

So whats wrong with this ferrite?
When looking up the Material data sheet of N30 which is used by this type the description is: Wideband pulse transformers up to 400 kHz. That may be a explanation why the dampening decreases above 7 MHz.

Ok the more interesting thing like the measurement in a 50 Ohms system will be how the inductors behave together with the LTZs compared to the CAN common mode choke.

with best regards

Andreas

[kleiner Rainer]

hello all,

contrary to popular belief, EMC work is NOT voodoo or black magic, but the correct application of well known engineering principles. I have been doing this for the last quarter of a century, successfully bringing many millions of € worth of devices through CE and FCC testing and certification.

First principle: know your enemy. In this case: which disturbance frequency? Many of the older equipment we collect and use was designed in a time, when AM and FM radio and CB radio was the main threat. Often it was simply assumed, that precision lab equipment would only operate in temperature and humidity controlled, well shielded rooms. Using a calibrator of 70s or 80s vintage in a home environment exposes it to strong fields in the 900, 1800, 2400MHZ and 5150 to 5700MHz range. Was it designed to cope with that? Maybe it has filtering against AM and FM radio, but not much more - mobile phones and WLAN were not in every household back then.

So what can we do? Throwing any ferrite we can find in our junk box at the problem does not help, as Andreas found out. Using the wrong ferrite can make it even worse, no joke! It must be the RIGHT ferrite material at the RIGHT place. How do we find out which one?

For a first look at the problem, it is useful to look what radio amateurs do in such a situation. I found a useful introduction to RFI problems and solutions:

http://audiosystemsgroup.com/PAARA-RFI-2011.pdf

The guy who wrote it, Jim Brown K9YC, was the EMC expert of the AES and has done extensive tests to assess the susceptibility of audio equipment to RFI.

Another great resource is the Book "Electromagnetic Compatibility Engineering" by Henry W. Ott. I have it on my lab bench during EMC work. Saved my ass on some occasions. It should be called "The Art Of EMC", really...

And now some hints: if you want to solve EMC problems, you should be able to measure/see whats going on - preferably up to 1GHz. I do quick checks with a handheld transceiver (Yaesu FT1XD) in scan mode. It also helps if you can listen to the disturbance - is it white noise, some kind of buzz or does it sound like radio or a data modulation (you all remember the sound of a fax or modem, do you?). Another low-cost option is the new TinySA. If it works as well as my NanoVNA, is a must for hobbyist EMC work.

For a broadband suppression choke, there is still the good old six-hole ferrite bead:

https://www.we-online.de/katalog/datasheet/7427503.pdf

As you can see, useful up to at least 1GHz. I keep a bag of them handy in the lab. Due to their construction, they do not show the parallel resonance of the classical choke. They are also very useful in filtering power supplies and brushed motors.

I would also recommend to have a look at the application notes on the Würth homepage:

https://www.we-online.de/katalog/de/pbs/emc_components  (switch to "english" left of the shopping cart symbol).

Hope that helps.

Greetings,

Rainer

[Andreas]

For a broadband suppression choke, there is still the good old six-hole ferrite bead:

https://www.we-online.de/katalog/datasheet/7427503.pdf

Hello,

good idea since I have them already here.
And they measured somewhat better than the Murata BLM31 which behaved better than the "small" CAN chokes which I used in my AD587LWdesign. See Measurement here:
https://www.eevblog.com/forum/metrology/emi-measurements-of-a-volt-nut/msg2684085/#msg2684085
(unfortunately the forum software has reordered all pictures to the end of the post).

And finally I have the NiZn Ferrites from 4A11 material used as high frequency cores for the AF2 network (Ferroxcube TN13/7.5/5-4A11).
They look very promising with 15.5 windings and are only a little larger than the cores from Frank.
I measured -40 dB @ 20 MHz and ~112uH @ 1 MHz. Starting already (-3dB) at 50 kHz.

with best regards

Andreas

[Andreas]

Some measurements on LTZ#9 with the EPCOS/TDK B64290L38X830 ferrites with different number of windings.

LTZ#9 has ADA4522 buffer with latest date code so around -238 ppm @ 60 MHz and 5Vpp without additional external filtering/coil.

a) with 2 x 10.5 windings
20201025_LTZ9_Buf_ADA4522_DC946_100nF_PGND_GNDs_C9_C11_TDK_10w5.PNG

around -12 ppm deviation @ 60 MHz so a improvement of factor 20 against the buffered reference alone.
So in practical application the ferrite behaves better than expected from the 50 Ohms measurement.

The EPCOS 51uH CAN choke had -3 ppm deviation in worst case under this conditions so only a factor 4 improvement.

b) with 2 x 5.5 windings
20201026_LTZ9_Buf_ADA4522_DC946_100nF_PGND_GNDs_C9_C11_TDK_5w5.PNG

around -38 ppm deviation @ 60 MHz so a factor 3 worse than above.


c) with 2 x 1.5 windings
20201026_LTZ9_Buf_ADA4522_DC946_100nF_PGND_GNDs_C9_C11_TDK_1w5.PNG

around -85 ppm deviation @ 52 MHz which seems to be a resonance.
Giving only a improvement of factor 3 against the naked buffer.

with best regards

Andreas

[Andreas]

And some further measurements with ferrites.

d) the Ferroxcube TN13/7.5/5-4A11 with 2x15.5 windings together with LTZ#9
nearly as cheap as the TDK ferrite core but a little bit larger allowing 15.5 windings on each side.
https://www.reichelt.de/ferritkern-material-4a11-ferr-tn13-7-5-5-p246049.html?&nbc=1

Diagram 20201028_LTZ9_Buf_ADA4522_DC946_100nF_PGND_GNDs_C9_C11_4A11_15w5.PNG shows around the same performance as the 51uH CAN choke. with -4 ppm @ 60 MHz and 5Vss.

e) the Würth 6 hole ferrites 7427503 (2 needed for the LTZ#9)

Diagram 20201029_LTZ9_Buf_ADA4522_DC946_100nF_PGND_GNDs_C9_C11_2x6hole.PNG shows -43 ppm @ 60MHz and 5Vss.
So for me it looks as if the 6 hole ferrites start working at around 20 MHz and may be a additional option wide above 60 MHz where I have my "blind spot" at the moment.

And now some hints: if you want to solve EMC problems, you should be able to measure/see whats going on - preferably up to 1GHz.

Agreed but what do you recommend for generating sine waves with at least +15 dBm up to 1 GHz (or at least 230 MHz) with computer interface and for a price which is adequate for a "secondary hobby" (< 200 EUR).

With best regards

Andreas

[dietert1]

Your TN13/7.5/5-4A11 common mode choke has about 80 uH (AL = 0,358 uH, *15*15). I measured the WE bead choke to be about 15 uH. So when you use two in parallel, you have 7.5 uH. This gives a factor ten and roughly explains the ratio of ten you find in your measurements. Your measurements are in a frequency region where this simple math still applies. It does not really tell you what happens with WLAN and the like.

We have a 7 KW photovoltaic generator on our roof with two switched mode power converters and i think i can see on my reference logs when the sun was shining. The effect is about 0.2 ppm on the daily averages. The problem is i can't turn it on or off, so it's a bit of try and error.

Regards, Dieter

[kleiner Rainer]

...

And now some hints: if you want to solve EMC problems, you should be able to measure/see whats going on - preferably up to 1GHz.

Agreed but what do you recommend for generating sine waves with at least +15 dBm up to 1 GHz (or at least 230 MHz) with computer interface and for a price which is adequate for a "secondary hobby" (< 200 EUR).

With best regards

Andreas

NanoVNA together with NanoVNA-Saver as PC freeware. The firmware also supports using it as a signal generator. You should still have enough cash left over for adapters, a set of good quality SMA cables and a Tiny SA. The output power of -18dBm is low enough for unlicensed use (+15dBm is QRP transmitter power!), but high enough for EMC work IMHO. Remember: Disturbance levels are commonly specified in dBuV... Optionally a power amplifier (MMIC-based) could be added. But as already mentioned - keep the licensing requirements in mind!

In an emergency, the second (receiver) port can be used as a crude SA - I demonstrated this at a club meeting with a rubber antenna connected to the input and showing all the GSM activity around 900MHz in the room. This use would also benefit from an LNA.

Introduction:

https://www.pvrc.org/Powerpoint/NanoVNA%20IntroductionCombined-2.pdf

https://www.bwcelectronics.com/articles/NanoVNA%20User%20Guide.pdf

Wahlweise in Deutsch:

http://www.gunthard-kraus.de/inhalt_de.htm

But be warned: VNAs and spectrum analyzers are another rabbit hole...

Greetings,

Rainer

[Andreas]

but high enough for EMC work IMHO. Remember: Disturbance levels are commonly specified in dBuV...

The standards to this theme tell something different.

If you have a look at DIN IEC 61000-6-1 together with the referenced standard IEC 61000-4-6 the level for household devices is 3V RMS on signal and power supply lines. (129.5 dBuV). Industry levels according to DIN IEC 61000-6-2 are even higher with 10V RMS and 140 dBuV.
Measured at the corresponding input with CW by replacing the device by a 150 Ohms load.
Additional you should have a 80% 1kHz AM modulation.

Most of the generator power is lost in the adaption network (CDN) to generate 150 Ohms common mode.

So it is no wonder why commercial test equipment for that test uses > 7W (38.5 dBm) linear amplifiers.
https://www.teseq.com/products/NSG-4070.php
https://www.theemcshop.com/16-emc-cdn-s-coupling-decoupling-networks

So +15 dBm generator power are in comparison at the low edge to keep the costs low.

with best regards

Andreas

[kleiner Rainer]

Andreas,

from your measurements and questions I assume you are a hobbyist with limited budget. In EMC work, 200€ is nothing - or the price for a single IEC standard. Luckily, the Indian government put their relevant standards on their server for everybody to use:

https://law.resource.org/pub/in/bis/S04/is.10052.1.1999.pdf
https://law.resource.org/pub/in/bis/S04/is.10052.2.1999.pdf

This is as close to the basic EMC standards CISPR 16.1 and 2 as you can get without paying.

IS10052.1 describes measuring apparatus and Annex N describes the coupling units for current injection. This part should be of interest for you.
IS10052.2 describes methods of measurement of disturbances and immunity. Again, Section 3, part 3.2 should be of interest to you.

With the help of these two standards you should be able to determine what to measure and with which equipment. DIY is possible, but then the question of calibration arises, as usual. Connections to EMC people or a university lab could help. The Nano VNA I recommended would at least help to measure the insertion and return loss of coupling networks.

Greetings,

Rainer

PS: more Indian Standards:  https://law.resource.org/pub/in/bis/manifest.litd.9.html

[Andreas]

Hello,

I have checked another 4A11 ferrite core.
TN 10/6/4 which is nearly as small as the TDK core.

With 0.5 mm wire I can get only 2*11.5 windings.
So with ~the same AL-value than than the TN14/7.5/5 core somewhat less inductivity.
Starting (-3db) near 200 kHz it reaches -32 dB at 20 MHz.

It gets better with thinner (0.28 mm) wire and 2*24.5 windings.
Starting (-3dB) near 20 kHz it reaches -45 dB at 20 MHz.

The picture shows the TDK/EPCOS core in comparison with the TN10 cores.

https://www.reichelt.de/ferritkern-170-nh-material-4a11-ferr-tn10-6-4-4a-p246048.html?&nbc=1

with best regards

Andreas

[Andreas]

Hello,

in the mean time I measured the smaller TN10 with 4A11 material cores on the now temperature compensated LTZ#9

with 0.5 mm dia wire I get 2*11.5 windings on the core.
with the smaller 0.28 mm dia wire there are 2*24.5 windings.

The 11.5 windings give somewhat weaker result near 20 MHz.
On 60 MHz the results are very similar.
So I believe that here the parasitic capacitance between input and output of the common mode choke plays more a role than the actual inductivity.

with best regards

Andreas

[Andreas]

Hello,

to put some light behind my "blind spot" I got one of those cheap NanoVNAs.
At least the frequency response of inductors above 20 MHz (the limit of the frequency generator of my scope) can be shown.

For each component I made 2 measurements to increase frequency resolution at least at the lower frequencies:
100kHz - 250 MHz and 100kHz- 3000 MHz.

the candidates are some of the previously known common mode chokes:
a) Ferroxcube TN10/6/4 4A11 core with 2*11.5 (0.5 mm dia) windings.
b) Ferroxcube TN10/6/4 4A11 core with 2*24.5 (0.28 mm dia) windings.
c) Ferroxcube TN13/7.5/5 4A11 core with 2*15.5 (0.5 mm dia) windings.
d) Epcos/TDK B64290L38X830 core with 2*10.5 (0.5 mm dia) windings.
e) the Würth 6 hole ferrite (2.5 windings)

with best regards

Andreas

[dietert1]

Today i found this (german) video on EMI measures with power supplies.


And  how these elements look like in the 1 to 2 GHz region:


Regards, Dieter

[Andreas]

NanoVNA together with NanoVNA-Saver as PC freeware. The firmware also supports using it as a signal generator.

Hello,

some questions to this?
How are the commands to get a sine wave out of the NanoVNA2?
At least there is no option on NanoVna-Saver to switch from rectangle to sine.

with the supplied PC-SW I always get only pulsed outputs and never a CW signal.
Is this possible with some commands?

Can I also do 1 kHz 80% AM modulation with the NanoVNA2?

with best regards

Andreas

[dietert1]

Can confirm this. Our nanoVNA V2.2 at 100 MHz to 100 MHz produces the expected peak at 100 MHz and about -10 dBm minus cable loss and a second peak at 300 MHz about 10 dB below, so it's probably a rectangular output signal. Don't know whether it's a problem though. As far as i understand the two receiver channels in a VNA are selective and even phase sensitive.

Regards, Dieter

[Andreas]

Don't know whether it's a problem though.

Hello,

I think its no problem for the usage as VNA. But if I want to use the signal generator for EMI measurements I would like to have a sine wave since the "receiver" in this case is not frequency selective.

with best regards

Andreas

[Andreas]

Hello,

after having added the 100nF capacitor between Output Ground (connector) and Power Ground (Buffer OP-Amp) I have no longer a significant difference between the ADA4522-1 and LTC2057 as buffer. (LTZ#3 and LTZ#7 shown as example).
This capacitor gives a improvement of about factor 6 (ADA4522) - 10 (LTC2057) against only having the output capacitor.

So the assumption that the EMI hardened ADA4522-1 is much better than the LTC2057 was only due to a "wiring fault" in my 2 layer layout with a long route from output connector to the buffer ground.  (so in this case it is really "better").

Attached a table with measurement results of all my buffered LTZs after adding the "ground shorting" capacitor.
So the "typical" result is a deviation of 60-90 ppm when 60 MHz and 5Vss at the unloaded function generator output is applied to the output of the LTZ via coupling network.

On LTZ#9 where the deviation is 213 ppm I still do not have all possible EMI capacitors populated on the PCB.
I will have to examine which of the missing capacitors contributes to the missing factor 2-3.

with best regards

Andreas

[kleiner Rainer]

Andreas,

the output cap soldered to the banana jacks looks like a class 2 MLCC in disguise to me - 100nF is in that range of size. Be careful - most class 2 ceramics are piezoelectric: if you tap them, they give you a voltage impulse, nothing you want in a precision reference. Better use a class 1 ceramic like C0G/NP0 or a film cap. They are recommended for that application. Bob Pease wrote about this problem in "Troubleshooting Analog Circuits", pages 45 and 111.

In my opinion, the board could use more ground pour around the opamp. And dont get me started about ESD...

Greetings,

Rainer

[kleiner Rainer]

Andreas,

sorry, I overlooked your question regarding the nano VNA. Here my answers:

- if you need a CWsignal, open the menu on the VNA, go to "Stimulus", then "CW freq", then enter the frequency you want. This I meant with "firmware". I use an older version of nanoVNA Saver, that cannot do it - will have to check the newer versions.
- yes, it is a square wave generator. The receiver part is tuned and only looks at the frequency it wants to measure in amplitude and phase, it ignores the harmonics. Think SDR.
- no, you cannot amplitude modulate it - at least not without external help (PIN diode modulator?).

Greetings,

Rainer

[Andreas]

the output cap soldered to the banana jacks looks like a class 2 MLCC in disguise to me - 100nF is in that range of size.

Hello,

of course I know that. On the first 2 samples I just added a capacitor for testing. (and "forgot" to replace it).
On the other side: the output is rather low impedant so the effect of a parallel capacitor should be low.
I think I should make some tapping tests before I replace them on LTZ#3 and LTZ#5.

with best regards

Andreas

[Andreas]

On LTZ#9 where the deviation is 213 ppm I still do not have all possible EMI capacitors populated on the PCB.
I will have to examine which of the missing capacitors contributes to the missing factor 2-3.

Hello,

attached the current schematic of LTZ#9 with the 213 ppm EMI influence.

The usually populated but missing in LTZ#9 components are circled in the diagram.
The dashed components are optional (not populated in my LTZs)
Additionally the 100nF output capacitor (GNDO) is added.
And the 100nF capacitor between the grounds (GNDs) which are connected to each other via R10 star ground at LTZ.

So the non populated parts are:

C12 (Zener base emitter)
C13 (Heater AMP feedback)
C14 (Heater AMP inputs)
C15 (Current AMP inputs)
R13 (startup-resistor)
R17 (Op-Amp stability with FET)
T2  (FET for low voltage operation)

so the next test will be with C12 added which is shurely the next sensitive point at the LTZ1000 after C11 which is already populated.

with best regards

Andreas

Edit: attached pictures of current population.
what is missing in the schematic is the capacitor from LTZ9 housing (internal substrate) to ground.

[Andreas]

Hello,

C12 100nF 0603 added (directly at the LTZ-pins 4+7 so in the center of the LTZ pins).

Result is reduction from 213 ppm deviation to 95 ppm @ 5Vss and 60 MHz.

I think there will be not much improvement by adding the further capacitors (but who knows).

with best regards

Andreas

[Andreas]

Hello,

next test with C13 + C14 populated.
As expected there is not much improvement from 95 ppm to 87 ppm.
So this could als be due to measurement variations and is no clear sign for improvement.

with best regards

Andreas

[Andreas]

and finally C15 ...

resulting in only 34 ppm deviation at 60 MHz and 5Vss.

This is much better than all my previous LTZs where the FET T2 is populated. (around 80 ppm)
So the question is: is there a sensitive point at T2?

Further tests will be necessary ...

with best regards

Andreas