EMI-Measurements of a Volt-Nut

[Andreas]

Overview:
=========
In metrology we try to measure precision voltages with stabilities/accuracies in the sub ppm range.
But usually the environment conditions regarding RF-Noise (EMI-Noise) are not ideal.
So readings can differ from location to location depending on RF-sources like:
- switch mode supplies (LED-lamps) (some 10kHz to some 100kHz),
- conducted emissions from USB/network cables, DC-motors, relays (contact bounce "fire" up to 60 MHz)
- and radiated emissions from key-fobs, radio stations, mobile phones (usually above 30 MHz).

https://www.eevblog.com/forum/metrology/metrology-grade-lighting-i-am-in-the-dark/msg2225853/#msg2225853
https://www.eevblog.com/forum/metrology/(ft)-ltz1000a-fairy-tale-or-the-story-of-little-jumper/msg1140683/#msg1140683
https://www.eevblog.com/forum/beginners/oscilloscope-interference/msg2530602/#msg2530602
https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg846835/#msg846835
https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg369383/#msg369383

Even heavy shielded commercial devices can be affected by EMI and demand for "highly controlled" RF environments:
E.g. Fluke 732b is specced according to the data sheet for 0.18V/m max field strength.
https://xdevs.com/doc/Fluke/732B/FLUKE_732B_734A_INST.pdf
House hold appliances have to withstand > 3V/m and in industrial environment devices have to withstand > 10V/m.
Of course a professional metrology lab will keep all those influences away from sensitive equipment.

But in a hobby lab we usually do not have perfectly shielded rooms and no influence what the neighbour does behind the wall.
So the devices have to be "better" to keep their stability in unknown conditions.
Shielding is one possibility which works good for radiated EMI at higher frequencies (above 30 MHz).
But the power supply/output lines are "antennas" through the housings so they have to be treated with filters.
Filtering possibilities are using capacitors to "short" the RF frequencies and using chokes / ferrites to increase the line impedance to keep the RF out.


cellularmitosis uses EMI-ferrites+capacitors with his "LTZ1000 board (PX-ref v2.4)" to increase immunity against EMI noise comming from supply lines or reference output wiring.

https://www.eevblog.com/forum/metrology/usa-cal-club-round-2/msg1501783/#msg1501783
https://www.eevblog.com/forum/metrology/usa-cal-club-round-2/?action=dlattach;attach=418882

Dr Pyta uses SMD ferrites on his board
https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg878662/#msg878662
https://www.eevblog.com/forum/metrology/ultra-precision-reference-ltz1000/msg941288/#msg941288

TIN widely uses multi-layer which gives some additional shielding.

To check the effectiveness of filters we need a possibility to test it.
Otherwise we do not really know wether the measures are effective or if they only quiet conscience.

EMC immunity is tested in several ways.
Above 30 MHz up in the GHz range usually a antenna and a capable ~1000W RF-amplifier is used to test against radiated emissions.
In the lower frequency range the test is done either as current injection (BCI Method) or as capacitive injection to test conducted immunity on the power supply and signal lines.

To keep the costs low I decided to use the capacitive injection on signal lines which is usually done from 150kHz up to 80 MHz or even up to 230 MHz in some applications.
So the ideal signal generator has at least up to 20Vpp output voltage and a frequency range from some 10 kHz up to 230/300 MHz, and is able to produce CW + AM modulation (typ. 80% AM @ 1 kHz)
The AM modulation may be demodulated by input protection diodes and give a "offset" on the input signal with the modulation frequency. So AM modulation is usually the test which is harder to survive.

The upper frequency ranges can usually handled by a metal housing and ferrite beads on the signal lines.
So I will try (in the first step) to find improvements for the lower frequency range.
And of course I need to have a possibility to measure the improvements.

Caution: these tests may not be always destruction free!!!

work in progress, will be updated ...

[Andreas]

Setup:
=====

The whole setup is similar to standard IEC 61000-4-6:
As I do only comparative measurements I have done some simplifications.

The table surface is covered with overlapping tinned steel sheets (RS-Components 682-472) to have a common ground.

The coupling decoupling network (CDN) is self built and similar to a CDN-AF2 network according to the standard.
I used NiZn ferrites similar to those from the standard and changed the number of windings to get the necessary inductivity.
According to standard a 6dB attenuator is to be used directly at the RF input of the CDN.
As I feared that the signal level of the signal generator would be too low I decided to include a 3 dB fixed attenuator within the CDN to protect the signal generator somewhat from short cirquits and maintain a higer level.
The CDN has 2 ports:
1 port for the DUT/EUT (equipment under test) where the RF is coupled in.
And another AE port (auxiliary equipment) which is decoupled from the RF by a common mode choke so that the reaction of the EUT can be measured eg. by a DMM.

DUT/EUT and AE have to be placed with 10 cm space on the common ground.
In my setup I use 5 cm card boxes instead to save overall space.
Conection between CDN and EUT is kept short (30 cm).
EUT may be a AD587LW, a LM399 or a LTZ1000(A) reference cirquit.

The AE is either a K2000 or a 34401A which measures the output of the DUT (e.g. voltage reference).
Fortunately I use batteries on my voltage reference devices. Otherwise I would need 2 CDN devices: one for the power supply port and one for the voltage output.

The RF is supplied from a Feeltech FY6800 DDS signal generator.
With sine waveform we can reach up to 60 MHz frequency range.
Typically the sensitivity of analog devices is relatively small band.
E.g. several resonances between (parasitic) capacitors and wiring inductance.

So we need a logarithmic sweep with a small step width of e.g. 1%.
Some devices are more sensitive when a AM modulated signal is used.
So typical testing is done with 1000 Hz 80% AM modulation.
But I can also think that the 1000Hz could be filtered away by the 10 NPLC of the DMM which is used to verfy the voltage reference.
So it might be that a lower frequency like 7 Hz AM modulation harms more in this case.

[Andreas]

FY6800:
======

The Feeltech FY6800 is a entry level DDS signal generator.
For the price we have some limitations.

The frequency range is up to 60 MHz with 1uHz resolution for sine wave generation.
Amplitude is up to 20Vss (+/-10V) without load up to 20 MHz.
Above 20 MHz the amplitude is limited to 5Vss (+/-2.5V).
The generator is controllable by USB (Virtual Com Port) and also has the possibility to modulate with AM.

Unfortunately there are some bugs in AM-Implementation:

https://www.eevblog.com/forum/testgear/fy6800-dds-signal-generator-questions/msg2502990/#msg2502990

- dialing 80% AM results in 66% modulation effectively and also the amplitude is reduced ("down modulation") instead of maintaining the average 100% level
- when changeing the carrier frequency the AM modulation is switched off and has to be reactivated every time the frequency changes.

I decided to ignore the down modulation bug for my measurements (this is also against the standard).

In the mean time there is a successor FY6900 available (with increased +/-12V amplitude up to 20 MHz).

[Andreas]

CDN-AF2:
=======

commercial available CDNs can be got from several EMI sources eg. here:
http://www.schwarzbeck.de/Datenblatt/CDN_AF2.pdf

When looking at the price I decided to build my own CDN-AF2.
According to standard the common mode choke consists of a low frequency part  (>280uH @ 150kHz) and a high frequency part (2-4 toroids).
My local catalog dealer offers following NiZn ferrites which are close:

For the low frequency part I choose a 25 mm core with 13.5 windings (13 outside/14 inside core) which gives ~350uH measured inductance @150kHz.
https://www.reichelt.de/ferrite-core-material-3f3-ferr-tn25-15-10x-p246055.html?

For the high frequency part I choose 4 pcs with 0.5 windings. (only feeding through the core).
https://www.reichelt.de/ferrite-core-material-4a11-ferr-tn13-7-5-5-p246049.html?

Housing is made of the following:
https://www.reichelt.de/metal-shielding-housing-162x68x28-mm-teko-394-p21199.html?

All resistors are 0.6W 1% metal film.
All capacitors are WIMA MKS-2 (polyester foil with 5 mm pin distance)
As already mentioned I include a 3 dB divider into the CDN-AF2.

Attached the "cirquit diagram" including discharging resistors (10 Meg to keep leakage currents low)  for the AE port capacitors.
And some pictures from my build.

16.09.2019 Edit: unfortunately I made a topology fault in the build:

The low frequency common mode choke belongs to the AE side
and the high frequency choke should be on the DUT side of the CDN.
So I built a 2nd device with correct topology. Starting from picture IMG2771w.jpg on.
(Here it is the "right" one which is correct).
So now I can compare if the topology really has an effect (at least up to 60 MHz).

[Andreas]

Comparing some chokes:
================

Real inductors are rarely ideal devices.
The parasitic capacitances lead to resonant frequencies.
Above the resonant frequency the inductor behaves like a capacitor.
So practically the chokes are only capable to filter a relative small frequency band.

Here I measure the inductors with my oscilloscope with integrated frequency generator in a 50 Ohms system.
On the right side the generator output is fed to channel B.
The inductor is connected between channel B and channel A.
Channel A is terminated with a 50 Ohms terminator.

The bode plot/dampening curve is done by a 3rd party software for the scope.
https://bitbucket.org/hexamer/fra4picoscope/wiki/Home
Thanks Aaron.
Since the function generator in the scope is limited to 20 MHz I have to live with that until I write my own program for the FY6800.

First inductor is a Murata BLM31PG601 1206 ferrite bead.
Which is a population option in my AD587LW 10V reference design.
These are specified for 100 MHz.
This explains also that the inductor starts (-3dB) at 850 kHz.
The maximum in the measurement is -17 dB at 20 MHz in a 50 Ohms system.
Usually we have higher impedances up to 377 Ohms (the impedance of "free air") so the real dampening is to be expected even lower at these frequencies. When looking at the result the ferrite beads seem to be better suited for higher frequencies.

In my AD587LW cirquit I filter the GND and 10V output are filtered as a PI-Filter (with capacitors between the lines).
So I also have to regard 2 inductors in parallel. Here the filtering starts at 2 MHz and reaches only -12 dB at 20 MHz.

Single BLM31PG601 measurement:




Dual (parallel) BLM31PG601 measurement:


The 2nd option for population in my AD587LW design is a Würth 51uH common mode choke. (SLM 744242510)
Here we start (-3dB) at 30 kHz and at 20 MHz we have -34 dB dampening. And that nearly independant of the number of signal lines which are filtered (1 or 2).
When looking at the results it would have been better to use both filtering options in series for the AD587LW design.
The Würth common mode choke for the lower frequencies and the BLM for the higher frequencies.

Single line SLM 744242510 measurement.




Dual line (parallel) SLM 744242510 measurement.


Jason uses EMI ferrite cores with 3.5 common mode windings in his LTZ1000.
I tried to get the same cores. But of cause there is no warranty that those are really identical.
Again like on all common mode chokes there is nearly no dependancy on single or double line filtering.
Filtering starts (-3dB) at 50 kHz and reaches -24 dB at 20 MHz in a 50 Ohms system.

Dual line EMI core 3.5 windings.




Single line EMI-core 3.5 windings.


Of course I also measured the NiZn ferrites of the CDN-AF2 device.
Here we need a significant impedance already at 150 kHz. (>= 280 uH).
This is done with the low frequency inductor (13.5 windings in my case).

The inductor starts (-3dB) already at 20 kHz.
At 150 kHz I have calculated 350 uH for the 13.5 windings.
At 2 MHz the maximum dampening of -42 dB is reached (resonant frequency).
At 20 MHz the dampening is reduced already to -28 dB.
This explains why the CDN is built with high and low frequency inductor.



The high frequency inductor of the CDN is built with 4 NiZn ferrites in a row.
(the signal lines are only fed through = 0.5 windings).
Here the dampening starts (-3dB) at 3 MHz and reaches -8 dB at 20 MHz.



And I also tested a 6-hole ferrite bead with 2.5 windings. A Würth 7427503.

https://uk.rs-online.com/web/p/ferrite-beads/2606830/

Filtering starts (-3dB) at 900 kHz and reaches -20.5 dB at 20 MHz.
So just a little bit better than the BLM31 SMD ferrite.



Update: 20.09.2019

In the very beginning when not having the standard available I also thought of using off the shelf single inductors.
But these are relative small band:

Fastron XHBCC 330uH
https://www.reichelt.de/fixed-inductor-axial-xhbcc-ferrite-330-h-l-xhbcc-330-p138551.html?

Starting (-3dB) short above 20 kHz. Peak -65 dB at 2 MHz. And -19 dB at 20 MHz



Fastron HBCC 47uH
https://www.reichelt.de/fixed-inductor-axial-hbcc-ferrite-47-l-hbcc-47-p86464.html?

Starting (-3dB) around 150kHz. Peak -59 dB at 9 MHz. And -30 dB at 20 MHz.



Fastron 09HCP 470uH
https://www.reichelt.de/vertical-inductor-09hcp-ferrite-470-h-l-09hcp-470-p138662.html?

Starting (-3dB) below 20 kHz. Peak -72 dB above 2 MHz. And -22 dB at 20 MHz.



Fastron 07HCP 10uH
https://www.reichelt.de/vertical-inductor-07hcp-ferrite-10-l-07hcp-10-p86398.html?

Starting (-3dB) at 900kHz. Peak -54 dB just below 20 MHz. -47 dB at 20 MHz.



Fastron 11PHC 220uH
https://www.reichelt.de/vertical-inductor-11phc-ferrite-220-h-l-11phc-220-p138677.html?

Starting (-3dB) at 40 kHz. Peak -68 dB just below 3 MHz. -20 dB at 20 MHz.



Fastron 11P 330uH
https://www.reichelt.de/vertical-inductor-11p-ferrite-330-l-11p-330-p72996.html?

Starting (-3dB) at 25 kHz. Peak -70 dB just above 2 MHz. -20 dB at 20 MHz.



Fastron 11P 47mH
https://www.reichelt.de/vertical-inductor-11p-ferrite-47-m-l-11p-47m-p73008.html?r=1

This one has already a high resistance at DC. So it is not really suited for precision measurements.
Starting already at DC. Peak -95 dB at 200 kHz. -24 dB at 20 MHz.



Update 22.09.2019:

Fastron SMCC 47uH
https://www.reichelt.de/choke-coil-fixed-inductor-axial-47-smcc-47-p18207.html?

I use this one in the charger for my LTZ1000 references. Intention was to make the output voltage immune against the switcher noise from the external 24 V DC (swtichmode) adapter. But with low success. The reason is that this choke does not cover the  ~100 kHz range of the usual switchers.

Starting (-3dB) around 200 kHz. Peak -56 dB at 7 MHz. And -23 dB at 20 MHz.



Update 01.02.2020:

Some EPCOS coils which seem to be very promising:
First a common mode signal line choke (CAN choke) with 51uH
https://www.reichelt.de/smd-power-induktivitaet-51-h-epco-b82790-s513-p245682.html?&nbc=1

Starting (-3db) around 35 kHz already going up to -34 db at 20 MHz without resonance.



further some axial coils with self resonance in the 100 MHz range.
first with 40uH
https://www.reichelt.de/inductor-axial-ferrite-40-h-epco-b82111-e-c5-p245652.html?&nbc=1
starting (-3dB) at 200 kHz going up to -40dB @ 20 MHz



2nd  with 22uH
https://www.reichelt.de/inductor-axial-ferrite-22-h-epco-b82111-e-c4-p245651.html?&nbc=1
starting (-3dB) at 400 kHz going up to -34 dB @ 20 MHz

[Andreas]

Automation of test sequence:
=====================

Automation is done by 3 programs:

The first program (FY6800.EXE) controls the FY6800 frequency generator and is responsible
to have synchronized readings between frequency output power and the DMMs.
Frequency sweep is done logarithmic with e.g. 256 frequency steps per decade.
This corresponds to around 1% relative frequency change per step.
After some tests I have decided to start with 10kHz and go to 60 MHz.
Note that the Inductor in the coupling network (CDN) works only above ~100 kHz.
So the 10 kHz signal is dampened significantly. But this way I want to see if there is
some sensitivity to signals below 100 kHz.

After setting the new frequency it needs around 420 ms (CW) - 700 ms (AM)
until the generator output is stable. So I introduced a "ti hold" which is
set typically to 2 seconds (= 3 seconds total cycle time) to allow settling
of the frequency generator and the DMMs.
So each frequency sweep from 10kHz to 60MHz needs ~ 50 minutes.
The latest measured value of the DMMs is recorded together with frequency
and output voltage of the FY6800 into a log-file before the next frequency step is applied.

After each sweep with the same output power/amplitude the next power increment "P Incr" is applied.
I am starting e.g. with 12 dbm. This value is calculated as corresponding power after
the -3dB dampening device which is within my CDN.
Reference level is: 0 dbm = 1 milliwatt into 50 Ohms = 0.2236 V RMS.
So e.g. 15 dbm correspond to 1.26 V RMS after the -3dB dampening device or ~5Vpp as set value
for the FY6800 generator as unloaded (high load impedance) peak to peak output voltage.
The levels might sound somewhat "high" but those are "common mode" coupled on both pins of the DUT.
And the reference or DMM is usually floating against mains earth ground. So the actual influence
on the DUT is rather low as they are usually designed to withstand up to 1000V
common mode voltage at low frequencies.
With 4 power levels the total sweep time is ~ 4 hours.
So that a measurement over night can do 2 total sweeps.

I can also choose AM modulation instead of CW.
I want to use 1000 Hz or 7 Hz with a modulation depth of 80%.
As already mentioned above the output power on AM is too low according to definition
and also the modulation depth is only 66% when setting 80%.

The 2 other programs (K2000.EXE and HP34401A.EXE) can read the results with a 6.5 digit DMM on
the AE port of the CDN from the reference which is connected on the DUT port of the CDN.
To keep the measurement time short but the "noise" of the readings not too high I choose 10 NPLCs.
This leads to 400 ms reading duration (with auto-zero) on the HP34401A and 600 ms on the K2000.
I am using a virtual file to transfer the readings between the programs. So each program
can be used independantly and all results can be logged by the "master" program.

But since even the 30 cm short connections on the CDN can act as antenna the readings
have to be taken with care. You never can shurely know wether the DUT is actually
influenced, or the DMM on the AE port. So the sensitivity of the DMMs has to be tested too.

[Andreas]

K2000 results:
==========

edit 17.10.2019:

The first picture confirms also for the K2000 the "law" that +3dB more EMI-Voltage gives around doubling the drift

3.5Vss: -  7.5 ppm
5.0Vss: - 14.1 ppm
7.1Vss: - 27.8 ppm
10Vss:  - 55.0 ppm

The measurement of 13.10.2019 shows the K2000 on the DUT side and the AD587LW#04 on the AE side both witout any additional EMI-core.

14.10.2019: a additional EMI-core on the AD587LW side shows no improvement -> the K2000 is the device which drifts.

16.10.2019: shifting the EMI-core to the K2000 side gives a little imrovement on the dip.

15.10.2019: again a additional EMI-core on the AD587LW gives no change to the previous measurement.

And obviously the high impedance on the K2000 input is the reason that the improvement of the EMI-core is very moderate on the K2000 side.

the EMI sensitivity of my K2000 is around a factor 3 higher than my HP34401A.
So for future measurements I will use the 34401A.

[Andreas]

HP34401A results:
=============

edit 15.10.2019:

The first picture confirms also for the HP34401A the "law" that +3dB more EMI-Voltage gives around doubling the drift (except for the last step from 7 to 10 V)

3.5Vss: +  3.9 ppm
5.0Vss: +  6.9 ppm
7.1Vss: + 11.7 ppm
10Vss:  + 18.6 ppm

The measurement of 09.10.2019 shows the HP34401A on the DUT side and the AD587LW#04 on the AE side both witout any additional EMI-core.

10.10.2019: a additional EMI-core on the AD587LW side shows no improvement -> the 34401A is the device which drifts.

11.10.2019: shifting the EMI-core to the 34401A side gives a little imrovement on the peak. But the dip gets larger. So obviously there are 2 independant receivers with opposite direction which are dampened differently by the EMI-core.

12.10.2019: again a additional EMI-core on the AD587LW gives no change to the previous measurement.

And obviously the high impedance on the 34401A input is the reason that the improvement of the EMI-core is very moderate on the 34401A side.

[Andreas]

AD587LW results:
=============

Edit: 29.09.2019

I have measured 2 devices of my AD587LW samples:

AD587LW#03 is equipped with Würth SLM 744242510 51uH common mode chokes.
On the measurement of 21.09.2019 we see a dip of -15 ppm @ 5.7 MHz when setting the (unloaded) amplitude of the FY6800 to 10Vss in CW mode (no modulation).
There seems to be a smaller dip at ~2 MHz.

AD587LW#04 is equipped with Murata BLM31PG601 600 Ohms ferrite beads.
The measurement from 26.09.2019 shows -4.7 ppm @ 2.5 MHz at 10Vss.
The surprising result for me is that the ferrite beads deliver a much better overall performance. Even at frequencies (5.8 MHz) where the common mode choke had a much larger dampening than the ferrite beads. Perhaps the self resonant frequency of the 100nF WIMA capacitors which should be somewhere near 5-10 MHz is a possible explanation.

Another result is that doubling the amplitude on the signal generator gives excess change in error voltage.
For the AD587 measurements each factor 1.4 (3dB) signal level changes nearly a factor of 2 in the error.

When looking at the AM modulation (1kHz or 7Hz) then the error voltage is generally lower than with CW mode.
This can be explained by the erroneous implementation of the AM modulation in the FY6800.
The down-modulated signal has less energy than a true AM modulated signal.
The corresponding pictures are marked as AM (for 1 kHz) or AM7 (for 7Hz).
The 7Hz modulated AM modulated files look more "agressive or noisy" as expected (no averaging over integration time).
But the peak error voltage is nearly independant of the modulation frequency.

The most disturbing result was that a measurement nearby (LTZ6 on K2000 with 0.3 m distance between the wiring on the DUT-side and the cable to the K2000) had more error over frequency than the AD587 measured.
So it looks like my LTZ-references are much more sensitive to EMI than my AD587LW references.

But this result also shows: all measurements have to be taken with a grain of salt.
You never know how the coupling of the RF-energy is done: Either over the CDN (where it should) or "over the air" which is not wanted.
Over the time we should get a feeling which instrument/reference is sensitive to which frequencies.
So many of the effects will be explainable if we have enough measurements.

Edit 12.10.2019:
As overview I add the measurements with 10Vpp in ppm for all 4 AD587LW 10 V references.
AD587LW#01 and AD587LW#03 both equipped with Würth SLM 744242510 51uH common mode chokes show dips of -14 and -17 ppm near 6 MHz.

AD587LW#02 and AD587LW#04 equipped with Murata BLM31PG601 600 Ohms ferrite beads have a much larger stray in values of -4.4/+7.6 ppm and -5.8/+1.3 ppm. Note that the peaks above 20 MHz are clipped by reduced output voltage of 5Vss instead of 10Vss.

One result is that the stray from device to device can be rather large even with same population.

[Andreas]

LM399 results:
==========

Hello,
the LM399 results were still missing here the first part of it:

19.04.2020 first measurement of a "naked" LM399 with only a 6K8 resistor to a stabilized (LT1763) 14V supply from 12 NiMH cells.
So there are only bypass capacitors at the voltage regulator with about 40 cm wiring to the LM399.
result with very low level of 0.63 Vss at the FY6800 Generator:
-1150 ppm @ 22.7 MHz and
-554 ppm @ 117 kHz

21.04.2020 same with 2 100nF SMD 0805 bypass capacitors near heater and zener pins:
result with 0.63 Vss level:
-503 ppm @ 52 kHz

so the 22 MHz peak has been filtered by the bypass capacitors but the low frequency peak is still there.
since I feared that the LT1763 voltage regulator also could contribute to the peak I tested with 10 NiMH cells without stabilisation

22.04.2020 same without stabilized supply:
result with 0.63 Vss level:
-548 ppm @ 52 kHz
so no significant change.

Mhm: the optimum capacitor for 52 kHz would be around 400 uF lets see what I have in the drawer ...

Update 28.04.2020
============

I tried two 47uF capacitors one at the heater and one at the zener.

measurement of 24.04.2020 with 0.63Vss showed: nothing!!!!
I first checked if the function generator still works with the oscilloscope: all working fine.

so I increased the level up to 5.02Vss on 25.04.2020
now I see two peaks:
-2.5 ppm @ 77kHz
-2 ppm @ 60 MHz

the 51uH common mode choke (CAN-choke) removes the 60 MHz peak on 26.04.2020
but the -2.5 ppm @ 79 kHz remain
I fear for the low frequency part a buffer OP-Amp like AD4522 may be better to isolate the LM399

on the other side: -2.5 ppm @5Vss is a very good result for the little effort of some bypass capacitors.

Update 16.05.2020
============

Measurement of 01.05.2020 is a repeated measurement of 25.04.2020 with the 47uF on the heater side removed (so only 100nF on heater and 100nF + 47uF on the zener side) this gives slightly worse results on the 60 MHz peak.
So the main effect for EMI hardening is from the 47uF + 100nF on the zener side.

Measurement of 02.05.2020 with a ADA4522 added as output buffer. (47uF still in charge)
Of course I have a 100nF across the output and decoupling against capactive loads with 22R 10nF and 4K7 on the output of the ADA.
The low frequency peak is gone but now the ADA4522 itself is affected by the EMI giving a positive peak at 60 MHz.

Measurement of 04.05.2020 with a additional 100nF directly at the positive input of the ADA4522 buffer.
Practically no change against previous measurement so the ADA EMI is from the already filtered output.

Measurement of 06.05.2020 with 47uF removed from the zener output.
Practically no change against previous measurement so the ADA is isolating the LM399 sufficiently for low frequencies.

Measurement of 07.05.2020 with additional 51uH common mode choke (CAN-choke) on the output.
Practically zero EMI influence.

Conclusion: The LM399 either needs a 47uF capacitor or a ADA4522 buffer on the output. (the 100nF is someting that I will always add for stability). A additional common mode choke (or perhaps some ferrites) improves the behaviour above 20 MHz.

with best regards

Andreas

[Andreas]

LTZ1000 results:
============

Edit: 29.09.2019

First measurement on LTZ#8 (buffered output).

being warned by the "nearby" measurement I thought that I have to reduce the amplitude from 10Vss to 0.89Vss as lowest level.
I wanted to have a maximum ~10 ppm change of output voltage by the RF voltage.
But the first "dry run" showed already a much larger (-42 ppm) change at 60 MHz.
Fortunately the output voltage resumed immediately to the nominal output voltage after changeing to 10 kHz.
(So no permanent damage).

But I had to do further reduction of the amplitude steps starting from 0.32Vss to 0.89Vss since I did not want to kill the buffered output of the LTZ.

See measurement of 28.09.2019:

Again it looks like every 3dB step of the FY6800 output voltage doubles the error voltage.

For 60 MHz the ferrite cores of cellularmitosis or the ferrite beads of Dr Pyta should give a good supression.
It looks like I will have to do a rework for my LTZ1000 PCBs.

By the way: adjusting a voltage reference (JJA) by RF (60 MHz) is now possible for the LTZ1000 too.
(I should make a patent for this). 

Update 02.10.2019:

Time to look how we can get a improvement.
So I tried the core recommended by cellularmitosis with 3.5 windings as adapter outside the LTZ#8.
(see photo img2777w.jpg).

with the same maximum output voltage (0.89V) there was merely a 1 ppm dip visible.
The dip is now near 25 MHz on the measurement of 29.09.2019

Increasing the power carefully to 2.5Vss and finally 5.0Vss on 01.10.2019 now shows the 25 MHz dip at -32 ppm and a -21 ppm value at 60 MHz.
So compared to the measurement without EMI-core (-21 ppm @ 0.63Vss @ 60 MHz) we have a improvement of factor 8 at 60 MHz.

[Andreas]

Reserve 1
=======

[Andreas]

Reserve 2:
========

[Andreas]

Reserve 3:
========

[3roomlab]

for some reference, this interesting 28 page article with noise numbers in E n H field
(some lights can be really noisy!)

https://biblio.ugent.be/publication/8519431/file/8519433.pdf

[SilverSolder]

for some reference, this interesting 28 page article with noise numbers in E n H field
(some lights can be really noisy!)

https://biblio.ugent.be/publication/8519431/file/8519433.pdf

Looks like it is safe to use a Hair Removal Device in the lab? 

[Andreas]

Hello,

don´t know if some one already has recognized this:
the schematics and the build of the CDN differ in one detail.
The schematics is correct but the device is not.

Since I do not know if this has a influence on the measurement results I will have to correct this and repeat the measurements up to now.

with best regards

Andreas

[MiDi]

the schematics and the build of the CDN differ in one detail.
The schematics is correct but the device is not.

You mean that the cores are placed in reverse order?

Thanks for this very interesting topic and investigation 

[Andreas]

Hello,

yes you are right the high frequency cores should be on the DUT side,
and the low frequency core at the AE side.
I have updated the previous post with a corrected build:

https://www.eevblog.com/forum/metrology/emi-measurements-of-a-volt-nut/msg2684082/#msg2684082

and also updated the choke measurements with a 6-hole ferrite bead.

https://www.eevblog.com/forum/metrology/emi-measurements-of-a-volt-nut/msg2684085/#msg2684085

with best regards

Andreas

[Andreas]

Hello,

just a note that I have updated further choke  measurements and the description of the test-automation

https://www.eevblog.com/forum/metrology/emi-measurements-of-a-volt-nut/msg2684085/#msg2684085

https://www.eevblog.com/forum/metrology/emi-measurements-of-a-volt-nut/msg2684088/#msg2684088

with best regards

Andreas

[Andreas]

As already mentioned in the AD587LW measurement I got a very disturbing result:
https://www.eevblog.com/forum/metrology/emi-measurements-of-a-volt-nut/msg2684097/#msg2684097

A simultaneous measured LTZ6 on K2000 in a 0.3 m distance had much more error voltage than the AD587LW which was tested.
Attached 2 measurements in CW-mode.
You can see that the setup delivers 3 critical (resonance) frequencies: 12.5 MHz, 15 MHz and 43.7 MHz.
An note: above 20 MHz the output voltage of the FY6800 is limited to 5Vss so the 43.7 MHz result would be a factor 4 larger if the amplitude was 10Vss.

Of course the coupling "over the air" is much less reproducable than the coupling via CDN.
But it also shows that we have a very high sensitivity on the LTZ1000.
This is also verified by the first LTZ1000 measurement.

https://www.eevblog.com/forum/metrology/emi-measurements-of-a-volt-nut/msg2684103/#msg2684103

[3roomlab]

the xtal in the K2xxx is 12Mhz
the wire become HF antenna?
dont know the xtal in 34401a ?

do you have a chance to try tiny bead like the picture?

[Andreas]

Hello,

Ferrites are a good idea (especially for higher frequencies) to determine which device is the guilty.

I prefer to attach the EMI-cores outside the device. (so it is reversible).
see updated photo of LTZ1000 here:
https://www.eevblog.com/forum/metrology/emi-measurements-of-a-volt-nut/msg2684103/#msg2684103

with best regards

Andreas

[Andreas]

Hello,

this time I tested a 2nd LTZ (LTZ#7) with different population.

LTZ#7 is populated with LT1013  (and 2 additional EMI capacitors) and 7 mm shortened legs of LTZ1000.
LTZ#8 is populated with 2*LTC2057 and long legs of LTZ1000.

LTZ#7 measured factor 1.6 more sensitive to EMI than LTZ#8.

Now the question arises for the reason.
- stray in sensitivity of LTZ1000 with same date code?
- shortened legs more sensitive than long legs?
- LT1013 more sensitive as LTC2057?
- the additional capacitors?

I need more samples.

with best regards

Andreas

Edit:
What also can be seen: the 2 runs of LTZ#7 did not meet the same point at 60 MHz. The temperature changed by about 2 degrees during the measurement. So possibly the EMI behaviour is temperature dependant.

[Kleinstein]

The large EMI effect seems to be really high frequency effect. The early data from the LTZ6 suggest that the that there are resonances and both #7 and #8 seem to have a resonance (just) higher than the frequency range tested. So the difference between the 2 may be just a minimal different resonance frequency.
The interesting part may be a slightly higher frequency (FM radio).