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/HomeThanks 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=1This 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.