1. What they are What they do How to apply
Ferrites
What they are
What they do
How to apply

2. What Ferrites are Ferrites are made of magnetic material
10700nH
5750nH
1520nH
12340nH
708nH
Inductance give in the picture: for 1 turn, at 1kHz.
Inductance goes up with the square of the number of turns.
You can already see that the open ferrite has a factor 2 less inductance compared with the next one higher.
Not shown here: There are also flat ferrites for flat cables.
I will treat ferrite as an induction creating device. This is not complete, for the have losses too, but in most cases this is sufficient.
Ferrites are made of magnetic material
to create an inductance in a current path

3. Ferrites: What can they do? Keep noise at bay. Noise from:
Switched mode power supplies
Variable speed drives
Stepper motors drivers
Ground currents
Most of these devices (switched mode power supplies, variable speed drive, stepper motor drivers) generate a broad spectrum of noise:
1kHz to 100kHz switching frequency, and 1MHz to 100MHz because of the sharp edges in the switching.
They can be applied to keep noise out of your signals: signals you monitor
And they can be applied to keep noise in your device: driving signals

4. How do we get ground currents?
Generator
Receiver
ground currents
magnetic field
By any changing magnetic field in a loop (nearby currents)
By capacitive coupling (nearby changing voltages)
Shielding of capacitive coupling is rather easy: any conductor can do this. Magnetic fields are way harder to get rid of. Most of the time you cannot, and you will have to kill the current it causes.

5. How do ground currents generate voltage in cables?
Generator
Receiver
resistance
ground currents
In cables we can distinguish two modes: differential mode: the voltage between the core and the return lead.
And the common mode: the net current through the cable. In the ideal situation there is no common mode current and the current that flows through the core is exactly the same as the return current: net current zero.
Due to cable resistance and inductance, common mode current can generate differential mode voltage. This is an impedance: a current generates a voltage: this is called transfer impedance.
Resistance in the shield/return lead
Inductive coupling one lead to another (coax is much better but not perfect)

6. Ferrites: What they do How to keep noise out.
There goes the RF you think. At first glance you think you will kill the RF in this way. But the coax has both the signal and the return current, which means the net current will be zero: no magnetic field and no inductance due to the ferrite. However for ground currents, going through the shield there is an inductance.
This is the main idea behind ferrites. Applied correctly they form no obstacle for the signal, but only for the ground currents.
The signal has no net current through the ferrite: this is the differential mode current.
The ground current is common mode current.
This common mode current generates a differential voltage in the signal. This is how noise gets in coax cables.
RF goes through without any obstruction.
Ground currents see the ferrite.

7. Ferrites: Test on network analyzer
A network analyzer puts a signal in the cable, receives it and shows what the cable did to the signal.
It puts in different frequencies and shows what the cable did to it. Usually it shows attenuation in the higher frequencies.
Here the network analyzer is set up to test a simple cable. And because it is a good cable I had to zoom in to show the attenuation of the cable, at high frequencies. here is scans from 300KHz to 3GHz.
The maximum attenuation here is 3.5 dB at 3GHz.
The next slides I will show a cable without a ferrite, with a ferrite around both conductors and around the core alone.

8. Ferrites: Test on network analyzer
Here you see a ferrite is applied on the coax, and no change to the signal.
The coax goes five times through the ferrite.

9. Ferrites: Test on network analyzer
To demonstrate what the difference is between putting a ferrite on a single conductor and both conductors, I split a coax cable. Now I scan from 300KHz to 20MHz. And you see a little attenuation.

10. Ferrites: Test on network analyzer
Now I put a ferrite on both: no change with previous screen.

11. Ferrites: Test on network analyzer
Now the ferrite goes over just one conductor: here you see the change.

12. Ferrites: What they do How to keep noise in
Variable speed drive
Motor
As ferrites can be used to keep noise out of a cable, it can be used to keep the noise in the cable.
Important is to make sure the protective ground is outside of the ferrite, and the wires that go through the ferrite have a total current of zero.
Ferrite

13. How to apply ferrites Ferrites inside or outside the shield?
It may have raised some questions that I said the ferrites should be applied outside of the protective ground.
This is when the ground is not used a return path. With heavy duty equipment this is always the case.
With small equipment this is different, as can be seen on this picture.
The next slide will show the difference.
Ferrites like these are applied to prevent the switching noise generated by the electronics from getting out.

14. Ferrites how to apply them Difference between in and outside shield You are outside of the shield
small device
Computer
Ferrite
Situation with small devices. You are outside of the shield. The signal, the return path and the shield can act as an antennae, and the ferrite stops this.
You are here

15. Ferrites how to apply them Difference between in and outside shield You are inside of the shield
BIG
DEVICE
Variable speed
drive
Ferrite
You are here
Situation with a big device. You are between the currents and the shield. Now it is important to keep the protective shield outside the ferrite. To make sure there path from current to the shield is blocked by the ferrite.
When it comes with delivering variable speed drives, it is really amazing what manufacturers can get away with. You connect a variable speed drive, and suddenly equipment around you starts making noise, and gets an offset. You look in the manual, and as a casual remark it says the if you experience noise problems you may apply ferrites to get rid of it.
And indeed that works.

16. Ferrites how to apply them Difference between in and outside shield
Different ways of applying ferrites:
With coax there is little choice: the braid is the return path of the signal current, and so not to disturb the signal you have to apply it over the shield.
On the right you see power leads that could be used for a variable speed drive: also here there is no choice the protective ground lead should go out. Otherwise the ferrite is useless.
On the right you see a cable with multiple cores: now you need to know if the shield is used as return path.
If not you can keep the shield outside the ferrite.
For stepper motor, and variable speed drivers you can apply the ferrite inside the box before the signals get in the connector.

17. Ferrites: how to apply them How many turns?
How many turns is useful? You may find in manuals that going higher than 5 turns is useless because of the parasitic capacitance between turns. It all depends on what frequency you deal with.
First it is important to realize that the inductance goes with the square of the number of turns, so it pays off rapidly to increase the number of turns.
And indeed, the capacitive coupling between turns also rapidly increases with the number of turns, as they get closer and their number increases.
The ferrite on the right will have a very complex behavior for high frequencies, and will be rather useless there. But I made to kill 50Hz common mode, with a few kHz signal going through it. Solving the problem with the ground current like this saved me making an differential amplifier.
Also I if you want to kill high and low frequency noise: nothing stops you applying ferrites in series : First a small number of turns through a ferrite, to kill the high frequencies, then a larger number, to kill the low frequencies. Or even the number of turns applied on a second ferrite.
Be creative with ferrites!

18. How to determine noise sources
If you make a coil like on the picture, you can measure noise generated by equipment in a (more or less) reproducible way.
This coil is made from thick solid wire, so it won’t change shape easily.
One can argue that a pickup coil like this has a strong bias towards higher frequencies: the voltage measured is proportional to dB/dt. So linear with the frequency. However the noise induced in any wire is also linear with the frequency, and in this way you just simulate a wire.
With this it is easy to test if something generates noise, and if a ferrite did change anything to reduce it.
A simple coil of a few turns, will do the job.

19. Check the noise level on a VSD
Here I measured the noise coming from a VSD.

20. Check the noise level on a VSD
Notice: no power lead to the scope, to be sure I don’t pick up noise via that.

21. Check the noise level on a VSD
Notice: with only 4 turns already more than 2V amplitude.

22. Ferrites: some examples
Here is a messy example of how ferrites can be applied.
An (extraction) kicker which has a current spike of about 1000A and a sharp rise time is connected to ground on one side of the vacuum chamber.
This give a huge noise spike in the current sensor of adjacent equipment.
You see some ferrites put in series. Even putting a ferrite on a opto-coupler input did help!
Apparently the capacitive coupling is so strong.

23. Ferrites: applied before an opto coupler
It sounds unlikely but in some cases putting a ferrite before an opto-coupler can keep noise out.
The first part of the graph is without a ferrite on the input.
The second part is with.
Extraction septum: This device gives a current pulse, when triggered and it measures the peak of the current.
This is the graph of the current it measures. The trigger input has an opto coupler.
An adjacent device (extraction kicker) generates so much noise it easily gets in the sensor input of the septum electronics.
Putting a ferrite before the opto – coupler significantly reduced the noise picked up.
The capacitive coupling is apparently strong enough to the noise in the septum electronics.

24. Ferrites: typical noise
In this case it was sufficient to tell the signal from the noise.
Coax cable connected to a sensor, but what we see here is typical noise from a switched mode power supply. Because there is ground current through the coax shield through the protective earth of the scope.
A few turns of the coax through a ferrite did significantly reduce the noise.

25. Ferrites: applied to kill 50Hz
It seems a long shot, but you can kill 50Hz noise with ferrites. It took about 200 turns on a small ferrite core. And it passed the signal on up to about 2MHz. It saved us a differential amplifier to get rid of the 50Hz.

26. Ferrites: how to interrupt common mode currents
Example: a patch panel at the Australian synchrotron
booster vacuum chamber
coax cables
patch panel
Booster ring
common mode current
The vacuum chamber from the booster is a conductor. Together with the coax cables a ground loop is inevitable. Any trapped magnetic field inside these loops will generate a voltage, and because there is a short circuit there will be currents. Common mode currents in the coax generate a differential voltage in the coax between the inner and outer conductor. A patch panel will normally distribute the currents and form the node in many loops. So this is the place to interrupt them.

27. Ferrites: how to apply them. Patch panel
metal patch panel
N conn
SMA conn
Equipment
noise
plastic patch panel
Equipment
At a patch panel there is a perfect way for all noise to get everywhere. If you connect the shield of the coax before any ferrite to ground, the noise can easily get in all cables. That is the case on this picture: a metal plane that connects all grounds. For a ferrite to be useful it needs to be applied before the shield is connected to ground. You see it here once. On this patch panel all connectors are adaptors from N to SMA type, so behind this patch panel there are all SMA connectors, with a much small diameter coax: much easier to apply a ferrite.
Now if you replace the metal patch panel with a plastic one, you can apply the ferrite after the SMA connector. If the patch panel is located in an environment with much electrical noise, then you can think of feeding the cables through a metal panel first.

28. What ferrites cannot do. Kind of noise the ferrite cannot eliminate.
A ferrite is meant to isolate currents, it does not filter!
Once the common mode current created a differential mode voltage, the ferrite can do nothing about this.
In the case below there will be common mode current through the cable shield regardless the ferrites.
The main reason why ferrites work in a network of cables, is because you work in a low impedance network, and you apply a higher impedance in one cable.
Currents through the shield, due to capacitive and/or inductive coupling
Ferrites do not filter, they isolate currents. They are not a low pass filter.
They don’t kill the differential mode currents or voltages, so once a differential mode current or voltage is created, they can’t kill them.
This is the case with long cables in a noisy environment.
The main reason why ferrites work in a network of cables, is because you work in a low impedance network. There are many ways in which the currents can flow. And when you apply a higher impedance in one cable, this will force a redistribution of currents, and the current through the cable with a ferrite will be reduced.
So if the ground current you want to kill is in a high impedance network the ferrite would need to create an even higher impedance in order to eliminate the current. This can usually no be achieved. You will have to create a low impedance path for this current where it does not do any harm.
Noisy cable