Puzzled about ferrite choke behavior

[LightlyDoped]

I recently bought some ferrite clamp-on cores (a little over 1 cm inner diameter) to try to deal with some RFI. On a whim, I connected my function generator to my scope using an insulated wire to connect the scope probe to the center conductor of the function generator output. I connected the ground leads of the scope and function generator directly.

I then wound five turns of the insulated wire through the ferrite core and closed the halves. I generated a sine wave at 3 MHz. (My function generator only produces a maximum sine wave frequency of 3 Mhz.) I found that rather than reducing the amplitude of the voltage on the scope, the amplitude was actually higher than with no ferrite core at all! And the voltage actually increased as the frequency increased.

I don't understand this behavior. In fact, I could see the voltage increase over the base line as I slowly closed both halves of the ferrite core. I don't know the mix used in the ferrite core, but I bought it for ham radio use and it's made by Fair-Rite, which is a reputable manufacturer.

I watched a nice video by W2AEW, and his use of a ferrite core clearly showed a reduction of amplitude of the signal as the frequency increased. So what am I doing wrong?

[uncle_bob]

Hi

Ok, your scope probe looks like what?

Maybe 20 pf parallel 10 meg ohms?

So effectively you are running your generator into a 20 pf cap. As you increase the inductance in series with the cap, you will boost the voltage. Effectively you are building a matching network. The alternate interpretation is that you are building a series tuned circuit. Oddly enough this surprised me a bit back about 40 years ago ....

Bob

[T3sl4co1l]

RF systems are almost exclusively measured at 50 ohms.

Some exceptions are:
- Any special case (within circuit, perhaps?) where the impedance naturally isn't 50 ohms
- Most TV related things, where they use 75 ohms
- RFI filters, which are sometimes measured at 0.1/100 ohms, which gives a more realistic measure of performance in typical applications (e.g., attached to an SMPS).

Real wiring systems are in the 50-300 ohm range, depending on conditions.  In fact, most regulatory standards use 50 ohm models, mainly because it's close enough and there really isn't a better way to do it.

You'll get higher impedances for common-mode signals (the total cable, with respect to its surroundings, is being charged to some RF voltage, or by some current), because those act like transmission lines against the rather distant surroundings, or like antennas into free space (which is the real concern here, avoiding radiation and interference).  Random peaks and dips in the impedance, due to reflections and standing waves and radiation at some frequencies and not others, leads to a crazy impedance overall, but the frequencies where the impedance is most reasonable (in that 50-300 ohm range) are usually the frequencies where radiation will be worst.  So, that's the motivation.

For differential-mode signals (where two wires within the cable are moving equally and opposite), the impedance is lower (because each wire is acting with respect to a much closer neighbor), and there is less danger of radiation (that is, as long as it remains balanced, anyway!).  A ferrite bead or CMC (common mode choke) will do very little or nothing against this mode, which is why you usually see big film capacitors filtering it (0.1uF and larger "X2" caps).

So, as far as measuring it, you need to terminate the scope.  Patch the FB-inductor right into the scope's input jack, using a BNC tee.  On the other side of the tee, connect a 50 ohm terminator.  (Or if your scope has an internal 50 ohm setting, use that.)

Finally, mind that ferrite beads aren't really lossy at 3MHz.  Even the low frequency materials tend to start getting lossy only above 10-20MHz.  Otherwise, they act like crummy inductors: storing energy (rather than mostly dissipating it), resonating with capacitors, all that usual stuff.  (The crumminess comes when you try to store very much energy in them: without an air gap to store energy, they don't store nearly as much as their size should suggest.  So don't use ferrite beads for filtering DC power, unless it's as a CMC.)

Tim

[LightlyDoped]

Thanks for the replies guys. Here's why I was puzzled. Take a look at the first few minutes of W2AEW's video:

http://https://youtu.be/81C4IfONt3o

He seems to be doing the same thing, although with a much smaller toroid. His waveform shows a decrease in amplitude vs. frequency across the entire range of frequencies he's generating, starting at 10 kHz. I didn't terminate with 50 ohms, so maybe that's my problem, and I can't tell from the video if his scope is terminated with 50 ohms.

[uncle_bob]

Thanks for the replies guys. Here's why I was puzzled. Take a look at the first few minutes of W2AEW's video:

http://https://youtu.be/81C4IfONt3o

He seems to be doing the same thing, although with a much smaller toroid. His waveform shows a decrease in amplitude vs. frequency across the entire range of frequencies he's generating, starting at 10 kHz. I didn't terminate with 50 ohms, so maybe that's my problem, and I can't tell from the video if his scope is terminated with 50 ohms.

Hi

The one nice thing about circuit theory - you can predict what happens in some cases. Since I've also run the experiment physically that's sort of a spoiler as well. You *are* adding inductance as you put turns on the core. The inductance you add is fairly high Q down at the frequencies you are using. That plus any strays you have running around forms a classic L network. It is a very standard way to do a voltage step up and is used in a *lot* of designs. The approach has been around at least since the 1920's. The theory goes back to at least the 1880's.

Bob