JohanEricson

Joined Aug 30, 2017
32
Hi all!
Trying to read a schematic for the Moog Werkstatt synthesizer, but some things are confusing me.. and there will probably be more the more i look in to it.
Anyway, at the moment i'm trying to figure this out:
There's a symbol for a inductor with a magnetic core, but for value it says "100K @ 100MHz", and i can't figure out why and i can't find a inductor to buy that has those specs.

The only thing i could think of is to use
Z = jwL --> L = Z/jw = Z/(2piF)
the set
Z = 100k (ignoring complex number stuffs)
F = 100 MHz
and get L to about 16 mH.

But it seems kind of strange to mark it that way..?

What does it mean?

Thank you!

Joined Jul 18, 2013
22,857
100kohms inductive reactance (impedance) at a frequency of 100Mhz.
Usually marked in Millih/ μh inductance.
Max.

JohanEricson

Joined Aug 30, 2017
32
100kohms inductive reactance (impedance) at a frequency of 100Mhz.
Usually marked in Millih/ μh inductance.
Max.
Well yes,
but does that mean my way of calculating the value of L is the way to choose the right inductor?
Or can you buy one with the specification "100k @ 100Mhz"
But now the follow-up question is, why do they mark it like that?
There are some other inductors without an iron core that just says 15uH.
Why is there a difference in the marking?

Dodgydave

Joined Jun 22, 2012
9,939
I get 159uH for the inductance .

ebp

Joined Feb 8, 2018
2,332
Is this just on paper or are you looking at an actual circuit?
If the latter, it could well be 10 µH (10 x 10^0), 10% tolerance (K), with a self-resonant frequency of 100 MHz (which is extremely high for 10 µH; realistic for 10 nH, but a 10 nH inductor will be very unlikely to have a ferromagnetic core).
If the former, can you post part of the schematic that shows the circuitry around the inductor?

JohanEricson

Joined Aug 30, 2017
32
I get 159uH for the inductance .
Yup, me too. I did it to fast in my head yesterday and lost a factor of 100 somewhere!
Thank you for making me aware!

JohanEricson

Joined Aug 30, 2017
32
Is this just on paper or are you looking at an actual circuit?
If the latter, it could well be 10 µH (10 x 10^0), 10% tolerance (K), with a self-resonant frequency of 100 MHz (which is extremely high for 10 µH; realistic for 10 nH, but a 10 nH inductor will be very unlikely to have a ferromagnetic core).
If the former, can you post part of the schematic that shows the circuitry around the inductor?
No it's just on the paper, its for the powersupply.
I've added a picture. You can see its the first thing after the input in all three circuits (the one in the bottom is a 12 to -9v converter) but one in the bottom also has L2 with is labelled 15uH.

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ebp

Joined Feb 8, 2018
2,332
Very strange! It would seem reasonable to interpret that as reactance of the inductor at 100 MHz. Any way you look at it, three significant digits for an inductor is weird, be it reactance or inductance. They are decoupling inductors, which is something you don't see very often. Odd that they are drawn as having cores whereas the 15 µH are shown as coreless.

There is an element of archaic design to the whole thing. Were it not for the surface mount schottky diode at D2, I might think the schematic is for for a steam punk version.

If your intent is to build the circuit, I would be inclined to recommend simply using 15 µH (or 10) in those positions, too. I would also increase all of the capacitors labeled 0.01uF to 0.1 µF.

JohanEricson

Joined Aug 30, 2017
32
Very strange! It would seem reasonable to interpret that as reactance of the inductor at 100 MHz. Any way you look at it, three significant digits for an inductor is weird, be it reactance or inductance. They are decoupling inductors, which is something you don't see very often. Odd that they are drawn as having cores whereas the 15 µH are shown as coreless.

There is an element of archaic design to the whole thing. Were it not for the surface mount schottky diode at D2, I might think the schematic is for for a steam punk version.

If your intent is to build the circuit, I would be inclined to recommend simply using 15 µH (or 10) in those positions, too. I would also increase all of the capacitors labeled 0.01uF to 0.1 µF.
And a decoupling inductor is a way to stop ripple in the current? Or can we see it kind of like a decoupling capacitor but for the current, like a "buffer"?

I was thinking of trying to build parts of it, this is just the power supply part. And of course it can be solved in other ways.
Why would you use 15uH and not 150 uH (in line with the calulations above)?
And also what is the reason you would choose 0.1uF?

Thank you so much!