Inductors: buy or roll your own?

**John Sorensen** · 2nd January 2004 08:13 PM

I'm working on FM transmitters/receivers, and I see instructions for making one's own inductors for them. On one it says the inductor is about 0.1uH. Can I just buy a 0.1u inductor (or a 0.056uH in another design), SMT types, or is there something special about the handmade ones? If I'm picking inductors for RF applications, what should I look for? The one I picked out is 1210, and it says it is air core. Digikey p/ns PCD1119CT-ND and PCD1116CT-ND.

j.

**Russlk** · 2nd January 2004 10:44 PM

for low value inductors, surface mount is best because there is no lead inductance to account for. Designs are much more reproducable. In some cases the Q of surface mount inductors is not as high, but for .1uH it should not be a problem.

**Roff** · 2nd January 2004 10:49 PM

I looked at the Panasonic datasheet, and the "Q" for the 100nH part was 10 minimum. Of course, it may be quite a bit higher. I think a hand-wound coil will give you Q higher than 10, but I can understand why you want to use an SMD part. Maybe 10 is adequate for your application.

**John Sorensen** · 2nd January 2004 11:10 PM

Can you explain Q in a way so that even a digital guy can get it?

j.

**Dean Huster** · 3rd January 2004 12:20 AM

A handbell has a higher Q than a cowbell. The tone will sustain longer.

In coils, resistance of the wire contributes to a lower Q which in turn leads to a resonant circuit that has a sloppier slope, i.e., not as narrow of a bandwidth. It's pretty difficult to find high Q coils in large values of inductance, especially if something other than an air core is required to achieve that inductance value. High Q isn't always good. Some circuits won't work unless the Q is high; other circuits really screw up bad if the Q is too high. Lessee, what is it ..... Q = XL/R

Dean

**Agent 009** · 3rd January 2004 03:34 PM

Do you have something to test the inductance of the handmade coil? If you do, i think it's handy if you want specific -but not very precise- inductors at hands.

**John Sorensen** · 3rd January 2004 04:02 PM

No, I don't have anything to measure inductance... Sometimes I'll charge up a capacitor, remove the voltage, and put the capacitor in parallel with the inductor I want to test, then measure the frequency of the oscillations.

j.

**John Sorensen** · 6th January 2004 05:15 PM

Can someone help me with this inductor marking scheme. They are marked using a color band system, similar to resistors. But my research turns up that they should have three bands, but all the ones I've seen have four. For example, the one I have here is Br-Gr-Br-Go. Now, my guess would be that it is 150 somethings. uH? pH? mH?

Here's the weird part. I built one of those FM transmitters, with the hand wound air core inductor, like 8 turns of #22, with 36pF in parallel, it was broadcasting somewhere in the middle of the FM band. So I pulled out that inductor and replaced it with the one I described above-- thinking maybe I could mess with the cap to get it back into the band. But when I put the new inductor in-- it broadcasted in almost the exact same spot on the band! Weird, huh? And a great signal, too! What is the deal with that?

Nother question: that capacitor across the transistor, I think someone called it a feedback cap, it's like 4.7pF. Is that value dependent on the choke and it's parallel capacitor, or is it independent?

One more thing, doing my little experiment with the capacitor and inductor in the last post, I got the value of the Br-Gr-Br-Go inductor to be about 180-220uH. Which is why I am so suprised the FM transmitter worked with it.

j.

**Gandledorf** · 6th January 2004 05:22 PM

Originally Posted by John Sorensen

Can someone help me with this inductor marking scheme. They are marked using a color band system, similar to resistors. But my research turns up that they should have three bands, but all the ones I've seen have four. For example, the one I have here is Br-Gr-Br-Go. Now, my guess would be that it is 150 somethings. uH? pH? mH?

One more thing, doing my little experiment with the capacitor and inductor in the last post, I got the value of the Br-Gr-Br-Go inductor to be about 180-220uH. Which is why I am so suprised the FM transmitter worked with it.

j.

The bands typicall mean the following:

Black - 0
Brown - 1
Red - 2
Orange - 3
Yellow - 4
Green - 5
Blue - 6
Violet - 7
Gray - 8
White - 9

And the gold is an alignment band. To read the code, assign the bands the following variables: A B C Gold

This indicates the component has a rating of (A*10 + B) * 10^C

Thus Br Gr Br Go is: (1*10 + 5) * 10^1, or 150

**John Sorensen** · 6th January 2004 06:48 PM

Yeah, but 150 whats?

j.

**Russlk** · 7th January 2004 12:14 AM

The bands, A, B, C, D are decoded as follows:
A is the first significant digit
B is the second significant digit
C is the number of zeros
D is a tolerance indicator, Gold is better than silver, but I don't remember
what number to apply.

Br, Gr, Br = 150 micro henries.

Small inductors are always in microhenries, when the value is large, the value is printed on it, like: 100mH.

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