# How different is Q-factor of SMD inductor from regular one?

I am trying to figure out if it is possible to replace inductors like this wound by 0.1mm (diameter) copper wire with multilayer 0805 SMD inductors. Those are supposed to be used inside band-pass filters. Right now there are 2 coils but I am thinking if it is worth using SMD inductors + trimmer capacitors there. The main goal is to reduce footprint on the board. I want to make cheap mobile rig so am looking for ways to make it lightweight and compact.

• Q is typically a result of ohmic resistance – and it's kind of hard to predict what you'll get without knowing the datasheet of both your previous choice and its potential replacement. The good news is that a datasheet is sufficient to tell! – Marcus Müller Jun 25 '18 at 5:26
• The problem: I am not qualified to figure out if they will work or not. Right now it is simple band-pass filter for signal from antenna. I have shown schematics but I don't think it will tell you much. I don't have RF engineering degree :( but I like to build new things. I came here to ask more experienced people about typical Q for regular coils and how is it different from SMD ones. – zoonman Jun 25 '18 at 5:54
• well, there's no "typical" as far as I can tell – I don't even know what a "regular" coil is and what to generally expect from the coils depicted in your picture :( – Marcus Müller Jun 25 '18 at 5:56
• Marcus, the problem - my coils are hand-wound. I don't even know a ferrite parameters on them. And unfortunately I don't have any idea of measuring using basic tester. All of that requires lab equipment like milliohmmeter to measure active resistance of the coil and something to plot inductive resistance. I just can't go to MIT and tell "hey guys, measure this and that for me". This is why I came here - to rely on someone's else real practical experience. On people who did it already and know more than I do. – zoonman Jun 26 '18 at 18:22
• Aaaah well, so this boils down to a measurement problem! Maybe you could ask a separate question on how to measure q factor of self-wound coils – Marcus Müller Jun 26 '18 at 19:35

SMD chip inductors, compared to larger ones, will generally have:

• lower Q
• higher DC resistance
• higher SRF
• lower DC saturation current, if they're ferrite cored

But read the datasheet and see if they'll work for your application. Often the higher SRF, smaller size and lower cost are the most important factors.

For filter design trade-off, I'd suggest:
First design the filter with ideal components.
Then simulate it again with real inductor resistance and Q. (Choose inductors that have SRF several times your operating frequency). Adjust inductance to bring the filter back into tune.
Compare performance to the ideal filter.

This will give you an idea of the additional losses in the filter due to the small components.

For simulations, if you don't already have a tool, I recommend QUCS. It's old and slightly cranky, but it's free and can do almost anything. Others might know better free software.

• Thank you for Qucs! I will try to learn how to model BPF in it with q-factor. Does it support stuff like this? – zoonman Jun 28 '18 at 20:51
• I think you can put in a DC resistance, which can be used to lower the Q. $R_{series} = X(f) / Q_{desired}$ – tomnexus Jun 29 '18 at 10:31