# Q factor in inductors

I am looking at the Q factor of certain inductors from the datasheet - inductor datasheet

Now, I am trying to analyse the Q-factor for the 2.2uH inductor. The chart shays that the Q factor is 48 measured at a frequency of 7.9MHz. But, if I look at the graph it says a different picture. At 7.9MHz its is almost 70.

How is this possible ?

Also, my need is for the frequency range 300MHz and 2.4GHz (different inductors ofcourse). So based on the graph the Q factor is not available for those frequencies. Do Q factors decay away to very low values after the peak or will it rise again at other frequencies ?

• Well regarding the Q factor, the data sheet does say Q Min, in other words the minimum Q factor is 48. Basically they are guaranteeing that the minimum value that the Q factor can be, but they are not stating the typical or maximum value you can expected to be. Sep 17, 2015 at 14:11
• No, thats not right. As the frequency increases, there will be an increase in the parasitic resistance (due to skin effect), inter winding capacitance,...etc.These will raise the resistance and decrease the Q factor. Also, the Qmin is at a specific frequence(in may cases i9ts 150MHz).That cannot be valid for 300MHz.So, it cannot be a valid argument. Sep 17, 2015 at 14:14
• They ARE stating the typical Q, it is in the graph. The Qmin is measured at 7.9 MHz which is a frequency chosen by the manufacturer for this specific model of inductor. That also means that the inductor is intended to be used in that frequency range. From the graph I would say the 2.2 uH inductor is usable up to 20 MHz, 40 MHz maybe if Q is not critical to the application. But not more than that. Sep 17, 2015 at 14:32

The graph shows the nominal or typical Q value that you can expect for a certain inductor at a certain frequency. Unfortunately not all these inductors will be identical, due to small variations in the materials and inaccuracies the Q will vary. Sometimes Q will be higher, sometimes it will be lower.

The table shows the minimum value of Q meaning that the manufacturer guarantees that the value of Q should never be lower than stated value (48 at 7.9 MHz).

Indeed for an inductor working at 300 MHz you will need a different type. No, the Q factor will not "recover" for higher frequencies. If there is a graph of the inductor's impedance over frequency you will see that the impedance will decrease over frequency at frequencies above the "peak Q" frequency so the inductor will behave like a capacitor ! That means that actually it does not matter what the Q is for higher frequencies as the Q is only relevant when the inductor behaves as an inductor.

The bottom line on this supplier's offerings is that the graph has anomalies that cannot be explained. The table shows an inductance range from 10nH to 10uH yet none of the graphs cover the range below 1uH.

Next, how can a 100uH inductor have a Q factor graph that peaks at a higher value than 1uH. This is rubbish from the part of the supplier. Also, what is happening with the 270uH - it seems to be much better in terms of frequency response than the 2.2uH.

Basically ignore the graph and possibly find a different supplier. As soon as anomalies start occurring in a data sheet as bad as this I find I cannot trust the supplier.

Basically at 7.9MHz, inductive reactance is 109 ohms and if you use the DCR you get a Q factor of 84. But it will be lower than that due to SRF, core losses, skin effect and proximity effects so the figure of 48 as a minimum value for Q sounds reasonable but the graph is the big spoiler for me.

• I was intrigued by your remarks about the supplier and wondered who the culprit would be (so I can avoid them). So I found a datasheet here: datasheets360.com/part/detail/mpci10002200/3666535835183535133 and the supplier is Dearborn although the datasheet is from Microspire !? There the graph makes its appearance also. But there the top curve is for a 220nH coil, not 220uH is in the graph above !!! So that's a typo which does not inspire confidence indeed. Better get your coils from Coilcraft or Murata ! Sep 17, 2015 at 14:47
• @FakeMoustache I use coilcraft, coiltronics, Wurth generally!! (or roll my own) Sep 17, 2015 at 14:50
• I also note that one of the curves say 100uH to 180 nH LOL Sep 17, 2015 at 14:52