Question about Q

Been away for a while in digital and software land. But soldering iron is on again and RF/electronics is back on.

So here is a silly little question:

I created a little test jig for when I have an occasional access to a spectrum analyzer. BNC in , BNC out and the device under test is connected to ground and the 2 BNCs. When I connected this setup to a SA with a tracking gen, I got the expected result:

a 82 pF cap, exhibits self-resonance around 142 MHz. This yields an inductance of about 15nH (assuming the cap is really 82nF at such frquency).

Here is my problem: the resonance peak looks pretty sharp. From my reading about loaded Q:

fc Rp QL = ------ = ---- BW XL

at resonance: XL = XC = around 13 Ohms and for Rp I'm assuming it's dominated by the input/output impedance of the SA so that's 25 Ohms (50 in parallel with 50).

This gives a loaded Q of about 2 so the bandwidth should be about 70 MHz.

On the spectrum analyzer display, the resonance peak looks much charper than this and the 3dB point is definitely not at 35 MHz away from fc (approximately).

Where did I go wrong??


Reply to
M. Hamed
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Chip or disc type?

I wonder if it might be resonating with poorly formed traces (or through-pins?? but you didn't make this for THT, did you?).

If it's disc, it might be the common mode resonance of disc against free space, lightly coupled by just one leg.

Chip should be more like 3nH (depending on size, of course).


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Reply to
Tim Williams

This is ideal to look at using free LTspice. Then just go through a checklist of parasitics, etc. You will have difficulty including the eddy current and skin depth effects, but analyzing in 'narrowband' sections, you can gain a lot of understanding. If you're skilled at LTspice already and stick to .ac; you can use the Laplace equation to represent frequency dependent skin effect losses pretty well.

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Thanks. I used LTspice and figured out what I was doing wrong. I was modeli ng the inductance as parallel to the capacitor it should be in series.

Similarly I put a 2 uH inductance in series with the same capacitor and saw a similar dip around 15 MHz. I don't know how to calculate the Q though wh en inductance is in series so I improvised. I tried to calculate the Q base d on the power at resonance as shown by the Spectrum Analyzer relative to t he power at DC but I got a value of 10000. For an air inductor at 10 MHz th at seems excessive, so I must have done something wrong.

Reply to
M. Hamed

LTspice is a 'dumb' simulator, so you must make cetain ALL the parasitics are properly represented at each frequency. It's especially not easy to include skin effect losses that vary with sqrt(frequency) and 'launching energy' where radiation takes power out of your circuit.

I highly recommend getting a free copy of femm 4.2 to calculate skin effect losses, then put those type values back into your LTspice model.

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That's probably too advanced for me. I just need a general idea of how things work. But this is good to know if I ever wanted to go that route.

PS: I corrected mistakes in my Q calculation and the new value I came up with is 150. That's a lot more reasonable even though I can not tell for sure if it's correct.

Reply to
M. Hamed

yes, much more realistic.

Actually, don't get too complex with femm 4.2 [only model a single wire, or some such simple structure] and you will learn a lot about the charactristics of skin effect and the true meaning of eddy current losses. AND you will get a 'general' idea of what's going on.

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