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??