74HCU04 LC Osc

I built this 5 MHz oscillator using a Toko KANK4174 inductor, which is
specified as having a Q of 100:



74HCU04
|\
.------| >O-----o----
| |/ |
| .-.
| | |
| | | 3k3
| 4.8uH '-'
| ___ |
o------UUU------o
| |
--- ---
--- 390p --- 390p
| |
| |
=== ===
GND GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

Peak-to-peak voltage at the output is about 3V. Peak-to-peak voltage across
either capacitor is about 0.5V

The 3k3 forms a potential divider with the impedance across the capacitor:

0.5 / 3 = Z / (3k3 + Z)
Z = 660 ohms

Dynamic impedance across the capacitively-tapped tuned circuit = Rp = 4*Z =
2640 ohms

Q = Rp/wL = 2640 / (2*pi*5e6*4.8e-6) = 17.5

Why is my calculated Q so low?

TIA
Andrew


.

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