Simple bandpass filter, problems

[matise]

This is so weird.
I build this simple bpfilter.
I used t50-6 amidon.
I want it for 18.1 Mhz.
So, in spice its perfect with either 3300pf and 24nH... Or 2200pf and 35nH.
(2 or 3 turns, inductor).
I tried both.
As you se it works over 18 Mhz in spice but then i build it for real the bandpass filter is 10Mhz.
What happens?
Whats going wrong here?

 

[betwixt]

You have an unusually high C to L ratio and I suspect other factors, particularly stray inductance is lowering the resonant frequency. I suggest values around 47pF & 1.645uH would be more sensible.

Brian.

 

[matise]

Mmm, you were wright.
I tried different values up to 47p and it come closer to the correct bp-freq.
Great. I learned something!

 

[betwixt]

Hey - I'm always right! (well... some of the time anyway)

General rule is pick mid-range values, if the capacitor value is too small, the capacitance of nearby objects becomes significant, if the inductance is too small the effects of connecting wires becme significant. Keeping them both in proportion doesn't stop those effects but it helps to minimize them.

Brian.

 

[D.A.(Tony)Stewart]

Looks like you didn't make it right.

THe wire and capacitor lead length is significant. Try full leads shorted where it enters core body. Then find fP. The cap leads have inductance. Calculate what it is.

Then design it so lead inductance is negligible and interwinding capacitance is negligible.
This region of values in the middle will likely be in the 100 to 1k Ohm range. not 3 Ohms.

- - - Updated - - -

Also the data sheet indicates///
6 Material
T50-6
Frequency Range 10 Mhz - 50 Mhz
Color: Yellow (3 Sides)
U = 8
Inner Diameter: 0.303 inches
Outer Diameter: 0.500 inches
Height: 0.190 inches
AL Value: 40 µH/100 turns

so is 3 turns 1200 nH?
and 2 turns 800 nH?