Strange response of 4-pole gaussian BPF

[Circuit_seller]

Strange Filter Response!

Hi

I have designed a 4-pole gaussian BPF arround 455kHz with 10kHz 3dB BW. I have tested it response and found that the sharpness is good but the insertion loss is about 30dB.
I guessed this insertion loss was due self resonance of the 680nF capacitor, but i have tested it response and found the resonance frequency is equal to 3.5MHz.
What is my fault can anyone help me?

Best Regards

 

[flatulent]

Re: Strange Filter Response!

Make C11 and L11 a series resonant circuit. In their parallel form they block the signal flow at their resonant frequency.

 

[Circuit_seller]

Re: Strange Filter Response!

Hi

Sorry, I have mistake. C11 and L11 are in series form.
Is there any mistake?

Regards

 

[IanP]

Re: Strange Filter Response!

What is the value of C12? Is it 680nF?
Could you please re-draw the actual diagram of your filter with correct values !!! 8)

 

[Circuit_seller]

Re: Strange Filter Response!

Hi

Here is the corrected version of the schematics.
The value of the C12 is 680nF.

Regards

 

[IanP]

Re: Strange Filter Response!

You have imbalance in the parrallel cell: the C value is to big and the L value is to small.
Try to balance them in such a way that their impedances at 455kHz are comparable.

 

[Circuit_seller]

Re: Strange Filter Response!

Hi

Thanks for replies.
I can not balance value of the L and C, as you know the ratio of the L/C change the BW. So i can not change value of the L and C.

Regards

 

[echo47]

Strange Filter Response!

Those are very high-Q resonant circuits. You'll need super parts!
With one volt input and ideal components (unlikely), you get up to 30 volts at node 1, and one ampere through L12.

PSpice simulation

Vin   5  0  AC 1.0V
Rin   5  6  50
C11   6  1  180pF
L11   1  2  680uH
L12   2  0  180nF
C12   2  0  680nF
Rout  2  0  50

.AC DEC 5000 400kHz 500kHz
.PROBE
.END

 

[Circuit_seller]

Re: Strange Filter Response!

Hi

Thank you very much.
what can i do to design such a filter?

Best Regards

 

[echo47]

Strange Filter Response!

Maybe use a ceramic filter. I've had good luck with small muRata filters. They make many types, including 455kHz with 10kHz bandwidth. Catalogs here:
https://www.murata.com/catalog/index.html