433 Bandpass filter

[dmowziz]

Hi,

Trying to design a 433 MHz filter, BW = 28 MHz... Top -C
The filter is just for learning exercise


The first image (top schematic) is the derived filter from analysis. But simulation got it at 435 MHz, 27.3 MHz bandwidth
The series coupling capacitor are small, so I changed to the bottom schematic which is equivalent.
38.77 nH


The 2nd image, is the schematic on the board.
 I did not want to "add" capacitors to get a single value so used 1 capacitor (s parameters) then adjusted the inductor values.


The major concern, is what the prototype is saying.
Which is the 3rd image. Shows the response on board and response from the schematic from 2nd image.


Please how can I do it better? and for a given frequency for a filter, what inductor value (/impedance) to use?
If you have the first schematic to build, how will you do it?




Thanks in advance for any replies.

[mvs]

Simulation is an ideal case. In reality you have non-ideal components which do have some tolerances and parasitics.
Even your PCB is not an ideal conductor. If you route 1cm² copper area on both sides of 1.5mm FR4 you get a capacitor with around 2.5pF.

Since you already know, that you are off by around 10% - just scale your design accordingly.
Another approach would be to identify most relevant parasitic elements and add them to simulation.

If you have the first schematic to build, how will you do it?

Sometimes it is better to construct own components, rather then buy them.
You can build 0.16pF cap using PCB traces. You can build even whole bandpass filter out of FR4.

[fourfathom]

When you find that your component values are getting impractical, consider alternate topologies.  For example, instead of top-coupling, try bottom-coupling.  Here are a couple of examples, with values pulled out of thin air.  Note that the bottom-coupling element can be either an inductor or a capacitor, depending on how you want the filter skirts to behave.