Re: LF: RE: Filter expert needed - Mystery of the lost real zero.

["Andy Talbot" <andy.g4jnt@googlemail.com>]

hi Mike -

No I don't want a zero there.  For all practical purposes I'm more than happy with what the filter actually does at the moment.

 

My puzzle is that there should be one and there isn't.  

 

An inductor coupling non-adjacent resonators appears to put a zero into the pass band;  at least it puts a huge dip there - the last thing I want.

 

Andy  G4JNT

 

On 27/04/07, Mike Underhill <mike@underhill.co.uk> wrote:

Andy

 

A correction: I should have said 'in series with' and not 'across'.  So that my post should read:

 

"For a second zero on the HF side try an inductor 'in series with' the 'zero' capacitor.  The trouble is that it will have to have the same reactance as this 1.5p capacitor and that may not be easily realisable in practice.  The inductor could also go between resonators 1 and 3 or 3 and 5 I think.  If the inductor value is too high you can use a small 'link' coil on each of the resonators to be inductively coupled and connect these by a pair of twisted wires.  Getting the coupling right can be fiddly. You can get another LF zero rather than the wanted HF zero, I think, if you get the phasing wrong."  . 

 

I hope this helps but as I said it is some time since I taught this stuff!  Good luck.