EMRFD Message Archive 2665

Message Date From Subject
2665 2009-01-31 18:43:01 la3pna Filter design.
I have been building an 42MHz crystal oscilator on an PCB, with an
band-pass filter on the output.
When buliding the filter on PCB I get an nice low frequency slope, but
the high frequency slope and center frequency are way off.
The Inductors are TOKO screened coils with ferrite core and the
screens are grouned.

The filter simulation (using AADE filter designer) is as I would
expect the filter to be.

The input and output of the filter is not thar far apart, could this
be part of the problem?

The shematic, board layout and 2 sweeps with spectrum analyzer can be
found at http://www.la3pna.com/filter/

Any suggestions please?

73 de Thomas LA3PNA.
2666 2009-02-01 07:00:39 Paul VDP Re: Filter design.
The filter is clearly a bandpass configuration.
But this is a huge pcb-design error... The input and output of the filter are barely seperated by a hairwidth (so to speak..)!
Remember, the higher the frequency, the higher the blowby... That's why the high-frequency side is totally spoiled...
Especially on a pcb without extra screening walls :
1) the filter should be laid out in a straight line
i.e. :  input-C -> 1st Toko-can -> coupling-C -> 2nd Toko-can -> output-C 
2) Input and output should be on opposite sides from the Toko-cans (no 'visual contact' between input & output)
3) don't bring in the output from the 2nd MMIC as you did... in the middle of the bandpass-filter, just 1 millimeter away from the output...!!! RF blowby will make the filter totally useless.... Output from the second MMIC is the highest-power point on the pcb. It should be as far away as possible from the output of your filter.
I think this needs a totally different pcb design to get things working.
A pcb is to be considered as a 'custom made rf-part'.... It's an art in it's own right to design a good rf-pcb....
73, Paul - on4bxz


2668 2009-02-03 12:21:46 ha5rxz Re: Filter design.
I would also pay close attention to grounding. Ground loops in the
oscillator and amplifiers will couple with ground on the filter unless
you are very careful, reducing filter performance. Try building your