I am trying to build a set of filters for the ham bands for use with
the 600W motorola designed fet amplifier described in last months qsl.I
have sucessfully modelled suitable filters using pspice using 3 series
inductors with 2 parallel caps for each band but I notice the current
in the caps can be several amps peak. Values range from 150 to 3000pf.
Can Ceramics be used? Or do I need several parallel silver mica caps?
How much power can one dissipate in these caps? Near the corner
frequency there is as much as 6db gain under low load conditions and
this exacerbates the problem. I could move the corner further up so
that the filter is not excited at the peak current, but then I will
probably have to increase the number of poles to 7 from 5 to get enough
attenuation at the third harmonic. Is there a better filter
architecture that minimizes cap current?

Have a look at http://www.avxcorp.com/docs/catalogs/hi-q.pdf -- there
are likely other manufacturers who have lines of high current RF caps,
too. The caps in the referenced PDF are rated for max current about
15A at 150pF, 13MHz; 11A at 150pF, 64MHz; over 20A at 3000pF at 13MHz.
High enough for you? Parallel a couple if not! These are SMT-style
ceramics--a 3300pF 1000WVDC will be 9.4mm long, 9.9mm wide, 3.3mm max
thick. A 150pF 3000WVDC will be 5.8mmx6.4mm, max 3.3mm thick. (Those
are the only two len * width in that series.)

Cheers,
Tom

wrote:
I am trying to build a set of filters for the ham bands for use with
the 600W motorola designed fet amplifier described in last months qsl.I
have sucessfully modelled suitable filters using pspice using 3 series
inductors with 2 parallel caps for each band but I notice the current
in the caps can be several amps peak. Values range from 150 to 3000pf.
Can Ceramics be used? Or do I need several parallel silver mica caps?
How much power can one dissipate in these caps? Near the corner
frequency there is as much as 6db gain under low load conditions and
this exacerbates the problem. I could move the corner further up so
that the filter is not excited at the peak current, but then I will
probably have to increase the number of poles to 7 from 5 to get enough
attenuation at the third harmonic. Is there a better filter
architecture that minimizes cap current?

Thanks, Tom for the input. I looked up the AVX caps and they would do
the job, but I find them difficult to get. They seem not to be in stock
anywhere at retail, and are quoted at 11 weeks leadtime. Somewhere
there must be a supplier who caters to small quantity orders I would
think. I just haven't found them!
Paul
K7ITM wrote:
Have a look at http://www.avxcorp.com/docs/catalogs/hi-q.pdf -- there
are likely other manufacturers who have lines of high current RF caps,
too. The caps in the referenced PDF are rated for max current about
15A at 150pF, 13MHz; 11A at 150pF, 64MHz; over 20A at 3000pF at 13MHz.
High enough for you? Parallel a couple if not! These are SMT-style
ceramics--a 3300pF 1000WVDC will be 9.4mm long, 9.9mm wide, 3.3mm max
thick. A 150pF 3000WVDC will be 5.8mmx6.4mm, max 3.3mm thick. (Those
are the only two len * width in that series.)

Cheers,
Tom

wrote:
I am trying to build a set of filters for the ham bands for use with
the 600W motorola designed fet amplifier described in last months qsl.I
have sucessfully modelled suitable filters using pspice using 3 series
inductors with 2 parallel caps for each band but I notice the current
in the caps can be several amps peak. Values range from 150 to 3000pf.
Can Ceramics be used? Or do I need several parallel silver mica caps?
How much power can one dissipate in these caps? Near the corner
frequency there is as much as 6db gain under low load conditions and
this exacerbates the problem. I could move the corner further up so
that the filter is not excited at the peak current, but then I will
probably have to increase the number of poles to 7 from 5 to get enough
attenuation at the third harmonic. Is there a better filter
architecture that minimizes cap current?