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?

If you can, get a copy of the book " Radio Frequency Transistors" by
Norm Dye & Helge Granberg. There is a chapter specifically about
designing and building Filters to handle the output of Motorola based
transistor RF amplifiers. To quote from the book:
"Paralleling of multiple capacitors provides a means to increase
current carrying capabilities and allows the use of inexpensive disc
types.
......Typically the RF voltage ratings of such capacitors are
approximately 30-35% of their D.C. voltage ratings."
High voltage ceramic disc caps are widely available ( 1,000 to 3,000
volts).
The book also has formulas for calculating various types of filters,
estimating RF currents and voltages, and choosing components, etc.
Highly recommended!

Jeff





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?

Is there a better filter
architecture that minimizes cap current?

======================================

There is no alternative to analysing the circuit and working it out.

Use your slide rules and pocket calculators. You ARE educated
engineers, or are you just typical, unreliable Google and newsgroup
addicts?

Thanks Jeff for the information. However I think it is dielectric loss
and not working voltage that may give me trouble here. Even with 3kV
disc ceramics the loss is quoted at 2% so these will get hot at a few
amps. The low loss AVX hi-q ceramics do appear to be better but are not
east to find. I am new to amateur radio so just learning these things.
I did order the book you suggested. Thanks,
Paul.
Dr. Jeff wrote:
If you can, get a copy of the book " Radio Frequency Transistors" by
Norm Dye & Helge Granberg. There is a chapter specifically about
designing and building Filters to handle the output of Motorola based
transistor RF amplifiers. To quote from the book:
"Paralleling of multiple capacitors provides a means to increase
current carrying capabilities and allows the use of inexpensive disc
types.
.....Typically the RF voltage ratings of such capacitors are
approximately 30-35% of their D.C. voltage ratings."
High voltage ceramic disc caps are widely available ( 1,000 to 3,000
volts).
The book also has formulas for calculating various types of filters,
estimating RF currents and voltages, and choosing components, etc.
Highly recommended!

Jeff





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?

American Technical Ceramics (ATC) makes capacitors specifically for high
power, low loss RF applications. You might see if you can get some of
their products.

My guess is that you'll find ordinary NPO ceramics to be good enough if
you use multilayer ones, rather than disks, of decent physical size. The
critical spec is the ESR if you can get it for candidate ceramics. I
don't, however, have personal experience at running them at this power
level.

Roy Lewallen, W7EL

BTW, in the "RF Transistors" book, on page 103 there is a photo of a
large RF Power Amplifier that has 6 filters built using large Ceramic
Capacitors & Toroid forms.
Quote: ".... a 2KW prototype RF amplifier for a period of ten years
with over 5000 hours of operation without failure."

wrote:
Thanks Jeff for the information. However I think it is dielectric loss
and not working voltage that may give me trouble here. Even with 3kV
disc ceramics the loss is quoted at 2% so these will get hot at a few
amps. The low loss AVX hi-q ceramics do appear to be better but are not
east to find. I am new to amateur radio so just learning these things.
I did order the book you suggested. Thanks,
Paul.
Dr. Jeff wrote:
If you can, get a copy of the book " Radio Frequency Transistors" by
Norm Dye & Helge Granberg. There is a chapter specifically about
designing and building Filters to handle the output of Motorola based
transistor RF amplifiers.