RF PA low pass filter component ratings.

[Paul Taylor]

Hi All,

Anyone know how you go about calculating the component ratings (not
values) for use in RF PA low pass filters as I would like to design
some filters for low power (5 watts) HF use using surface mount
components.

I guess you could take worse case operating conditions ie open or
short circuit load providing a full reflection and then perform some
kind of nodal analysis but not sure.

Any help would be much appreciatted as relevant info seems very sparse
or havn't I looked hard enough.

Regards,

Paul.

 

[Rene Tschaggelar]

Hi All,

Anyone know how you go about calculating the component ratings (not
values) for use in RF PA low pass filters as I would like to design
some filters for low power (5 watts) HF use using surface mount
components.

I guess you could take worse case operating conditions ie open or
short circuit load providing a full reflection and then perform some
kind of nodal analysis but not sure.

Any help would be much appreciatted as relevant info seems very sparse
or havn't I looked hard enough.

The power rating is dependent on the components.
Usually when they become saturated or nonlinear
or simply too hot.
Capacitors have a max voltage and possibly a max
current rating. Likewise the inductors.

Another point may be the heat dissipation in those
components. That'd need extensive modelling plus a
few measurements.

Rene

 

[Paul Burridge]

The power rating is dependent on the components.
Usually when they become saturated or nonlinear
or simply too hot.
Capacitors have a max voltage and possibly a max
current rating. Likewise the inductors.

Another point may be the heat dissipation in those
components. That'd need extensive modelling plus a
few measurements.

Is it possible to get SM components suitable for HF?? I've built
several filters for that band and they all tend to be on the **large**
side.

 

[Rene Tschaggelar]

Is it possible to get SM components suitable for HF?? I've built
several filters for that band and they all tend to be on the **large**
side.

Whatever your HF is, yes, the mobile phones contain surface
mount components. You may have to verify the parameters for
your frequency band though.
Eg a 100nF 1206 is good to 10MHz, above which it becomes
inductive.

Rene

 

[Winfield Hill]

Rene Tschaggelar wrote...

Whatever your HF is, yes, the mobile phones contain surface
mount components. You may have to verify the parameters for
your frequency band though. Eg a 100nF 1206 is good to 10MHz,
above which it becomes inductive.

10MHz sounds pretty poor the way you say it, but with your
0.1uF, another way of expressing it is to say the series
inductance is 2.5nH, which doesn't sound so bad. But it's
likely less than that, depending on the measurement path.

 

[Paul Burridge]

Whatever your HF is, yes, the mobile phones contain surface
mount components. You may have to verify the parameters for
your frequency band though.
Eg a 100nF 1206 is good to 10MHz, above which it becomes
inductive.

And the corresponding L? Is the Q of those little buggers really up to
the job? You can't defy the laws of physics and I just can't see how
an inductor that physically small used at 10Mhz can be much use. :-/

 

[Rene Tschaggelar]

And the corresponding L? Is the Q of those little buggers really up to
the job? You can't defy the laws of physics and I just can't see how
an inductor that physically small used at 10Mhz can be much use. :-/

As Win said, the inductivity is in the oder of 2.5nH, or 1nH per mm
of its length. All parts exhibit that inductive per length property.
Well, 1206 is rather big for nowadays standards.

Rene

 

[Joerg]

Hi Paul,

Is it possible to get SM components suitable for HF?? I've built
several filters for that band and they all tend to be on the **large**
side.

For a few watts, maybe. But not for high power RF. There you need the
"door knob" caps or you'd hear a kaboom after extended exposure to RF.
There just is no big market for RF caps in the HF range.

What some folks do is parallel a whole lot of caps. That stretches your
mileage with SMT. But as to inductors it isn't likely going to buy much
because the little Panasonics and Sumida caps are either very small or
contain cores geared towards SMPS usage, generally below 1MHz. I have
always ended up rolling my own, on pot cores or preferably toroids.

Just as an example: Once I chose a cap on the skimpy side for a test.
Within less than 15 seconds that telltale hi-amp stench filled the room
and what used to be white ceramic was now green glass with bubbles in
there. Same with an inductor that already was a couple inches long.
Poof, and nothing of the winding was left. It taught me to wear safety
goggles during those experiments.

Regards, Joerg