#### QCX: Calculation of low pass filter for PA?

Axel

Hi,

for my QCX, I would like to build the NB6M Miniboots PA from
http://www.norcalqrp.org/nb6mminiboots.htm (~10 Watts) for 20 m. Now, I'm
very curious, how I could calculate the values of the there given lowpass
filter (20 m) on my own. It's the upper LP-filter in the top-most
schematic (capacitors: 500, 680, 360 pF).

Could anybody point me to a appropriate resource or calculation/calculator
or program?

Output load resistance of the Mosfet should be about 7...10 Ohms (from
Vcc^2 / 2P ).
Simulation of the given 20-m-lowpass with RFSIM99 says, that the IRF510
"sees" 19 Ohms in parallel with 570 pF (this equals 10 Ohms in series with
1.2 nF or 10 Ohms - j9.6 Ohms); I didn't expect that.

Kind Regards,
Axel, DF1ET

geoff M0ORE

You need to use Elsie, free to students.

Geoff

On 25/07/2019 18:44, Axel wrote:

Hi,

for my QCX, I would like to build the NB6M Miniboots PA from
http://www.norcalqrp.org/nb6mminiboots.htm (~10 Watts) for 20 m. Now, I'm
very curious, how I could calculate the values of the there given lowpass
filter (20 m) on my own. It's the upper LP-filter in the top-most
schematic (capacitors: 500, 680, 360 pF).

Could anybody point me to a appropriate resource or calculation/calculator
or program?

Output load resistance of the Mosfet should be about 7...10 Ohms (from
Vcc^2 / 2P ).
Simulation of the given 20-m-lowpass with RFSIM99 says, that the IRF510
"sees" 19 Ohms in parallel with 570 pF (this equals 10 Ohms in series with
1.2 nF or 10 Ohms - j9.6 Ohms); I didn't expect that.

Kind Regards,
Axel, DF1ET

Kārlis Goba

This might provide some answers: https://www.rcarc.org/presentations/AB2EW%20Class%20C%20club.pdf

Note the cap at the drain to suppress parasitic oscillations.

--
Karlis YL3JG

Axel

Thank you both for your replies!

As to Elsie, I tried it. The everybody's free version (student, thats looong ago) seems not support impedance transformation (and about 100 Euros for the pro version, that to much for a single problem, for me). But besides from this, it look very good.

Karlis, that document is very interesting. For example, it tells me, why to put and additional cap from drain to source and which value to choose. But for impedance matching, it uses an additional inductance (page 14, "output match").

Axel, DF1ET

Kārlis Goba

You can use various networks for impedance matching, see this: https://home.sandiego.edu/~ekim/e194rfs01/jwmatcher/matcher2.html (the best calculator on this topic, but sadly the author is no more with us).

Depending on their topology, these networks can often be combined with the following lowpass section, e.g. parallel capacitors or series inductors merged into one.

However, it is still not entirely clear to me how the authors of your original link arrived at their component values. I bet they did it with good old clever analog design methods :)

--
Karlis YL3JG

Elsie sucks.

Try this - rfsim99.

Works perfect if using Linux / Win or anything that will run 16 bit Windows stuff.

73 de Andy

Kārlis Goba

I'm sorry, I mistook the author of the linked calculator for another author. John Wetherell is apparently all well and with us, it's just that his personal page no longer works.

--
Karlis YL3JG

On Thu, Jul 25, 2019 at 08:27 PM, Andy Brilleaux wrote:
Try this - rfsim99.

Works perfect if using Linux / Win or anything that will run 16 bit Windows stuff.
Ooops, works perfect if u sing  version of Windows that supports 16 bit Windows apps, and works if using WINE on Linux.

Not many programs stand the test of time, but RFSIM99 has.

73 de Andy

Axel

Karlis, I did many trials with https://home.sandiego.edu/~ekim/e194rfs01/jwmatcher/matcher2.html , but I cannot get to the values of the NB6M miniboots PA. I think, you tried it, too.

RFSIM99: I have placed the parts into RFSIM99 and did successfully run the simulation. Now, how can I get the right parts' values? Just by manually setting different values and see what the simulation gives? Or, is there a design tool in RFSIM99 with which this can be done?

Axel, DF1ET

Alan G4ZFQ

is there a design tool in RFSIM99 with which this can be done?
Axel,

A search found "There are even built in calculators in the Tools menu which can automatically design and simulate filters for you."

Now to look at the program...

73 Alan G4ZFQ

On Fri, Jul 26, 2019 at 06:03 AM, Axel wrote:
RFSIM99: I have placed the parts into RFSIM99 and did successfully run the simulation. Now, how can I get the right parts' values? Just by manually setting different values and see what the simulation gives? Or, is there a design tool in RFSIM99 with which this can be done?
RFSIM99 does it all for you.

TOOLS > DESIGN > FILTER.

Tell it what filter you want, how many elements, freq and press the button.

It instantly give you an optimized circuit and s-parameter simulation.

Even a baby can use it ;-)

73 de Andy

Axel

Andy, I found it. That's really nice and easy! Thank you!

I did some simulations with RFSim99 now and found, that the original lowpass (see my drawing #1) could be devided in three networks (drawing #2):

The right part (360pF - 500nH - 360pF) transforms 50Ohms into 50Ohms (hence, no transformation, filtering only)
The center part transforms 50Ohms to a lower impedance for matching the Mosfets output impedance, it transforms into 18.5 Ohms in parallel with 200pF. I wonder about these values.

The left part is an additional capacitor of 500pF. This could be the additional capacitor shown in the document which  Kārlis mentioned, https://www.rcarc.org/presentations/AB2EW%20Class%20C%20club.pdf, page 4 "parasitic supression". Again, I wonder about the high value (500 pF).

73
Axel, DF1ET

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