EMRFD Message Archive 14595

Message Date From Subject
14595 2018-03-24 13:32:35 Ravi Miranda Low Pass Filters

While designing an LPF, there are several combinations of L and C that
can achieve the characteristics necessary for a good design.

For the 6M band I have the following values; for a 7 order LPF I'm
getting the following.

R1 50ohm
C1 25pf
L1 180nH
C2 104pf
L2 289nH
C3 104pf
L3 180nH
C4 25pf
R2 50ohm

I used https://www-users.cs.york.ac.uk/~fisher/cgi-bin/lcfilter

Would this be the optimum? or would there be a different combination
that might yield a better? result.

In the above case, I noticed the voltage magnitude dropping off at
34MHz even though I specified a cut off of 54Mhz.

Is there something that I'm missing here? Incorrect interpretation perhaps?

Many thanks for reading.

Best 73


I'm here to add more value to the world than I'm using up.
14596 2018-03-24 14:37:50 Clint Re: Low Pass Filters

Hi Ravi,


Have you tried the free Elsie app.  It will allow you to design low pass filters and then to optimize them for best operation.  The performance of the built filters is very close to the Elsie simulation.







14597 2018-03-24 14:49:18 Clint Re: Low Pass Filters

Hi Ravi,


I let the Elsie app do a design for a low pass filter.  The parts values came up to be very close to your values.  I also noticed that the Elsie design ended up with a cutoff frequency of about 34 Mhz.


I then let Elsie do an optimization to have minimal loss at 54 Mhz and to have a -40 db loss at 90 Mhz.  I then normalized the resulting capacitors to standard values and let Elsie do an optimization on just the inductors.


Here is what Elsie came up with (the spectrum curve looks very good now).


R1 50ohm
C1 25pf                    56 pf
L1 180nH                 224 nh
C2 104pf                  100 pf
L2 289nH                 260 nh
C3 104pf                  100 pf
L3 180nH                 224 nh
C4 25pf                    56 pf
R2 50ohm





14598 2018-03-24 15:29:48 Steve Dick Re: Low Pass Filters
Hi Ravi. You might also want to take a look at Tonne software’s optlowpass software for Windows.  It designs excellent filters with your choice of optimum components (best performance and return loss) or nearest 5% components (still works well but performance slightly worse).  Download at:
-Steve K1RF
14599 2018-03-24 17:21:44 Graham / KE9H Re: Low Pass Filters
On six meters, the board parasitic capacitance, and the parasitic inductance of the component leads will change the shape of the filter.

Plan to build one of the filters, measure the performance, then adjust the values to make the filter do what you want it to do.

Maybe twice.

--- Graham / KE9H


14600 2018-03-24 17:23:28 Clark Martin Re: Low Pass Filters
Rule of thumb is the filter will be 3 dB down at the cutoff frequency for every pole of the filter.  This is a 7 pole filter so it will be 21 dB down at 54 MHz.I don’t recall what effect it will have at 63% of the rolloff frequency but there will be some effect.  Remember that a low pass filter attenuates all the way down to but not including 0 Hz.  Conversely it passes some signal up to all but an infinite frequency.  In each direction the attenuation / passing at some point becomes negligible but until you do the math you won’t know.

You want to set the cutoff frequency of an LPF ABOVE the range of frequencies you wish to pass, likely well above.  In the case of a harmonic filter the cutoff frequency needs to be somewhere midway between the fundamental frequency an twice that frequency.

Note that for these purposes, midway should be the geometric mean, i.e. square_root(2) * Fundamental_Frequency.

Clark Martin

14601 2018-03-24 18:12:58 Ravi Miranda Re: Low Pass Filters
Thanks, Clint. 

I did visit the ELSIE site yesterday to download the software, but I'm getting a 'Harmful Website blocked' alert from my AV.

Not sure if this is normal? I'm trying AADE Filter design software also.

On a slightly different topic, the 6M band is 50-52Mhz, what is the cut-off frequency  should I aim for?

Best 73,


14602 2018-03-25 22:47:48 kerrypwr Re: Low Pass Filters
I can't imagine that there is a problem with Jim Tonne's site; the "harmful" message is probably because the site has software that was not written by Microsoft.  :)

I've used Elsie for a long time; she's a fun girl.  :)

Kerry VK2TIL.
14603 2018-03-26 02:30:58 Ravi Miranda Re: Low Pass Filters
Thanks All for your helpful ideas and suggestions, I will work on these. 

Have a great week ahead.