14595	2018-03-24 13:32:35	Ravi Miranda	Low Pass Filters
Message	Date	From	Subject
Hi, 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 Ravi/M0RVI -- 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. 73 Clint W7KEC
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 73 Clint W7KEC
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: http://www.tonnesoftware.com/optlowpassdownload.html -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 KK6ISP
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, Ravi/M0RVI
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. Ravi/M0RVI ok