Hello, can you please help me design a LPF for 25MHz (50R)
I need it to be quite sharp, in the sense that I need to attenuate the >= 28MHz quite a lot.
Thanks
View attachment 122035
you can try this toplogy first.
Because 25 MHz is too close to 28 MHz,
It is difficult to achieve low insertion loss for 25 MHz and high attenuation for 28 MHz simultaneously.
Thus, maybe you should use BPF rather than LPF.
You know that a passband ripple and stopband attenuation specification is necessary to finish the design.
No, not a bandpass filter. It has to cut-out all frequencies above 25MHz not below. It will be used as a filter in a 14MHz-25MHz 1W transmitter, so the second harmonic of 14MHz which is 28MHz (and frequencies above that of course), has to be cut quite well.
I think I have space onto the PCB for 2-3 T50-2 cores.
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See the post above, I do not have specific requirements, as long as the passband ripple is kept low (<0.5db or less?) and the stopband attenuation as high as possible. I consider the 28MHz first for the reasons explained above and because it is the first HAM band (harmonic of 14MHz) that needs to be cut first. The higher harmonics should be easier attenuated anyway because they are more within the filter's cut.
I misunderstood that your center frequency is only 25 MHz.
If so, you can try this BPF firstly.
View attachment 122038
Insertion loss of 14 MHz : 0.237 dB
Insertion loss of 25 MHz : 0.475 dB
Insertion loss of 28 MHz : 24.4 dB
Thus, this topology makes you posses acceptable insertion loss for passband and acceptable rejection for 28 MHz.
As mentioned above, 25 MHz is too close to 28 MHz.
Compared to BPF, it’s difficult for LPF to achieve low insertion loss @ 25 MHz and high rejection @ 28 MHz simultaneously.
Compared to BPF, it’s difficult for LPF to achieve low insertion loss @ 25 MHz and high rejection @ 28 MHz simultaneously.
Suggestion for Neazoi: Download from http://www.tonnesoftware.com/elsiedownload.html
The free "student edition" should be fine for what you want. Works on Windows, I haven't tried it on Linux yet.
Brian.
Thank you very very much for your reply!
Maybe I was not very clear in my description, the filter has to have a passband from 1-25MHz, whereas the one you have shown starts from 14MHz.
I apologize it was my mistake.
I thought in terms of harmonics not in terms of passband.
Can you alter this design for me so I can try it?
If it helps you, even a more relaxed attenuation curve (with less inductors used) would be ok as the transmitter harmonics are not very high anyway.
Thanks a lot!
If your passband is from 1MHz to 25 MHz.
And you hope for less inductors used at the expense of harmonics rejection.
Perhaps you can try the simple 5 order LPF firstly.
View attachment 122044
Insertion loss @ 1MHz : 0.0014 dB
Insertion loss @ 25 MHz : 0.0474 dB
Clearly written on the diagram: s11 (reflected signal), see https://en.wikipedia.org/wiki/S_parametersI have no idea what the blue line is.
It's apparently a butterworth filter or similar (no ripple in the passband) with standard cut-off frequency attenuation of 3 dB. Your specification seems to demand a lower cut-off frequency attenuation, if you use a Chebyshev filter prototype, you get cut-off attenuation = ripple maximum by default, e.g. 0.5 dB attenuation for your initial specification. Stopband attenuation does then depend only on the filter order, e.g. about 22.5 dB at 28 MHz for the 9th order filter.Eventhough complex, it does not meet the specs as it has too much loss even in the passband.
Clearly written on the diagram: s11 (reflected signal), see https://en.wikipedia.org/wiki/S_parameters
It's apparently a butterworth filter or similar (no ripple in the passband) with standard cut-off frequency attenuation of 3 dB. Your specification seems to demand a lower cut-off frequency attenuation, if you use a Chebyshev filter prototype, you get cut-off attenuation = ripple maximum by default, e.g. 0.5 dB attenuation for your initial specification. Stopband attenuation does then depend only on the filter order, e.g. about 22.5 dB at 28 MHz for the 9th order filter.
View attachment 122049
The latest filter is an elliptical (cauer) type which gives higher stopband attenuation by implementing real zeros.
Still not sure what's your actual stopband specification. But apparently you know now how to use the tools to calculate a filter. Very good.
I only do not know how this blue line (reflected signal) affects the filter and how it should better be. Should it better be at low values or at high ones? Or it does not matter for the filter response?
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