Electronics > Projects, Designs, and Technical Stuff
Low Pass Filter Designing
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Nikos A.:
Hi everyone,

I have a 10Mhz OCXO and I want to design a low pass filter to apply at its output. The reason for that is that besides the foundamental frequency (10Mhz) I can measure harmonics as well. I've never designed a filter before... Looking some tutorials on the web I calculate the RC values for a low pass filter with cutoff frequency of 15Mhz.

I tested the circuit on the breadboard and the results was frustratingly... The filter actually doesn't work at all..

Is ther any reliable free software/tool for filter desinging? Should I use LC instead of RC filter?

Thanks in advance
mikerj:
The performance of a passive filter is very dependent on the source and load impedance, did you take these into account?  Additionally a single pole filter only rolls off at 6dB/octave, so a third harmonic at 30MHz would only be 6dB down even if your filter was working correctly.
srce:
What values for the R & C did you use?

Useful filter design tool here: http://sim.okawa-denshi.jp/en/CRlowkeisan.htm

Along with some other types: http://sim.okawa-denshi.jp/en/Fkeisan.htm
MarkF:
And here is a LC filter design program with typical output:

https://www-users.cs.york.ac.uk/~fisher/lcfilter/

JagV12:
This LC filter adds the rejection of any chosen frequency. You can cascade several to reject various harmonics. It is often used as output filter on RF transmitters. R1 & R2 are not actual components, they symbolize source and termination impedance. Of course, L & C components value has to be calculated to match your needs...

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