Ripple specs of a filter - Page 1
Forum section: electronics / rf-microwave

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: September 21, 2019, 02:43:25 pm
content: Does anyone know how do we arrive at the ripple specs for a low pass filter. The bandwidth spec is easy as it is based on the signal of interest. However the ripple spec seems to elude me. I understand that too large a ripple will distort the signal and too low a spec will cause a design constraint. So how do we arrive at that magic number?

author: coppercone2
author_rank: Super Contributor
timestamp: September 21, 2019, 02:51:32 pm
content: if you experiment with a filter program like nuhertz it will have a option for passband ripple so you can see how the parameters of the components change as you mess with that to synthesize filters if you want a feel for passband ripple.magic number comes from a specification, or what you need for the filter to work correctly with the signal of interest. I assume you mean for information transfer, then that depends on your modulation schemes and system design as usual.

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: September 21, 2019, 03:04:13 pm
content: Thanks,I would like to think that the modulation would play a major role in the filter ripple spec. For e.g. an OFDM signal would place a tighter spec on the filter than say BPSK. If the ripple is too high, then the SNR will be degraded at a certain carrier of the OFDM signal. But for the BPSK it may not create a major problem. It may cause some waveform distortion, but not large to cause high BER. What I was looking for is a closed loop formula linking the ripple to the BER for a given modulation.

author: coppercone2
author_rank: Super Contributor
timestamp: September 21, 2019, 04:45:17 pm
content: That sounds complicated, I think you need to do signal loss in matlab or something for specific cases. not sure what a general solution would be useful for other then a RF textbook/cookbookwhich would be useful but I think if you are trying to solve a specific problem this way, you are going to be in for too much work

author: OwO
author_rank: Super Contributor
timestamp: September 21, 2019, 05:39:23 pm
content: Quote from: EEEnthusiast on September 21, 2019, 03:04:13 pmYou have it the wrong way around. A single carrier signal (PSK/QAM) is more sensitive to filter ripples because it means the filter "smears" the signal in the time domain and creates inter-symbol interference. OFDM is immune to this because each subcarrier sees a local "smooth" frequency response and can correct for it. Depending on the spacing and magnitude of the ripples and channel conditions (long distance over terrain vs indoor) the ripples may be completely insignificant compared to the frequency response of the channel. I see deep frequency-selective fading on FM radio here where you can have a notch 20dB down and 10kHz wide in the middle of a single FM signal.

author: OwO
author_rank: Super Contributor
timestamp: September 21, 2019, 05:49:31 pm
content: It's very easy to predict ISI by just simulating the filter in the time domain. You can use e.g. LTspice and send a single pulse through the filter, then look at the filter output and see how high the ringing is at t = 1 symbol period + delay. The effects of ISI can be modeled by adding a noise source. If this is for a real life communication system (and not academics), just keep the ripple below 1dB and make sure the OFDM subcarrier width is much smaller than the ripple width.