Electronics > Projects, Designs, and Technical Stuff
low pass filter for SPWM speech synthesiser
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Buriedcode:

--- Quote from: hussamaldean on October 31, 2018, 10:14:42 am ---I used passive RC, LC and active RC LPF with cut-off frequency of 20KHz directly on the output pin of the uC. when the filter is used, the output of the filter is zero regardless the input voltage i.e. the output is attenuated beyond usable range

--- End quote ---

Now we're getting somewhere.  Post a schematic of that filter you use - the "active RC LPF with cut-off frequency of 20KHz".   And the parts you used would be helpful (like the opamp part number).

If in doubt, the web has quite a few applets/design tools that can help: http://sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm (scroll down for getting part values for a given cut-off frequency).
hussamaldean:

--- Quote from: Buriedcode on October 31, 2018, 11:07:38 am ---
--- Quote from: hussamaldean on October 31, 2018, 10:14:42 am ---I used passive RC, LC and active RC LPF with cut-off frequency of 20KHz directly on the output pin of the uC. when the filter is used, the output of the filter is zero regardless the input voltage i.e. the output is attenuated beyond usable range

--- End quote ---

Now we're getting somewhere.  Post a schematic of that filter you use - the "active RC LPF with cut-off frequency of 20KHz".   And the parts you used would be helpful (like the opamp part number).

If in doubt, the web has quite a few applets/design tools that can help: http://sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm (scroll down for getting part values for a given cut-off frequency).

--- End quote ---
I used LM358 as voltage follower and basic RC LPF
I used 1KOhm resistor with 10nF capacitor which should me give around 17KHz
here is the schematics

OP-AMP is lm358
Buriedcode:
Given there are no capacitors in series with the input (that block DC) I'm surprised you get "no output" at all from that.  As a LPF it should pass DC.  How are you powering the LM358?  Its input can only sense within ~1.5V of the top rail (and same with its output) but as I said, you should still get some output with the input > 0V.
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