Hello,
I am designing a low pass filter for a dds system. The output waveforms of the dds range from 1-12 MHz, so the filter has to have an adjustable cutoff frequency for harmonics and spurs. With an RC filter, this could be done with a potentiometer, but how could this be done with an LC filter? I have tried using an nmos controlled capacitor bank but the capacitances of the mosfet are limiting my cutoff range. I have also looked into varactor diodes but i cannot seem to get them to respond in a simulation.

 

Hello,

The low pass filter did depends on the output waveform frequency but only on the clock frequency to remove image products. Others spurious cannot be removes only with a low pass filter.
What is the purpose of your DDS frequency?


Jean-Louis