I am designing a sine wave generator using a dds chip (ad9833). The system parameters are:
- frequency range of 1-12 MHz
- spurs and harmonics down to -50 dBc
- Output power atleast 10 dBm
Since the frequency is changing, the low pass filter cutoff has to be adjustable. My first thought was to have a mosfet controlled capacitor bank, but the capacitances of the transistor are limiting the range of the cutoff. I have also tried varactors but I am having trouble simulating them in LTSpice. I would like for this circuit to be an LC filter. Last resort is to have different filters for different key frequencies, like 1 MHz, 2 MHz, 4 MHz, 8 MHz, 16 MHz, but how would I switch between filters using a microcontroller?