I'm currently into designing passive LP filters such as RC topology and active filters such as Sallen-Key topology.
I'm fully aware that what matters in the choice of values for the resistors and the capacitors is their product so increasing one will result in decreasing the other in order to keep the same cut-off frequency.
What I have trouble with is knowing whether I should chose my capacitor first and adjust the resistor or the opposite. And for each option, how should I chose the component's value.
I'd like to know if there's any rule of thumb to chose quickly the values for my filters, knowing the impedance of the load or other parameters.
For example, I'm currently designing a filter (active or passive) for a PWM running at around 10kHz and I want my cut-off frequency to be around 1kHz, the load having an impedance of around 100 kOhms.