# Notch filter design - resistor value

I'm trying to design a notch filter to suppress an oscillator signal coming through a mixer (namely, 53.125 MHz), at the same time I must get the best possible signal at 60 MHz.

I've been running some simulations - it seems lowering the capacitor value and increasing the inductor value makes the "notch" steeper. It would seem in this example design, I would be losing a lot of signal on 60 MHz?

What confuses me, also, is the role of the value of the resistor here, I don't really understand what the lower blue line and the dotted green line represent.

As I lower the resistor value, the lower blue line moves up and the dotted green line curves.

Can someone give a basic idea of what's going on here?

• Your signal source is feeding power to something. Can you show a load on that dangling wire? Is the load resistive at 53.125 MHz? at 60 MHz? Commented Aug 8, 2017 at 17:03
• What does "best possible signal" mean? Unity gain? High gain? What is the bandwidth of the pass band at 60MHz? Is phase important to this signal? Commented Aug 9, 2017 at 4:07

I think the left side shows solid Blue = 20dB which I believe means 20dBV relative to 1V from 10V source.

The dotted blue line shows phase ref for 0 deg

THe dotted Green line show Phase going from -90 to +90 deg thru the peak notch.

The X(s)/R value gives the Q value and a simple 2nd order Notch will not give you the optimal pass/reject ratio. First define your goal (specs) for impedance , Zi, Zo at fo then the SNR ratio of 60/53.125, then the tolerance of your components to see what is needed. I suggest at least a 3rd order or 5th order LC filter.

From what I know about LTSpice, and this is weird how they represent it, the dotted green line you see is the phase. So that spike in the phase line is a phase shift of $180°$, which usually occurs at the cutoff frequency.

The resistor value helps with your $Q$ value in your curve which will affect the bandwidth of the curve.

The two blue lines must represent a DC signal since there is no change in phase or magnitude.