Electronics > Beginners

simulating insertion loss in LTSpice

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Simon:
So i did the attached circuit and using the formula 20 log⁡√(1+(2πf/((R_S+R_L)/L))^2 ) calculated that at 100KHz i would have an insertion loss of 56 dB and at 1MHz an insertion loss of 76 dB. The simulation comes out with about 8 dB more loss with a 50 ohm load but with a 1K load is bang on. I assume this would be because "insertion loss" is what the filter (inductor) is removing from the otherwise unfiltered circuit but the simulation is giving me the output to input ratio and including the source resistance in the "filter loss" as it is not doing a filter loss calculation just an out to in.

Is there a way of doing this as an insertion loss?

Re running with a 0 source resistance still puts the numbers out

Simon:
Right i worked it out! As the insertion loss is the circuit before the inductor was added versus after it was added i have to take the source impedance into account and lump it with the load impedance. Doing this brings about the expected results.

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