I have a AM modulator circuit and i want to measure using a spectrum analyzer with internal resistance 50 Ohm. This internal resistance is my problem because it is connected parallel and changes the operation of the modulator. I think that a isolator can be used in order to have the correct measures. Does anyone know the suitable isolator (buffer) circuit taking into account an operation at 1.5 MHz minimum (perhaps the buffer may limit the operation and cut high frequencies) ?