AM modulation and spectrum analyzer

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) ?

You need a buffer stage with a 50 ohm output, a single transistor or FET should be enough, for your low RF frequency it shouldn't be a problem.

Some scopes have FFTs that will allow the use of high input impedance so this wouldn't be a problem. A simple high speed op amp circuit could make a usable buffer for your spec analyzer.

How much do you want to load your AM modulator? Just use an appropriate impedance matching transformer. Or figure out how to remove the parallel resistor from the spectrum analyzer, if you can take the analyzer apart it should be a simple resistor shorting out the input leads to load the input line to simulate a dipole antenna or 50ohm coax transmission line.