Since the RF signal is being transferred in one direction from a 75 Ohm source to a 50 Ohm load it is only necessary to add a 25 Ohm (low Wattage) resistor in series with the 50 Ohm input to the spectrum analyzer. When using the added 25 Ohm resistor it will be necessary to multiply the RF Voltage amplitude displayed on the spectrum analyzer by 1.5 to determine the true amplitude of the RF Voltage from the 75 Ohm source.

Yeah, you can't go directly into the 50 ohms without putting something in between. The impedance mistmatch will screw up your spectrum due to reflections at the mismatch point. An L-pad or voltage divider network with 44 and 88 ohms would give you proper source and load impedance. And then you'd have to account for the 7.5dB loss through the L-pad.

Buying would probably be better than building. It all depends on how high the frequency of interest is. If you're working with audio frequencies or frequencies up to a few 10's of megahertz, you could probably get away with building one. But as the frequencies go up to hundreds of megahertz or gigagertz, a clean signal path(including constant controlled impedance and no sharp corners in your wiring) in your build becomes more and more critical. And building the converter becomes more and more of a project. Even carbon film resistors(which is what your common SMD resistors are) will have a frequency effect and you'd have to go with metal foils.

Put a 25 ohm non inductance resistor in series with 50 ohm input shall do. Compensate the signal loss due to 25 ohm was add by adding 33.33% more.



Or obtain or make your own 75 ohm into 50 ohm wide band adapter depending on the frequency that you are working on.

You can buy or build a 75 to 50 ohm adaptor. Link has one, but you have to find one that matches the connectors you are using, and also has a frequency band specified that covers the range you need.

PS, a 25 ohm resistor will only match in one direction. You need an L or T pad to match in both directions and prevent reflections.

Second link has resistor values needed

You can use an impedance matching transformer or add 25 ohms in series with the analyzer. In both cases you will have to calculate the actual signal based on what the analyzer shows.

I have a 50 ohm input spectrum analyzer to do a test of setting RF levels to a 75 ohm system. Is there some calculation changes I can make and still use the 50 ohms input spectrum analyzer? The company does not want to buy a 75 ohms spectrum analyzer.