What is the best signal source to connect to the vertical input of an oscilloscope for checking the quality of a transmitted signal?

To accurately assess the quality of a transmitted signal, the oscilloscope must display the final RF envelope exactly as it is being sent to the antenna. The safest and most accurate way to do this is to sample the RF output directly from the transmitter using a sampling device, such as a directional coupler or specialized RF sampler. Connecting directly to the audio input ignores the RF modulation and amplification stages where distortion, such as flat-topping, usually occurs. You must monitor the final product, not just the starting audio. Using the RF signal of a nearby receiving antenna or the IF output of a monitoring receiver introduces variables and potential distortion from the receiver's own circuitry. This masks the true nature of the transmitter's raw output.