IDT typically recommends that the 8V97051 RF_OUT output loaded with a 50Ω pull up resistance and with an AC coupling capacitor in series.
the main advantage of that output loading configuration is that it is a broadband termination that can be used for the whole frequency range of the 8V97051. So the user does not have to worry about having the right output load when changing the output frequency by a large amount.
Also, the output stage operates based on a constant current source. Therefore, in theory, the output power can artificially increase (be optimized) by increasing the value of the pull-up resistance, while keeping the same power setting, however, we recommend to change the RF output power settings.
The limitations of the resistive loading is that the amplitude might get attenuated at high frequencies (resistance becomes impedance with some L and parasitic capacitance – traces, input and output stages, etc.). The RF_OUT waveform is closer to a square signal (in theory) up to a certain frequency, but it starts to look more like a sine wave at higher frequency. This is typically not a problem in most applications, especially in RF card applications where the RF Synthesizer is used as the LO providing a reference to the RF mixer or the modulator / de-modulator.
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