The clock generator devices like Si5340/41/91 as well as the jitter attenuator devices like Si5345/95 are both designed to provide low jitter output clocks. But a major difference is that the jitter attenuators, as the name suggests, will attenuate the noise/jitter at the input while for the clock generators, the noise at the input passes through to the output and contributes to the output jitter value.
Therefore, in case of the clock generator devices, the total RMS jitter at the output will tremendously depend on the jitter of the input source and also on the input frequency. In order to get the best jitter performance, it is recommended to use an input frequency between 48MHz to 54MHz at the XAXB pins of these devices and also to avoid using low frequencies like 20MHz or 25MHz at the input. Lower input frequencies cause an increase in noise at the clock output which is seen as increased RMS jitter.
The above applies to both the XAXB pins and the clock inputs pins for the clock generators.
Also, the signal generator or the oscillator used at the input should have minimum possible jitter. The Rohde & Schwarz SMA 100 signal generator is one of the very low jitter signal generators available
Now in case of the jitter attenuating clocks, any allowed input frequency and any signal generator can be used to produce the input signal and it will not have any effect on the output clock jitter.