When using the 9DB202 in PLL mode, why do we see a 80 MHz signal on the output, even when there is no input provided?

When the device is in PLL mode, the VCO is always running.  There is no VCO shutdown pin on the part. If there is no reference input being supplied, the PLL reference is 0 Hz. The negative feedback loop then drives the VCO control pin until the VCO frequency is at the lowest frequency limit of the VCO. For this part, the lowest frequency guaranteed over processing, temperature and voltage is 450MHz, which when divided by 5 is 90 MHz.

 

The reason we see 80 MHz is because the VCO is operating at 400MHz with a 50 MHz guard band. Since the outputs are enabled, the VCO appears at the outputs; just as it should if the input reference were active.