Thanks for the guidance.
From Scope’s SM I read, that I have to connect CH1 and CH2 through a 10X attenuator to generator and set it output amplitude to MAX. In specs for the PG506 is written that max output signal is 100V.
That means, that the scope expects fast rise signal with amplitude 10V and unknown frequency.
I tried different ones from 1 kHz to 1 MHz with no luck.
I will keep use it that way until I find a suitable calibration device .