RIGOL DG4000 Series - Manual (Page 71)

RIGOL
Quick Guide for DG4000 15
 FSK：
If ASK, FSK or PSK is enabled for CH2 and external modulation source is
used, this connector accepts an external modulation signal (users can set
the polarity of the signal).
 Trig In:
If CH2 is in sweep or burst mode and external trigger source is used, this
connector accepts an external trigger signal (users can set the polarity of
the signal).
 Trig Out:
If CH2 is in sweep or burst mode and internal or manual trigger source is
used, this connector outputs a trigger signal with specified edge.
8. External Signal Input (Counter)
BNC female connector with 50Ω nominal impedance. It is used to accept an
external signal to be measured by the counter.