Electronic – HP8116A Function Generator Signal Problem

500kHz is a ways past the maximum specified frequency for square waves for this generator: 100kHz. It appears to have a single-pole RC of about 3.5µs, which would work great for a 0.35/(3.5µs)=100kHz square wave. The output may have a LPF for slew limiting. Also, it is a 50Ω source, so it should be terminated properly to avoid ringing. Try using the TTL and CMOS waveforms, too. B&K have put together this document: Function & Arbitrary Waveform Generator Guidebook .

The external CMOS inverter is not a 50Ω source -- it's source impedance is only a few ohms at most (for low currents) due to V_CC and ground impedances and FET R_ON equivalent resistances. Notice that the output duty cycle isn't 50%, and the edges are ringing.

Going purely by the (somewhat unlikely) results of connecting the trigger input, as reported in the question:

The problem is not a low output signal from the gate itself, but a lack of external pull-up resistor at the output. Note that the datasheet indicates an output current sink rating of 16 mA, but source rating of only -0.4 to -0.8 mA.

Add a pull-up resistor of around 1 kOhms from the output of the gate to Vcc, and the output should trigger the signal generator fine.