Signal generator – circularly polarised

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.

Let's call the top resistor of a voltage divider R1 and the bottom R2. The output impedance is R1 // R2, and the gain is R2/(R1+R2). Set these two equations equal to what you want. Now you have two equations and two unknowns. Solve accordingly.

Note that R1 encompasses the 50 Ω of the source, so the actual top resistor (what you add directly in series with the signal generator) will be 50 Ω less than what you solve for R1. At your very low gain, this will make little difference though.