[G0HZU_JMR]

I dug out a basic line driver chip, I think it is an old SMD 74HCT244 but the logo is very faint. This isn't the greatest line driver in the world but when I add a series 27R resistor at the output it does a pretty good job of driving a 2MHz square wave into a decent JFW 50 ohm step attenuator and this is then fed via about 4 feet of RG58 coax into a 1Meg scope input.

The square wave looks good at all attenuator settings with nice and crisp risetime and a flat top to the square wave. In other words, I can attenuate down from 5V pkpk in 1dB steps to just a few mV pkpk and I still see a classic square wave at the scope.

The scope is not terminated in 50R and the reason I get this good performance is because of the line driver + series 27R resistor and the 50 ohm step attenuator after it. They provide a good 50 ohm source impedance to the RG58 cable at all attenuator settings.