Tri-level composite signal from microcontroller

[epicedium]

What is the easiest way to generate this pulse from a microcontroller running at 3v3? I have IO pins and PWMs available.

https://en.wikipedia.org/wiki/Tri-level_sync

I haven't dealt with negative voltages before, and I'm not that familiar with analogue electronics (other than simple opamp circuits). My first instinct is to generate -3v3 (somehow?) then use 2 or 3 microcontroller pins to switch 2/3 mosfets or transistors with resistors dividing voltage, but I do wonder if there's a better/ simpler way.

It doesn't seem possible/appropriate to create a virtual ground unfortunately, as the composite signals go out over BNC using chassis ground.

 

[duke37]

The wikipedia article says there is no DC, perhaps a capacitor in the line would be sufficient.
There are integrated circuits which can give a low current negative supply, I have never used one.

 

[epicedium]

Thanks.

lm2660 seems a good place to start - http://www.ti.com/lit/ds/symlink/lm2660.pdf

Perhaps the best solution is to drive my microcontroller's 6bit DAC into an opamp.

 

[Alec_t]

The opamp would need a dual-polarity supply if you want it to put out a negative-going pulse.

 

[Alec_t]

Duke suggested a cap. Here's a sim. Good enough for your needs?

 

[epicedium]

Alec_t, Duke- the cap, it may well be acceptable, yes! I'll simulate it into a 75Ohm load and experiment- my experience with AC is pretty minimal so I don't really grasp how it works. I assume that I could clamp the spike with diodes and that the magnitude and duration of the spike are a product of the capacitance and load. I'll report back once I've had a play around. Thanks!