Assuming you are referring to a side hung fixture on a vertical truss?
If you want a continuous circle, the best thing is a fixture with continuous pan.
Otherwise, you can create an effect that goes within the full pan range. EX if the fixture is 540° of pan, setting a sawtooth on pan with 270° size and base of 0°, leave tilt at your preferred value. The caveat to this is that you will see your fixtures go the opposite way half of the time. You can counteract this by placing your fixtures PAN to ramp or Inverse ramp. the caveat to this is that you will probably want your intensity to turn off at the very end of the wave so the audience does not see the snapback. This does take a little bit of practice, as the timing will differ from fixture type to type. Make sure you set your lights to SYNC in the effects engine.
The general rule of thumb, if you want your lights to "Circle" and they are not contious P/T then you need to have your tilt value not go past 0° of tilt in the effect.
