The Team - Do you have a Scrum Master outside the Dev Team?

Pure Scrum dictates that the Scrum Master could be one of the developers on the team, but we find this is hard for a developer as he will be split between trying to do the work himself, while also ensuring that the Team still uses Scrum properly. We find that a better option is to use a separate Scrum Master so his only responsibility on the project is to ensure Scrum is used well and the Product Owner is informed of any developments.