The uncertainty relation for joint measurement of position and momentum
  We prove an uncertainty relation, which imposes a bound on any joint measurement of position and momentum. It is of the form $(\Delta P)(\Delta Q)\geq C\hbar$, where the `uncertainties' quantify the difference between the marginals of the joint measurement and the corresponding ideal observable. Applied to an approximate position measurement followed by a momentum measurement, the uncertainties become the precision $\Delta Q$ of the position measurement, and the perturbation $\Delta P$ of the conjugate variable introduced by such a measurement.   We also determine the best constant $C$, which is attained for a unique phase space covariant measurement. 
R.F. Werner