Charge Pump Phase locked loops are used in a variety of applications, including on chip clock synthesis, symbol timing recovery for serial data streams, and generation of frequency agile high frequency carrier signals. In many applications PLL?s are embedded into larger digital systems, in consequence, analogue test access is often limited. Test motivation is thus towards methods that can either aid digital only test of the PLL, or alternatively facilitate complete self testing of the PLL. One useful characterisation technique used by PLL designers is that of closed loop phase transfer function measurement. This test allows, an estimation of the PLL?s natural frequency, damping, and 3dB bandwidth to be made from the magnitude and phase response plots. These parameters relate directly to the time domain response of the PLL and will indicate errors in the PLL circuitry. This paper provides suggestions towards test methods that use a novel maximum frequency detection technique to aid automatic measurement of the closed loop phase transfer function. In addition, techniques presented have potential for full BIST applications.