RF Built-in-Self-Test requires generation and analysis of high frequency signals on-chip, which usually involves complex circuitry. Generally a high frequency to low frequency conversion, such as a peak or amplitude detector, is employed to analyze the output of the device under test (DUT). However, this conversion circuit is subject to similar process variations as the DUT, which has to be included in the measurement process. Moreover, despite affecting the accuracy of the entire measurement, the input generation for RF BIST is sparsely discussed. While relative measurements, such as gain, can be made without knowing the input signal attributes, other parameters, such as output power, IIP3, or IIP5 require absolute measurements, hence the knowledge of the input signal amplitude. In this paper, we propose a technique for on-chip amplitude measurement that is independent of process variations. The approach relies on on-chip generation of a square wave using an RF limiter with two different amplitudes. Using these two input signals and mathematical modeling, we extract the amplitude at the output of the DUT with high accuracy. The circuits for the BIST have been designed and simulated at the transistor level, including process variations. The concept of the proposed measurement technique is demonstrated in hardware using off-the-shelf components.