As we move towards the Tera era it is important to understand how to make measurements in this frequency domain. Over the past twenty years measurement techniques in this area have been developing. The main technique has been electro-optic sampling and probing. In this technique a fast laser producing a train of optical pulses (∼1ps). The train of pulses is split in two by a beam-splitter. One train acts to start a short electrical event. The second train of pulses is delayed in time with respect to the first and is used to probe the short duration electrical signal. The probing in the case of electro-optic sampling is achieved by measuring the polarization rotation of the probe beam as it passes through an electro-optic probe crystal placed in the fringing field of the short duration electrical signal. This is a complex system. When measuring the response of photodiodes a simpler alternative, electrical auto-correlation, is available. In this case the non-linear current response is measured as a function of the overlap of the two trains of optical pulses focused onto a photodiode. In this work we show our results using electrical auto-correlation in which we have made direct comparisons with electro-optic sampling.