Signal vs 1/SQRT(Bandwidth)
Here the FTMW signal of propyne (00 - 10 transition) is measured by linear frequency sweep and by a transform limited Gaussian pulse. In each case, the signal is measured as a function of bandwidth. This graph shows the advantage of linear frequency sweep for exciting larger bandwidths of spectrum. Because the signal from a linear frequency sweep scales with the inverse of the square root of the bandwidth, as opposed to the inversed of the bandwidth in the case of a transform limited pulse, the advantages to linear frequency sweep excitation increase dramatically with increasing bandwidth. The dots are actual data points measured in the laboratory, while the curves are the theoretical dependence of each excitation method on bandwidth. The divergence of the (blue) data points from theoretical predictions at low bandwidth (high 1/sqrt(bandwidth)) is due to saturation of the molecular transition. The transform limited excitation scheme saturates the transition with roughly the same signal, but at a lower bandwidth.