rize LPTV behavior an incoming signal U(f) that contains a wanted component near the carrier frequency and unwanted components near DC and twice the carrier frequency. These unwanted signal components are, for example, caused by nonlinearities in previous stages. As depicted in the left-hand side of Fig. 3.1, the downconversion stage is designed with the intent to transfer the input information content around the carrier frequency to baseband. However, due to the parasitic feedback, unwanted input signal content at DC and twice the carrier frequency also manages to make its way to the output frequency band at DC. The latter transfers are highly unwanted and must be suppressed beneath a certain level. So, in judging performance, a designer is interested to know both whether the downconversion stage properly passes the wanted signal component at the carrier frequency to baseband and whether it sufficiently suppresses unwanted signal transfers. Any framework that aims to assist designers in their task to develop the type of systems discussed above, should therefore be able to quantify all signal transfers that occur. Furthermore, all methods preferably operate in the frequency domain: this most closely corresponds to the way designers reason on mixers and PLLs. 3.1.2 Using harmonic transfer matrices to characterize LPTV be- havior Among all representations of LPTV behavior, harmonic transfer matrices [Maas88, Molloo, Vana02c, Were91a] are the only ones that directly meet the requirements stated above. They operate in the frequency domain and directly describe the signal transfers that occur between the different frequency bands. HTMs build on the observation that, as signal content can get transfered from one frequency band to another, a single scalar spectrum (s) is no longer efficient to de- scribe the content of a signal u(t). As illustrated in Fig. 3.2, it is natural to subdivide a signal's overall spectrum into components contained within evenly spaced frequency bands centered around multiples of fo. Here, fo r