TWT AM to AM and AM to PM conversion

TWT AM/AM and AM/PM conversion:  The TWTA is a common element in earth stations and  communication satellites.  For an input sine wave of frequency f and amplitude r, the TWTA is characterized by the relationship


The empirical relations


describe A(r) and φ(r).  The first term is called AM/AM conversion, and the second is AM/PM conversion. The four contestants can be determined from the actual TWTA tube measurements via a least square fit.  The term A(r)/r is the nominal gain.  A plot of A(r) shows the output power increasing with the input, and then leveling off and actually decreasing as the input power continues to increase into the overdrive region.  This is the saturation phenomena.  TWTA must be operated a little bit below saturation to control sideband .

Figure 3.  Power characteristics of a TWTA showing the saturation. The definition of the back off parameter is also shown.


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