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TWT AM to AM and AM to PM conversion | |
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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
Figure 3. Power characteristics of a TWTA showing the saturation. The definition of the back off parameter is also shown.
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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 .