ACWWong
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There are couple of reasons depending on the nature of the control line signal.
If the control lines signal is an analogue tuning voltage, then the 2.5kohm forms a high frequency pole for the tuning line and part therefore is part of the loop filter. It also acts as ESD robustness (the, especially if the control line goes to a pad/external world.
If the control line is digital and used to either completely switch in/out the FETs in switched tank structure, then again the resistor adds ESD robustness, and acts to attenuate noise on the control line.
cheers aw
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