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Purpose of this resistor in LC-VCO (Read 2327 times)

neoflash Community Fellow Offline Mixed-Signal Designer Posts: 397	Purpose of this resistor in LC-VCO Jan 01^st, 2007, 12:55am Why there is a 2.5k ohm resistor on the control line? What is that for?
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ACWWong

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Posts: 539
Oxford, UK

Re: Purpose of this resistor in LC-VCO
Reply #1 - Jan 1^st, 2007, 4:33am

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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rf-design

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Reiner Franke

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Germany

Re: Purpose of this resistor in LC-VCO
Reply #2 - Jan 5^th, 2007, 3:03am

I share the first view from the first comment

1. Possible additional pole in the loop transfer function which make a third order PLL. The question is that for the same phase delay at the critical loop bandwidth frequency this could be more efficient be done by an multi-stage RC filter.

2. Do not short the second harmonic current in the loop filter cap. This could possible be a negative effect for PN.

3. Isolating noise from the digital control is more effective by using a buffer inverter which is conneted to clean supply. So questionable!

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