What's the series resistor between the vctrl and the varactors used for in LC-VCO

In many LC-VCO circuits there is always a series resistor between the control voltage and the varactors, what's the intention of this design?

One guess is to prevent the high-frequency noise on the voltage control line to enter the VCO and complete a AM-FM process, but at the same time this resistor introduce thermal noise which deteriorate the phase noise.

There are two important reasons for a series resistor:
1) in circuits with a dual supply op amp driving the varactor, you can get a situation (during start up, etc) where the varactor diode becomes forward biased. If there is no current limiting resistor, then the varactor diode can actualy blow up.
2) A typical PLL has a "lowpass response" rolling off starting around 10KHz to 100 KHz. If you have a reference fequency of maybe 2 MHz, then that 2 MHz spike noise is not going to be very well attenuated by the standard loop filter. A common improvement to the loop filter is an R-C lowpass filter that will knock that reference spur down another 20 dB or so. To add one, though, you have to make sure the additional R-C phase shift at the loop bandwidth edge is not so big as to cause control loop instability.

Rich

thx for the reply, very helpful.

It also provides isolation, as the varactor is normally reverse biased it passes almost no current so very little DC drop occurs in the resistor and it appears almost as a DC short. As there is normally oscillation at one or both ends of the varactor, the resistance acts to prevent the circuit producing the tuning voltage from loading the oscillation too much. In other words it lets the DC through but blocks the AC.

Brian.

sincere thanks!
however, is there any methods to prevent the phase noise induced by the resistor?

yeah, keep the R value small. If you are in a 3.3 V system, and the varactor can safely handle 100 mA before vaporizing (a guess), you can live with 33 ohms.

Unless I am doing something odd, I usually set it at 120 ohms and forgetaboutit.