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VCO & Output Buffer

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Full Member level 4
Mar 4, 2004
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vco load pull

Hi Guys:

Fabrication of VCO, for measurement requires the integration of output buffers, usually a source follower stage for a VCO and open drain buffers for QVCO(N.J.Oh, S.G.Lee, "Current Reused LC VCO", IEEE Microwave and Wireless Components Letter", vol. 15, no. 11, Nov. 2005, pp. 736-738.). Why does this distinction exist?, Is the loading( Open drain buffer) and degeneration (source follower) resistance is equals to 50 Ω?. Thanks in advance


My guess be load pull

What do you mean by load pull?....Is the load is to be 50Ω (off-chip), at the open drain buffer. Thanks in advance


Let me take a crack at it. Your intended system usage dictates what you need to do.

VCO's determine their frequency with a resonator circuit. Usually some sort of L-C network, where the C is partially composed of a varactor diode. Where ever the reactance of the total L equals that of the total C, the VCO will oscillate.

If you have a nice resistive 50 ohm load, that is the end of the story. But if you have an unfriendly load, like a 0/180 degree switching phase modulator, then the varying load will somewhat load down the VCO's resonator circuit. A surprisingly little variation in this external reactance will cause a very big change in the VCO's operating frequency. Usually the phase locked loop does not have a big enough bandwidth to adequately supress this effect, and you end up with all sorts of phase errors, etc, in the communications link.

So, during your design of the vco, you determine how happy the vco will be with the load presented, and then usually conclude that you need a buffer amp with a lot of reverse isolation (s12) to keep the system happy.

The VCO frequency is depended on some parameters such as..

-Resonator components
-Transistor parasitic elements
-Tuning elements such varicaps
-Supply voltage

and Load..

If load is changed, the frequency is also changed. And if that load deviation has very fast changing in time, this will create Phase Noise.

Therefore after a the VCO it's quite classic to use a high isolation amplifier ( e.g. buffer amplifier ) to prevent the VCO from load perturbation.

And obviously buffer amplifiers have high input and low output impedances to drive lower value loads.

I think it should be 50 ohm

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