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VCO Varactor Tuning Range

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flames4791

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Hi, I am trying to design a voltage controlled oscillator with a frequency range of 2.35G to 2.55G. My problem is that I am trying to vary the tank capacitance by using a MOS varactor and I cant get more than a 10MHz tuning range when I have the tuning voltage swept between 0 and 1V. I have tried many different variations of connecting the transistors like grounding the bulk, connecting bulk to drain and source and connecting to voltage supply as well as pmos transistors hvt and lvt transistors. Can anyone help me get a good capacitance swing?
 

I don't think my school's model library has the MOS transistor that is needed for the accumulation MOS. Is there any other way of implementing this?
 

flames4791 said:
I don't think my school's model library has the MOS transistor that is needed for the accumulation MOS. Is there any other way of implementing this?
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yes, there are many VCO modules built with wide range freq coverage ... a few components on the outside to control it and you are in business

There's several in the Z-Comm range that would cover the range you need ...
eg the V585ME47 --- 1600MHz to 2700MHz

www.zcomm.com for more info

Dave
 

if you have to only move the tune voltage 1 volt, then you have to compromise. You need to resonate the tuning circuit (mostly very lightly couple it) so that a tiny change in capacitance makes a huge change in resonant frequency. For example, use a 100 nh inductance, and resonate it out with a 42 fF capacitance, or some other such relatively ludicrous thing. You will have poor loaded Q, and hence a lot of phase noise, but you will meet the tuning range.

Discrete VCOs use more like a 5 volt tuning range and a special varactor diode that has a big capacitance change over the 0 to 5 V range while having a good unloaded Q. THAT is how they get a big tuning range AND good oscillator noise.
 

biff44 said:
if you have to only move the tune voltage 1 volt, then you have to compromise. You need to resonate the tuning circuit (mostly very lightly couple it) so that a tiny change in capacitance makes a huge change in resonant frequency. For example, use a 100 nh inductance, and resonate it out with a 42 fF capacitance, or some other such relatively ludicrous thing. You will have poor loaded Q, and hence a lot of phase noise, but you will meet the tuning range.

Discrete VCOs use more like a 5 volt tuning range and a special varactor diode that has a big capacitance change over the 0 to 5 V range while having a good unloaded Q. THAT is how they get a big tuning range AND good oscillator noise.
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I was actually thinkin the same but thats too large of an inductor. I dont think my professor would like that. And update, I actually was able to get the frequency range I need by using the nmoscap. However, my phase noise is too high at the lower frequencies, but I should probably make a new thread for that. Thanks for everyone's help.
 

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