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Design >> RF Design >> dual resonance frequency tank
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Message started by raja.cedt on Jan 11^th, 2013, 6:19am

Title: dual resonance frequency tank
Post by raja.cedt on Jan 11^th, 2013, 6:19am

Hello,
I have strange application where i need a vco without any tuning range @ 60Ghz. I designed tank inductor which is fine at 60G from Q point of view, but it's self resonance frequency is around 90G unfortunately capacitor (not mos-cap, inter-digitized metal cap) is also having self resonance frequency near to this and hence around 104G inductor is behaving like cap and cap is like ind. So finally i have two frequencies.

So now my Question is when i am designing VCO, what kind of care should to make sure that it wont oscillate at 104G. I know transistor may not work at that frequency but i am interested to know more theory behind it like when you have two frequencies and in both cases if there enough gain then where oscillator locks stuff like that.

Thanks,
Raj.

Title: Re: dual resonance frequency tank
Post by rfcooltools.com on Jan 11^th, 2013, 12:07pm

raja,

You will want to separate the resonance of the capacitor by increasing the resonance frequency. To increase the resonance frequency of the cap you could make the interdigitated cap fingers shorter and add more in parallel to return to the same cap value.

In general the oscillator will resonate at a frequency where the gain is the highest. Gain will be construed as the gm*Re(Z(tank)). I am not sure which frequency will be the oscillation frequency if they two frequencies have roughly equal gain, possibly some intermediate one.

http://rfcooltools.com

Title: Re: dual resonance frequency tank
Post by raja.cedt on Jan 11^th, 2013, 2:45pm

hello,
yes i did it, now tanks looks fine. But can you tel me clearly why it oscillates at frequency where it has higher loop gain?

Thanks,
Raj.

Title: Re: dual resonance frequency tank
Post by rfcooltools.com on Jan 11^th, 2013, 4:40pm

Raja,

Consider two WEAKLY coupled tanks one resonant tank1 at freq1 and one tank2 resonant at freq2. Now consider the Q of the tank1 to be greater than tank2. Apply a step response and observe the voltage waveforms after a length of time. the tank with the higher Q will die down slower than the tank with the lower Q also note that the resonant frequency might be a combination at first, but later each tank will settle to a their own frequency separately. (this is not to say that the final frequencies of both aren't altered by the weak coupling element).

If you are with me so far...

Now consider this step response in reverse as energy is added to the tanks. The tank with the higher Q will increase amplitude at a faster rate.

Finally, the active part of the VCO will eventually become a limiter as the amplitude further increases. Now that the VCO is a full limiter the selectivity of that tanks resonant frequency is reinforced since the additive frequency of the weaker tank is only present on the through zero transitions (or when the limiter changes polarity). The stronger resonance is further reinforced by the periodic energy additions from the limiting action and eventually very little energy is at the secondary resonance and thus a single resonance oscillation occurs for the most part.

http://rfcooltools.com