Current starved VCO cadence design

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destro98

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Hi,

I am trying to design a current starved VCO in cadence, i tried some circuit and it is oscillating, but i don´t know how to vary the input control voltage to get the oscillation frequency from maximum to minimum. i am attaching the png of my design. Any kind of help is appreciated regarding this concept.

Thank you.
 

You want to feed "input" with a DC current source
if you want anything linear-looking. You can drive
it with a voltage source but should expect a response
between exponential and square-law in voltage-freq
transfer characteristic.

In a practical circuit you would probably take a control
voltage, run it through a linear-ish voltage to current
converter and push that current down "input".
 

thanks for the help, but i can't understand what do you mean by "running control voltage through in practical circuit" are you saying we should use a DC voltage source as a control voltage?
 

I'm saying that when you make a VCO as a "product"
(piece-part, IP chunk, whatever) the expectation is
going to be that it is (linearly) voltage controlled. Not
current, unless you advertise it as a CCO.

So then you would be tasked to take an input voltage
and do what you must, to get a reasonably linear
df/dV across the band of interest. This could be as
simple as a minimal op amp and current mirrors, one
of which drives a I*R feedback leg and another of which
drives the CCO core.
 
BigBoss said:
I think the input reference current changes the oscillator frequency.
Click to expand...

Yes, the input current source changes the oscillator frequency, when i change the current source value the frequency changes, but as it is a voltage controlled oscillators how can i know frequency variations with respect to the voltage variations. How can i know the voltage variation applied for the certain frequency? that was the main issue i am facing with this model.
 

I want to control the input current, but as it should be voltage controlled, i want to convert voltage to current and give the output current as the input, so that i can vary the voltage and can get the required frequency. Can you help me with how can i design a Vto I converter without using opamp, using opamp brings complications in cadence, can we design a converter circuit using MOSFET?

Thank You.
 

Op amp brings complications?

Even a little 5-transistor buffer? Or maybe 7, since you
might like common mode range to include ground?

If you expect circuit design to be free of complications,
expect much disappointment.
 

dick_freebird said:
Op amp brings complications?

Even a little 5-transistor buffer? Or maybe 7, since you
might like common mode range to include ground?

If you expect circuit design to be free of complications,
expect much disappointment.
Click to expand...

Yes OP-amp is the easy one, but we have to design the conversion without the op amp as per our project.
 

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