how to increase the input impedance for a ring oscillator

[beabroad]

hi all

I am designing a ring oscillator, working at 2.5GHz. The input impedance looking by the control voltage is only 100m ohm. I would like to know how to increase that.
My ring oscillator is a current saturated sigle-ended inverter ring oscillator. And the control voltage is applied on the current source above and below the inverter.

Thank you.

 

[analogTechie]

Re: how to increase the input impedance for a ring oscillato

Hi beabroad,

Did you mean current starved? Because, if your current inverters are, in fact,
current saturated then how could you possibly change the inverter delay, and thus
the frequency, by any significant amount ?

Maybe that is what your problem is ?

In any case, your Zin looking into the gates of the current source FET's should be
quite high. What else do you have hanging off of the control node ?

 

[beabroad]

Thank you for your reply.
how high the input impedance of ring oscillator shall be? several M?

 

[analogTechie]

Re: how to increase the input impedance for a ring oscillato

Hi beabroad,

After looking at your schematic, it appears that when you measure Zin of the
ring oscillator, you are measuring the Zout of the diode connected FET, which
should be of the order of 1/gm. So, 100mohm is a reasonable measurement.

Also, why do you want this impedance to be high ? What you want is to have
a large change in current for a small change in voltage at the control node,
which translates into a low impedance, not high. So, you should be OK.

What is it about the ring oscillator performance that does not meet yhour spec ?

Cheers,

 

[beabroad]

Re: how to increase the input impedance for a ring oscillato

thank you analogTechie.

sorry i made a mistake. I shall calculate the impedance at low frequency, for the Vctrl is low frequency. It is OK now, the input impedance at low frequency is more than 100M ohm.
what is the output impedance for the ring oscillator? I heard it shall be as small as possible to prevent parasitics to the following circuit. What frequency I shall choose when calculating the output impedance? Is that on 2.5GHz?

currently, my PLL has voltage ripple of 30mV when it is stablized. my colleage suggested me to use voltage divider to divide my 2.5GHz VCO output by 4, and I am trying that. My Loop filter is 3rd order passive one. The bandwidth for the loop filter is 2MHz.

 

[alvays]

beabroad, perhaps you can give analog a "helped" by pressing the "help me" button if you really think he helped you.
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