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vijay

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Ring VCO
Dec 27^th, 2005, 2:27am

Hi,
I have designed a symmetric load ring VCO with the bias control circuit. The circuit oscillates when the control voltage is from the range of 0.75V-4V.
The same circuit fails to oscillate when the control voltage is <0.75V.
Morever,the bias voltage required at the tail transistor of the VCO for such a low control voltage will put it in linear region always.
The circuit work fine for the required range of frequencies:-400MHz-600MHz.
I am worried about the start up frequency of the VCO,assuming there will be no control voltage when the circuit is poweredup for the first time.
So practically,whether the free-running frequency of the VCO is considered at Vcontrol=0V?
Thanks in advance.
V

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ywguo

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Shanghai, PRC

Re: Ring VCO
Reply #1 - Dec 27^th, 2005, 6:27am

Hi, vijay,

Of course we should not expect the VCO to work when the control voltage is near the power or ground rail because some transistors in the VCO must fall out of saturation region.

If you design a charge-pump PLL, forget the free-running frequency. If you design a traditional PLL using a multiplier type phase detector, the free-running frequency is that where the phase detector detects no phase error.

:)

Best regards,
Yawei

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vijay

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Re: Ring VCO
Reply #2 - Dec 28^th, 2005, 3:03am

Thanks Yawei,
Looking at the statement:-
"If you design a charge-pump PLL, forget the free-running frequency. If you design a traditional PLL using a multiplier type phase detector, the free-running frequency is that where the phase detector detects no phase error. "

But mathematically the VCO should have a free running frequency at Vcontrol=0.I know that this would not be the case for a PLL employing this VCO,but a stand alone VCO should have a free running frequency.
One more question:
For a 0.6um N-well process,with Fout=500MHz ,employing a symmetric load ring VCO, what should be the power of the oscillator.I am not looking at an exact answer but a round about value of power so as to have a value to refer to.
Regards,
V

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Visjnoe

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Re: Ring VCO
Reply #3 - Dec 28^th, 2005, 9:14am

Hi Vijay,

I think you're right a standalone VCO should have a free running frequency. The 'textbook' definition often defines this as the oscillation frequency @Vctrl = 0V. In practive, however, the circuit may not oscillate at such a low voltage (e.g. by transistors entering the linear region). It should be clear that the 'text-book' definition is indicative and that the free-running frequency of the VCO is the lowest oscillation frequency corresponding with the lowest control voltage (assuming a positive VCO gain) in the valid control voltage range [vmin,vmax].

Concerning the power dissipation of your ring oscillator:

1. what's the number of stages of your ring oscillator?
2. what's the required phase noise spec? A lot of papers (Hajimiri&Lee, McNeill, Razavi...) give nice formulae for the relationship between phase noise and power dissipation.

Otherwise you can easily derive a power estimate knowing the required output swing of each stage and a first-order estimate of the capacitive load C of each stage:

P = N x VDD x Istage

with Istage such that Istage (C*Vswing) > 500MHz

Kind Regards

Peter

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keble

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Re: Ring VCO
Reply #4 - Jan 9^th, 2006, 9:45pm

Not sure if this helps, but it might: always be sure to take care of your start-up conditions!
With any PLL you might find some set of conditions where the VCO is not controlled - particularly true of a Maneatis-style ring oscillator where control voltages below one threshold don't have any effect.
Also for this particular (self-biased) PLL you have to have a start-up circuit to ensure the bias op-amp is operating properly.
I've done quite a few Maneatis PLLs with a few different start-up circuits; they've worked OK, but they do need a lot of care. Generally the strategy is to make sure the control voltage to the VCO is at a sensible level (ie it is controlling the VCO frequency and not at one extreme or other) �when you power up, by biasing it to some voltage; then you need to detect that the VCO is running (could use a bias level or pulse detector), delay, then switch off the start-up bias. By this time the phase detector should be able to take over the control voltage.
Regards

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judean New Member Offline Posts: 3	Re: Ring VCO Reply #5 - Jan 12^th, 2006, 12:53am Thanks alot for the start up part.I was actually doing the statr ups circuitry. Could you let me know one thing..where do you actually require the OPAMP?If I am right Maneatis paper does not make use of opamps anywhere. Regards V
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keble Junior Member Offline Posts: 11	Re: Ring VCO Reply #6 - Jan 14^th, 2006, 1:16am The op amp can be seen in Fig 2 of the Nov 1996 paper - it's marked Diff Amplifier Rob
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