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sk.shawn

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Need help with my circuit
Jan 04^th, 2010, 1:30am

Dear all,

I'm seriously having problems with this circuit (as attached) that I'm designing at the moment. The topology is based on a paper reported in solid state journal and the working principle is to linearize my open loop VCO by negative feedback using a switched capacitor network that is acting as a frequency to voltage converter (FVC).

The issue is can Vin be a DC signal? As soon as I feed a DC input, the opamp output is immediately saturated. I'm just all confused about the type of input signal that should be fed. Ultimately, the range of voltage that I would like to have at the opamp output is 1.2V to 2.2V which I have no idea of how to achieve this.

I have tested all individual blocks separately and they are all working (opamp, FVC, open loop VCO). I feel that it could be just some minor thing that I need to get it figured out. So, I'm hoping someone here can shine some light to assist me.

P.S Vin is a negative input and Vp is grounded according to design in paper.

Sorry for the long type. Any help is appreciated greatly.

Thanks a million.

Regards,
Shawn

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mycircuit.JPG

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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: Need help with my circuit Reply #1 - Jan 4^th, 2010, 3:53am hi, can you please tell me the name of the pap,so that i can also read. The problem could be lack of -ve feedback (because you are opearting circuit in open loop), if that fvc block is providing -ve feedback then its fine. So just check whats that block polarity? Thanks, Rajasekhar.
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Mayank

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Re: Need help with my circuit
Reply #2 - Jan 4^th, 2010, 3:58am

Hi shawn,
i worked on a similar ckt....

Answers to your queries --
Quote:

As soon as I feed a DC input, the opamp output is immediately saturated.

Most probably because of Opamp's offset voltage, It gets integrated and saturates it. Place a high Resistance in parallel with C2 across your opamp output and pin Vn so as to provide a DC Feedback.

Quote:

I would like to have at the opamp output is 1.2V to 2.2V which I have no idea of how to achieve this.

Read up on OCMR & Max attainable Output Swing of an opamp.[Output Common Mode Range]

My assumptions from ur post :--
1. Quote:

The issue is can Vin be a DC signal

I dont see any wire / pin names Vin...Though pin Vn is clearly visible...I assume you are referring to Vn as Vin.

My Questions :---
1. Quote:

by negative feedback using a switched capacitor network that is acting as a frequency to voltage converter (FVC).

How much current are you feeding to the F2V converter and how much to the VCO ??

2. Why do you need such a linear VCO ??

General Practice :--- We avoid these kinda circtuits generally because of power and area overhead...If you wanna alter you Kvco, do it by directly altering the current fed to the oscillator. We generally dont need very high-linearity in Ring-Osci VCO's. [ Again I assume that you are using Ring Osci. ] We just ensure the VCO Frequency Tuning with Vcont is Monotonic ie varies in just one direction.

Hope it helps,
Mayank.

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sk.shawn

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Re: Need help with my circuit
Reply #3 - Jan 4^th, 2010, 6:15am

raja.cedt wrote on Jan 4^th, 2010, 3:53am:

hi,
�can you please tell me the name of the pap,so that i can also read. The problem could be lack of -ve feedback (because you are opearting circuit in open loop), if that fvc block is providing -ve feedback then its fine. So just check whats that block polarity?

Thanks,
Rajasekhar.

Hi Rajasekhar,

Thanks for the reply. I have attached the paper for your reference. Basically, my circuit is based on the topology mentioned in this paper. The FVC is implemented as a simple switched capacitor network. A lot of my peers have told me that this circuit is rather straightforward but I just can't get it to work in the way I want.

Anyway, appreciate your support. Hope to hear more views and opinions.

Regards,
Shawn

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Linearization_of_Voltage-Controlled_Oscillators_Using_Switched-Capacitor_Feedback.pdf

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sk.shawn

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Re: Need help with my circuit
Reply #4 - Jan 4^th, 2010, 6:38am

Hi,

Thanks a million for replying in sequence. I have attached the paper here in my earlier reply to Raja. My circuit is based on the topology discussed in that paper.

Quote:

Place a high Resistance in parallel with C2 across your opamp output and pin Vn so as to provide a DC Feedback.

I thought of this too and actually included this into the circuit. However, as reported in the paper, the feedback resistor was not included by the author. So, I was contemplating if this was a correct move. So, in this case, a feedback resistor is required then?

Quote:

I dont see any wire / pin names Vin...Though pin Vn is clearly visible...I assume you are referring to Vn as Vin.

Perhaps, the image I attached was unclear. Vin is a negative input applied to one of the terminals of the resistor. It will be clearer if you look at the circuit in the paper. Sorry about that.

Quote:

How much current are you feeding to the F2V converter and how much to the VCO ??

The FVC is implemented as a simple switched capacitor network. Will current be an issue here? Sorry I didnt really check this out. But i will perform a simulation and see. Thanks!

All in all, I need a highly linear VCO to realize a time based ADC where other constituents blocks will come together as well.

Once again, thanks for the help and effort in replying. Please continue to assist me. I will be more than glad to provide more information if required.

Thanks a million

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Mayank

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Re: Need help with my circuit
Reply #5 - Jan 4^th, 2010, 10:39am

Hii,
@ raja : I came up with this idea on my own some time back, frequency controlled resistor using S-C and resistive comparison....It's cool as well as disheartening to know someone's already worked on it. :( Although, the ckt i came up with is a lot different frm this. But linearity was my secondary objective.

Quote:

I thought of this too and actually included this into the circuit. However, as reported in the paper, the feedback resistor was not included by the author. So, I was contemplating if this was a correct move. So, in this case, a feedback resistor is required then?

In the paper, he didnt mention anything about opamp offset...So i guess the author ignores it dere / assumes it to be negligible.

Quote:

Perhaps, the image I attached was unclear. Vin is a negative input applied to one of the terminals of the resistor. It will be clearer if you look at the circuit in the paper. Sorry about that.

Ahh, i got it....

Quote:

The FVC is implemented as a simple switched capacitor network. Will current be an issue here? Sorry I didnt really check this out. But i will perform a simulation and see. Thanks!

The S-C will draw some current from Vref...This current will decide the gain of the ckt....Author's VCO was only 10kHz VCO & external opamp IC LM324 which is high power...Author doesnt elaborate on power dissipation point of view. At GHz range VCO's, this power is comparable to total VCO oscillation Power itself, hence this topology i nt desirable....

Quote:

All in all, I need a highly linear VCO to realize a time based ADC where other constituents blocks will come together as well.

Are you using VCO's o/p clock as sampling clock for ADC ?? Are you using a PLL Loop around the VCO ?? I didnt understand the fundaa of time-based ADC ??

--
Mayank.

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