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gaom9
Joined: 08 Oct 2007
Posts: 146
Helped: 3
Location: China
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 02 Oct 2008 2:52 Question about the switch cap in VCO design |
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Hi,
I am designing a VCO and using the switch capacitor to change the VCO output frequency range.
I have a question about the switch capacitor array. I have read some paper about the VCO design, and the circuit they used for switch capacitor is as Figure 1, which contain two capacitor and one switch, but why do they use the circuit as shown in Figure 2, which contain one capacitor and two switches?
As the circuit in Figure 1, the equal value of the capacitor is only a half of one capacitor, and it will cost twice layout area.
Is that because of the capacitor quality factor? As the MOS switch will introduce some resistance to the circuit, so we should decrease the number of switch?
Can anyone give me some advice about this, please?
What is the difference between the two circuit in Figure 1 and Figure 2?
Thank you!
Best regards!
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jecyhale
Joined: 19 Feb 2008
Posts: 442
Helped: 46
Location: China
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| 02 Oct 2008 8:33 Re: Question about the switch cap in VCO design |
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To improve the Q of capacitor array! There is more resistor series than figure1 if the switch is same each other.
Higher Q of capacitor array is better for VCO.
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gaom9
Joined: 08 Oct 2007
Posts: 146
Helped: 3
Location: China
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| 02 Oct 2008 12:45 Question about the switch cap in VCO design |
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Thank you for your reply.
I try this two kind of circuit, the circuit of Figure 2 will make the VCO work badly.
But how to design the MOS switch size to ensure Q of the capacitor?
Thank you!
Best regards!
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jecyhale
Joined: 19 Feb 2008
Posts: 442
Helped: 46
Location: China
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| 03 Oct 2008 4:33 Re: Question about the switch cap in VCO design |
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| gaom9 wrote: |
Thank you for your reply.
I try this two kind of circuit, the circuit of Figure 2 will make the VCO work badly.
But how to design the MOS switch size to ensure Q of the capacitor?
Thank you!
Best regards! |
You know there is parasitic capacitor and parasitic resistor of switch, and they are both bad for VCO design.
So must trade off between them.
First, we should use the minimum length, then larger width will be lower resistor, but larger parasitic capacitor. And larger parasitic capacitor leads to lower tuning range.
But if the tuning range of VCO is enough, you should increase the Width of switch.
Besides, the width of switch should be in direct proportion to the capacitor value.
For example, 10/0.18 for 0.4p capacitor, then 20/0.18 for 0.8p capacitor. Then generate a capacitor array.
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gaom9
Joined: 08 Oct 2007
Posts: 146
Helped: 3
Location: China
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| 03 Oct 2008 5:23 Question about the switch cap in VCO design |
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Thank you for your reply, jecyhale.
Your reply provides some key information for my design.
Especially "the the width of switch should be in direct proportion to the capacitor value", I used the same switch width for different capacitor before.
I will try again.
Thank you!
Best regards!
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jecyhale
Joined: 19 Feb 2008
Posts: 442
Helped: 46
Location: China
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| 03 Oct 2008 7:45 Re: Question about the switch cap in VCO design |
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| gaom9 wrote: |
Thank you for your reply, jecyhale.
Your reply provides some key information for my design.
Especially "the the width of switch should be in direct proportion to the capacitor value", I used the same switch width for different capacitor before.
I will try again.
Thank you!
Best regards! |
Welcome.
I think we should ensure all parasitic capacitor should be twice and the Q of capacitor+switch should not change when the value of capacitor increase to twice.
In other words, no matter what the capacitor is, the Q does not change.
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