Voltage Controlled Capacitor

Thread Starter

lkgan

Joined Dec 18, 2009
117
Lkgan, If you can be happy with a tunable filter that will not work past 100kHz, have a look at LMF100. You probably won't find a simpler way to tune a filter. Tuning is done by varying the clock frequency.
Hi Ron H,

I scanned through the LMF100 document, I don't quite understand how the block diagram works. You mean the bandwidth or cut-off frequency of the filter can be tune by varying the clock frequency? Is the LMF100 applicable for PLL?

Take a look at GYRATOR CIRCUITS , ask google about it.


good luck
Lineman
Thanks for introducing something new for me, but it's not controllable, unless you got some ways to do it? :D
 

Ron H

Joined Apr 14, 2005
7,063
Hi Ron H,

I scanned through the LMF100 document, I don't quite understand how the block diagram works. You mean the bandwidth or cut-off frequency of the filter can be tune by varying the clock frequency? Is the LMF100 applicable for PLL?
Read the application notes in the datasheet.
Also, see AN-779.

What are you going to be filtering with your tunable filter? Is this for the loop filter in a PLL?
 

kubeek

Joined Sep 20, 2005
5,794
No I am not just using buzz phrases. I was trying to say that you most probably can make a variable capacitor (but with quite big series resistor) using a variable gain amplifier in a capacitance multiplier circuit. I will study and simulate it a bit more and post back.
 

Ron H

Joined Apr 14, 2005
7,063
No I am not just using buzz phrases. I was trying to say that you most probably can make a variable capacitor (but with quite big series resistor) using a variable gain amplifier in a capacitance multiplier circuit. I will study and simulate it a bit more and post back.
You could be right. I'm sure we'll hear about it if you are.:D
 

kubeek

Joined Sep 20, 2005
5,794
You could be right. I'm sure we'll hear about it if you are.:D
I am sure you are right as well.

Anyway, here is a schematic of a capacitance multiplier equal to a grounded 20uF capacitor with 1ohm ESR. I simulated that if you change the gain of the amplifier the capacitance changes as well, so for example amplifier gain of 10 makes the capacitor 200uF. You could set the resistors lower for lower ESR, but the buffer amplifier has to be capable suplying the needed current.

Don´t know what is the frequency of the signal used, but using for example LM13700 in variable gain configuaration with TL082 should work pretty well in place of the amplifier in the simulated circuit.
 

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Ron H

Joined Apr 14, 2005
7,063
I am sure you are right as well.

Anyway, here is a schematic of a capacitance multiplier equal to a grounded 20uF capacitor with 1ohm ESR. I simulated that if you change the gain of the amplifier the capacitance changes as well, so for example amplifier gain of 10 makes the capacitor 200uF. You could set the resistors lower for lower ESR, but the buffer amplifier has to be capable suplying the needed current.

Don´t know what is the frequency of the signal used, but using for example LM13700 in variable gain configuaration with TL082 should work pretty well in place of the amplifier in the simulated circuit.
It didn't work when I simulated it. If the gain is anything greater than one, the positive feedback results in the output running off to one of the supply rails.
Can you post your simulation results?

Also
 

kubeek

Joined Sep 20, 2005
5,794
In my simulator Microcap 9 it doesn´t stick to the rails and acts somewhat like a capacitor, but anyway the circuit doesn´t behave exactly as the real cap with ESR does, even in the original cap. multiplier configuaration where the negative input is tied to the output like in normal voltage follower.
 

Ron H

Joined Apr 14, 2005
7,063
In my simulator Microcap 9 it doesn´t stick to the rails and acts somewhat like a capacitor, but anyway the circuit doesn´t behave exactly as the real cap with ESR does, even in the original cap. multiplier configuaration where the negative input is tied to the output like in normal voltage follower.
I see you are driving it with a voltage source. What evidence did you have that it acted as a capacitor? You post your results, and I'll post mine.;)
 

kubeek

Joined Sep 20, 2005
5,794
That was the original schematic, when I added some series resistance from the voltage source it started to behave strangely. I was simply trying to bring this to life and make it variable, but after some trying it seems it doesn´t really work as expected to begin with.
 

Thread Starter

lkgan

Joined Dec 18, 2009
117
Thanks for giving so much information. I will need some time to digest those information. Will get back if there are any questions. Thanks.:)
 

Thread Starter

lkgan

Joined Dec 18, 2009
117
Hi Ron H,

I have gone through the documents. My analysis on switch capacitor approach is that it's not suitable for PLL applications. Switch capacitors filter has more noise compare to the active filter. In PLL, the usual approach is using passive components. Active components are used when wide tuning voltage is needed for the VCO to cover wideband application. In this case, I wish to use a simpler approach such as parallel connection of varactor diodes. What you say?

The LM13700 is something that I can explore on. Have you tried using this chip before? What does it mean "A signal voltage applied at Rx generates a VIN to
the LM13700"? Isn't the control voltage suppose to be at node Vc? Rx is the output desired resistance right?
 

Ron H

Joined Apr 14, 2005
7,063
Hi Ron H,

I have gone through the documents. My analysis on switch capacitor approach is that it's not suitable for PLL applications. Switch capacitors filter has more noise compare to the active filter. In PLL, the usual approach is using passive components. Active components are used when wide tuning voltage is needed for the VCO to cover wideband application. In this case, I wish to use a simpler approach such as parallel connection of varactor diodes. What you say?
When I suggested the LM100, you had not yet told us that you were trying to implement a PLL filter. You might have saved us time if you had mentioned this in your first post.
Varactors are not suitable for PLL filters because they are two-terminal devices, which means their capacitance will change as a result of signal voltage as well as control voltage.

The LM13700 is something that I can explore on. Have you tried using this chip before? What does it mean "A signal voltage applied at Rx generates a VIN to
the LM13700"? Isn't the control voltage suppose to be at node Vc? Rx is the output desired resistance right?
A signal voltage applied at Rx generates a VIN to the LM13700
Where did you see this quote? I don't feel like searching through documents about LM13700 to find it.


Why do you need a tunable PLL filter?
 

Thread Starter

lkgan

Joined Dec 18, 2009
117
When I suggested the LM100, you had not yet told us that you were trying to implement a PLL filter. You might have saved us time if you had mentioned this in your first post.
Varactors are not suitable for PLL filters because they are two-terminal devices, which means their capacitance will change as a result of signal voltage as well as control voltage.
Hi Ron H,

I apologize for it. The reason I did not mention the purpose because I wish to open up my mind into various approach on voltage controlled capacitor and explore alternative solution. Most of the PLL literature do not have information about it.

Where did you see this quote? I don't feel like searching through documents about LM13700 to find it.

Why do you need a tunable PLL filter?
It's in Voltage Controlled Resistors section (Page 11). A tunable filter is required for more flexibility. I am still exploring how can I change the capacitance using any control (voltage, current or .......) , of course simple implementation would be great as it lower the input noise. :)
 
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