LC oscillator configuration in PLL

[lucky112]

Hi, I am a student working on a PLL and down-converter circuit.
Attached pic is a simplified drawing of the circuit.
VCO needs to generate 480 MHz, and I need to find out the value of two inductors.

In the schematics, the weird thing is there are two inductors that are connected to ground.
Is it different from normal parallel-LC configuration?
I guess it's similar to Hartley oscillator config. because of two inductors.
But the base of TR is not connected to LC tank.. this is one of discrepancies.

Please help me understand this LC oscillator..
Thanks in advance.

 

[BigBoss]

These inductors are connected in fact series.So one inductor value will be 11.68nH excluding varicap zero bias capacitance.This value will absolutely be lower than this because varicap zero bias capacitance and mid-voltage capacitance ( for a proper VCO operation) will be added so inductor value will be quite lower than that value.

 

[lucky112]

These inductors are connected in fact series.So one inductor value will be 11.68nH excluding varicap zero bias capacitance.This value will absolutely be lower than this because varicap zero bias capacitance and mid-voltage capacitance ( for a proper VCO operation) will be added so inductor value will be quite lower than that value.

Thanks a lot. Could you please help me understand how you derived the value?
If I use the equation 2*pi*f=1/root(L*C), and let c 4.7pF, then L is about 23.4nH, which is approximately (your value*2).
Then, can I neglect c1 and c2 (270 pf)?

 

[BigBoss]

Thanks a lot. Could you please help me understand how you derived the value?
If I use the equation 2*pi*f=1/root(L*C), and let c 4.7pF, then L is about 23.4nH, which is approximately (your value*2).
Then, can I neglect c1 and c2 (270 pf)?

I have already given that value for single inductor.If you would use 2, of course the total value will be 11.68x2.
You can principally not neglect the coupling capacitor because they will bring also some additional capacitance coming from ICs internal circuit.If you don't know these capacitance values, you should try and see.
But nevertheless, you shouldn't underestimate those factors..
-Varicap zero bias capacitance ( parasitic and intrinsic) and mid-point cap value for proper operation
-ICs internal capacitance
-Layout parasitic capacitances

All those capacitances will define your oscillator frequency

 

[lucky112]

You can principally not neglect the coupling capacitor because they will bring also some additional capacitance coming from ICs internal circuit.If you don't know these capacitance values, you should try and see.

Once again, thank you so much.
I'm sorry, but I still don't understand what you said.
You were talking about the additional capacitance. Is it different from the capacitance of coupling capacitors(C1,C2)?
If I can't neglect the coupling capacitors (270pF), I think their capacitances are quite higher than 4.7pF(C3)

And may I ask you one more question?
What topic/concept do I have to search/study in order to understand that two inductors are connected in face in series.
Sorry to keep bothering you.

 

[BigBoss]

Once again, thank you so much.
I'm sorry, but I still don't understand what you said.
You were talking about the additional capacitance. Is it different from the capacitance of coupling capacitors(C1,C2)?
If I can't neglect the coupling capacitors (270pF), I think their capacitances are quite higher than 4.7pF(C3)

And may I ask you one more question?
What topic/concept do I have to search/study in order to understand that two inductors are connected in face in series.
Sorry to keep bothering you.

Yes, additional capacitances which will come from internal circuitry of the IC and these capacitances will be reflected/influenced somehow into the calculation.The inductors are series connected because GND connection is a "pseudo ground" so tank circuit loop current will terminate its path from capacitor through inductors. Connection to GND of the inductors has been done due to symmetry.Some VCOs use VCC instead of GND connection to supply VCO itself, so it's the same thing.It's just initiative interpretation.

 

[lucky112]

Yes, additional capacitances which will come from internal circuitry of the IC and these capacitances will be reflected/influenced somehow into the calculation.The inductors are series connected because GND connection is a "pseudo ground" so tank circuit loop current will terminate its path from capacitor through inductors. Connection to GND of the inductors has been done due to symmetry.Some VCOs use VCC instead of GND connection to supply VCO itself, so it's the same thing.It's just initiative interpretation.

Thanks a million! You've helped me a lot.