LMX2491: Fin coupling RC value

Yoshino Yuji

Prodigy 10 points

Part Number: LMX2491

I would like to know how to determin RC values to be cuppled to Fin.

Figure 14. Typical Schematic on the datasheet shows the values as 36ohm and 10pF.

Are those values not necessity to change for any frequency in range of fFin?

In addtion, why is 36ohm necessity?

over 4 years ago

0 Noel Fung over 4 years ago

TI__Guru** 121260 points

Hi Yuji-san,

those resistors around Fin are used to split the VCO signal to Fin and to the device following the VCO (To Circuit).

Fin is not 50Ω, assuming the output impedance of the VCO is 50Ω, the input impedance of "To Circuit" is 50Ω, then the resistor network will make the impedance looking outward from Fin 50Ω.

0 Yoshino Yuji over 4 years ago in reply to Noel Fung

Prodigy 10 points

Hi,

Thank you for taking your time.

In my understanding, two 18ohm and 68ohm (50 + 18ohm) make a divider.

I think the impedance looking outword from Fin seems to look 23ohm, because from Fin view, three 68ohm are conected in parallel.

So I was wondering why it puts the 36ohm resister instead of a 27ohm that would be 50ohm (23 + 27).

0 Noel Fung over 4 years ago in reply to Yoshino Yuji

TI__Guru** 121260 points

Hi Yuji-san,

So the impedance on the left hand side of Fin will be 22.67 + 36 = 58.67Ω.

0 Yoshino Yuji over 4 years ago in reply to Noel Fung

Prodigy 10 points

Hi Fung-san,

I am cofused.

On your first reply, you said 'the resistor network will make the impedance looking outward from Fin 50Ω.', but on your second replay, said 'So the impedance on the left hand side of Fin will be 22.67 + 36 = 58.67Ω.'

Which is correct and I think I need more explanation from you for my understanding.

Thak you.

0 Noel Fung over 4 years ago in reply to Yoshino Yuji

TI__Guru** 121260 points

Hi Yuji-san,

58.67Ω is closed to 50Ω, we don't need to make it exactly equal to 50Ω.

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LMX2491: Fin coupling RC value