LMX2491: Could you help me review the LMX2491 SCH if it is ok?

Eggsy Pang

Part Number: LMX2491

Hi Team,

Could you help me review the below LMX2491 SCH if it is ok?

In the datasheet, I sawthere is a 68Ω pull down resistance in OSCin pin of LMX2491. What is effect of 68Ω resistance here? Why is 68Ω resistance rather than 50Ω resistance?

Thanks!

over 5 years ago

0 Noel Fung over 5 years ago

TI__Guru** 107030 points

Hi Eggsy,

If Y2 is a clipped-sine wave TCXO, then R30 cannot be populated because this kind of TCXO is not able to drive a 50Ω load.

If the VCO requires 5V tuning voltage, pin 22 should be connected to a 5V supply.

0 Eggsy Pang over 5 years ago in reply to Noel Fung

TI__Expert 4985 points

Hi team,

Customer used in Radar use PLL and VCO to realize 26Mhz to 24Ghz output.

One more question, why does CPout need give feedback to Fin pin? Fin pin should be given feedback from VCO output.

Thanks!

0 Noel Fung over 5 years ago in reply to Eggsy Pang

TI__Guru** 107030 points

Hi Eggsy,

CPout should go to a loop filter and then followed by a VCO. In the diagram, the symbol at the top left corner represents a VCO.

0 Eggsy Pang over 5 years ago in reply to Noel Fung

TI__Expert 4985 points

Hi Noel,

Below is my block diagram.

Do you think it makes sense?

What about the M , N Divider and Gain value of LM2491 which I input into below?

Thanks!

0 Noel Fung over 5 years ago in reply to Eggsy Pang

TI__Guru** 107030 points

Hi Eggsy,

Max. gain is only 31, 47 is not correct.

Others look good.

0 Eggsy Pang over 5 years ago in reply to Noel Fung

TI__Expert 4985 points

Hi Noel,

If the max. gain is olny 31. I can't get the 2.4Ghz output. I just get the 1612Mhz (31*52) which can't meet customer requirement.

Do you have other suggestions?

Thanks!

0 Noel Fung over 5 years ago in reply to Eggsy Pang

TI__Guru** 107030 points

Hi Eggsy,

The equation is:

OSCin * M / R = phase detector frequency = Fin / N.

The gain = charge pump gain = current setting of the charge pump.

0 Eggsy Pang over 5 years ago in reply to Noel Fung

TI__Expert 4985 points

Hi Noel ,

I understand "OSCin * M / R = phase detector frequency = Fin / N"

So what is the equation betwwen the gain, the phase detector frequency and the Cout frequency?

Thanks!

0 Noel Fung over 5 years ago in reply to Eggsy Pang

TI__Guru** 107030 points

Hi Eggsy,

CPout is a current signal, not frequency signal. The current is set by charge pump gain.

0 Eggsy Pang over 5 years ago in reply to Noel Fung

TI__Expert 4985 points

Hi Noel,

Understand.

So I set the Gain = 31, the max, which is ok for our application, right?

If I set the gain=1, what happen will be?

Thanks!

0 Noel Fung over 5 years ago in reply to Eggsy Pang

TI__Guru** 107030 points

Hi Eggsy,

PLL's applications require loop filter design. Design requirement is determined by system as well as the user. Several PLL parameters will affect the design, charge pump gain is one of those.

We have an offline design tool to help loop filter design, please has the customer downloaded it and use it. The tool is called PLLatinum Sim.

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LMX2491: Could you help me review the LMX2491 SCH if it is ok?