LMX2571 with external VCO Loop Filter

user4536882

Other Parts Discussed in Thread: LMX2571, LMX2571EVM, CODELOADER

Hi team,

Anyone can help to explain more on how to calculate the resistor(R) and capacitor(C) for external loop filter.

From evaluation kit note, says that the loop filter RC value is tile closely to the intended VCO output frequency.

I'm evaluating the LMX2571 with external VCO output frequency => 380MHz, the VCO not lock but tuning voltage goes to maximum 5V.

Please advise.

Thanks and best regards,

Yaakub

over 8 years ago

0 Julian Di Matteo over 8 years ago

TI__Expert 4085 points

Hi Yaakub,

Thank you for your interest in our products. We received your question and will be answering it as soon as possible. We appreciate your patience.

Best Regards,
Julian

0 Noel Fung over 8 years ago in reply to Julian Di Matteo

TI__Guru* 81400 points

Hi Yaakub,
Would you please try our newly released design tool named PLLatinum Sim? this tool supports LMX2571 loop filter design.

0 user4536882 over 8 years ago in reply to Noel Fung

Prodigy 210 points

Hi Noel/Julian,

Thanks for your replied.

Appreciate if you can provide the link for the PLLatinum Sim.

Regards,

Yaakub

0 Noel Fung over 8 years ago in reply to user4536882

TI__Guru* 81400 points

www.ti.com/.../pllatinumsim-sw

0 user4536882 over 8 years ago in reply to Noel Fung

Prodigy 210 points

Hi Noel,

Thank you very much for your help.

I will make use given tool to continue test my VCO design with LMX2571.

Regards,

Yaakub

0 user4536882 over 8 years ago in reply to user4536882

Prodigy 210 points

Hi Noel,

Beside the loop filter components I believe there is a calculation for external charge pump components for LMX2571, appreciate you advise on this as well.

Maybe you have certain formula or tool in Ti to calculate the charge pump components value for LMX2571 => CPout.

Please advise.

Regards,

Yaakub

0 Noel Fung over 8 years ago in reply to user4536882

TI__Guru* 81400 points

Hi Yaakub,

PLLatinum Sim support both internal and external VCO design for LMX2571.
When you choose the part in step 1 of the simulation, select LMX2571-ExtVCO.

0 user4536882 over 8 years ago in reply to Noel Fung

Prodigy 210 points

Hi Noel,

Thanks for your replied again.

Yes I agreed, PLLatinum Sim support both internal and external VCO design for LMX2571; next question is how to calculate the charge pump components value for LMX2571 => CPout.

Refer to the schematic of LMX2571EVM, how to calculate the value for C1LFA, C2LFA and R2LFA.

Please advise.

Regards,

Yaakub

0 Noel Fung over 8 years ago in reply to user4536882

TI__Guru* 81400 points

Hi Yaakub,

The loop filter component values depend on your choice of loop bandwidth, charge pump gain as well as the phase detector frequency.

We have a couple of blogs you may be interested in to find out some basic technique in PLL design.

e2e.ti.com/search

0 user4536882 over 8 years ago in reply to Noel Fung

Prodigy 210 points

Hi Noel,

For your information, I'm using discrete VCO frequency 380MHz in my application and loop filter components are calculated using Ti tool as per given link above.

I have checked the feedback level of RF power to LMX2571 is about 0 dBm but it still not lock to intended VCO frequency; it tuned to 5V maximum tuning voltage and f = 402MHz, any advice or comment on this.

Regards,

Yaakub

0 Noel Fung over 8 years ago in reply to user4536882

TI__Guru* 81400 points

Hi Yaakub,

Are you using Codeloader or your own software to program the device? Could you send me your register configuration setting? If you use Codeloader, please provide the .mac file (File --> Save) or screen shot of each tab, thanks.

0 user4536882 over 8 years ago in reply to Noel Fung

Prodigy 210 points

Hi Noel,

I'm using Codeloader application to program the PLL.

TI PLLatinum Sim is being used to calculate the loop filter components.

Please advise.

I have no idea how to attach the file here.

Thanks and best regards,

Yaakub

0 Noel Fung over 8 years ago in reply to user4536882

TI__Guru* 81400 points

Hi Yaakub,

After clicking the red Reply button, a regular small white window will pop up . on the right hand side, click the Use rich formatting link, a new big window will pop up. you can attach file or image with the new widow.

0 user4536882 over 8 years ago in reply to Noel Fung

Prodigy 210 points

Hi Noel,

I have tried to attached the .mac file but it seem like photo logo is attached, it not the actual .mac file.

Appreciate it you can share email address so that I can send the file directly to you or alternatively you can send an email to my email yaakub@ste.com.my.

Regards,

Yaakub

RF & microwave

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LMX2571 with external VCO Loop Filter