LMX2592: LMX2592 giving wrong frequency on output

aleksandar vujic

Part Number: LMX2592

Hi,

I am using LMX2592 to produce output frequencies in range of 50-100MHz. When i programmed registers according to datasheet and TICS Pro to output 57.5 MHz with Fosc of 25MHz i get output of 57.5MHz, as i wanted. But when i change R45,R44 and R38 to output 58.5 MHz, on output i get 57.8166 MHz. I would appriciate your help.

Thanks, Aleksandar57.5.tcs 58.5.tcs

over 5 years ago

0 Noel Fung over 5 years ago

TI__Guru** 120060 points

Hi aleksandar,

After programming R38, R44 and R45, did you program R0 to initiate a VCO calibration?

0 aleksandar vujic over 5 years ago in reply to Noel Fung

Prodigy 30 points

I programmed every register from R70 to R0. With same values like i provided in attachment from TICS Pro software

0 Noel Fung over 5 years ago in reply to aleksandar vujic

TI__Guru** 120060 points

Hi aleksandar,

Please make FCAL_EN = 1 all the time to ensure when you program R0, the VCO gets calibrate.

0 aleksandar vujic over 5 years ago in reply to Noel Fung

Prodigy 30 points

Already programmed it. Doesnt help. I guess i cant choose larger numerator than denominator. I wanna figure out how to change output frequency by 1Hz in range of 55-85 MHz. I cant figure out how to auto calculate register values with my software. Is there some kind of formula

0 Noel Fung over 5 years ago in reply to aleksandar vujic

TI__Guru** 120060 points

Hi aleksandar,

I don't see a problem with your denominator, you should be able to make it lock to 58.5MHz. To change the frequency from 57.5MHz to 58.5MHz, program R38, R44, R45 and R0 (with FCAL_EN = 1).

Your current configuration will change the output by 1Hz when you change NUM by 1, you've done the calculation correctly.

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LMX2592: LMX2592 giving wrong frequency on output