LMX2571 Fractional Spurs problem

Vishnu R

Other Parts Discussed in Thread: LMX2571EVM, CODELOADER

Hi,

I'm presently evaluation LMX2571EVM and stuck with the fractional spurs problem.

I tuned the synthesiser to 840MHz and it showed very nice spectrum, with phase noise agreeing to the datasheet value.

But when I tuned it to 840.025MHz (25kHz step), the spectrum is corrupted with fractional spurs, at +/-25kHz, +/-50kHz, +/-75kHz offsets. The worst case it is -43dBc relative to carrier.

Kindly advice on how to proceed with this. Is there a configuration setting to reduce the fractional spurs. Since the loop bandwidth is ~200kHz, all these spurs will be passed by the loop filter.

Regards,

Vishnu

over 9 years ago

0 Julian Di Matteo over 9 years ago

TI__Expert 4085 points

Hi Vishnu,

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.

Regards,
Julian

0 Noel Fung over 9 years ago in reply to Julian Di Matteo

TI__Guru** 110840 points

Hi Vishnu,
If 840MHz is an integer multiple of your oscin frequency and/or fpd, then the 25kHz spurs are expected. Would you let us know your configuration so that we could check if there is room to improve the spurs?

0 Vishnu R over 9 years ago in reply to Noel Fung

Prodigy 180 points

Hi ,

I've attached a snapshot of the Codeloader GUI.
This is show the configuration which I'm using right now.

Regards,
Vishnu

0 Noel Fung over 9 years ago in reply to Vishnu R

TI__Guru** 110840 points

Hi Vishnu,
All the spur frequencies you saw were came from the cross talk or mixing between OSCin and VCO/fout. The best way to resolve this problem is change the OSCin freq. Is this possible in your application?

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LMX2571 Fractional Spurs problem