LMX2820EVM: Using PFD input and external mixer

Callum Austin

Part Number: LMX2820EVM
Other Parts Discussed in Thread: LMX2820

Tool/software:

Hi,

I am currently using the LMX2820 with the internal VCO, however, using external feedback. Therefore, the output of the LMX2820 is down converted using an external mixer and fed back to the PFDIN pin.

My current settings are –

PFD Ref = 200MHz

PFD External Feedback = 1200MHz

Internal VCO Output = 7900MHz

External signal generator = 6700MHz

I am able to get the LMX2820 to lock with these settings however, I would like to adjust the external signal generator +- 300MHz. When I do this, the LMX2820 loses lock and the output is rubbish.

I was hoping the LMX2820 PLL will act as a tracking filter and ultimately adjust the VCO to keep the same PFD External Feedback frequency, without the need to constantly reprogram the LMX2820.

Can this be done? If so what am I doing wrong? Am I missing something?

Regards,

Callum

over 2 years ago

0 Noel Fung over 2 years ago

TI__Guru** 121260 points

Hi Callum,

+/-300MHz change is too large for the VCO without a re-calibration.

Without re-calibration, approximately, the VCO can change +/-50MHz.

0 Callum Austin over 2 years ago in reply to Noel Fung

Prodigy 150 points

Thank you for the reply Noel.

Does this rule also apply to using the RFIN pin?

0 Noel Fung over 2 years ago in reply to Callum Austin

TI__Guru** 121260 points

Hi Callum,

This is not related to any mode of operation, it is the characteristic of the VCO. We have 7 VCO cores to support 5.65G to 11.3GHz, we need calibration to put the VCO in the right place. After calibration, we can change the VCO frequency by approx. 100MHz without recalibration. Beyond this range, the PLL may unlock, we need to recalibrate the VCO to make it lock again.

0 Callum Austin over 2 years ago in reply to Noel Fung

Prodigy 150 points

Hi Noel,

Thanks for the reply, and sorry for the delay in reply. If we were to use an external VCO with the LMX2820 PLL section (Internal VCO Turned off), would the PLL still need to VCO calibrate?

0 Noel Fung over 2 years ago in reply to Callum Austin

TI__Guru** 121260 points

Hi Callum,

External VCO operation does not need calibration, just like traditional PLL+VCO.

Clock & timing

Clock & timing forum

LMX2820EVM: Using PFD input and external mixer