LMX2491: lock time too long

Samuel Lehner53

Part Number: LMX2491

Hi,

I have built a fast stepping Synthesizer based on LMX2491 and a Crystec VCO. The frequency range is from 1640MHz to 2960MHz with 40MHz step sizes. The PLL bandwidth is around 1 MHz and the resulting lock time < 5us. So far so good.

I found a Problem at 1760MHz. At this frequency lock time is sporadically (likelihood about 10%) much longer. The PLL jumps a completely wrong frequency and stays there for exactly 100us. Someone an idea how to fix this problem?

Regards Samuel

This picture shows the voltage at the VCO (sweep from 1640MHz to 2960MHz) input with the glitch at 1760MHz.

The length of the glich is exactly 100ns

over 6 years ago

0 Dean Banerjee over 6 years ago

TI__Mastermind 28021 points

Samuel,

Indeed this is a strange problem and I'm not really sure what is going on. TICSPro has a RAMP Calculator tab that is much more visual that might be useful if we could see that. Have you tried changing the modulator order and seeing if there is any impact?

Regards,
Dean

0 Samuel Lehner53 over 6 years ago in reply to Dean Banerjee

Prodigy 160 points

Hi Dean,

The frac order was set to integer mode (I sent a wrong screenshot). Now I changed it to 1st Order Modulator and the PLL locks in < 5us for all frequencies.

Thank you

Regards Samuel

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LMX2491: lock time too long