LMX2592: Vref removed but PLL still shows locked

Hi Shaun,

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

Hi Shaun,

 How about when you make REF_EN=0 (register R9[9])?

Regards,

Brian Wang

Hi Brian,

Same issue when REF_EN=0.

Thanks.

Hi Brian,

Any feedback?

Thanks.

Any feedback?

Hi Brian,

Please advise the soonest.

Thanks.

Hi Shaun,

I found out what's going on. So the lock detect method used in this device is digital lock detect. What that does is the device keeps counting the phase comparison fails/successes during the frequency calibration sequence and then indicates the lock when accumulated a set number of successes. This is an accurate way of indicating if the devices is locked to the intended frequency. The other type of method is the analog lock detect, which senses the tuning voltage and claims that the device is locked if it is within a boundary of voltage, since when a device locks after calibration it should be in the center of the tuning range (for example the tuning range of LMX2592 is 0 to 2.5V, after calibration it settles around 1.3V, so the boundaries may be around 1.1 to 1.5V). For the LMX2592, the method used digital and not the analog lock detect. so it will show lock correctly after calibration, but it doesn't change the status when you remove the reference (which the analog lock detect can catch because the vtune falls when you lose the reference). What you do to see the status of the lock in the digital detect method is calibrate again, then it does not reach the needed successes, and LED will go off.

Regards,

Brian Wang