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Carrier synchronization

Sam Dexter
Prodigy 90 points
Other Parts Discussed in Thread: LMX2531, LMX2485

Gentlemen,

We are trying to achieve an over-the-air receiver LO synchronization
using a few PLL chips - LMX2531 and LMX2485. We are using SA606 with
450 kHz ceramic filters as AGC amplifier followed by a 74LVC1G04DBV
Schmitt Trigger. Please see the block diagram attached.

When LMX2485 is driven from an external signal source we can get
both PLL#3 and PLL#2 to lock. However, when the incoming RF signal
is present PLL#2 goes off lock and its output becomes a wide-band
noise and PLL#3 never locks again.

I wonder if anybody has done something like this before and can
suggest an appropriate course of action?

A question for the TI professionals (if they read this):
1. does TI have a low cost, small form factor, single chip
reference-less wireless receiver which recovers the LO
frequency directly from the received RF signal?
2. is it possible to get a reference design that accomplishes
such a task?

What is an OSCin jitter spec for LMX2531 and LMX2485?

Please help.

  • 0
    TI__Mastermind 27581 points
    Hi,
    I dont' know how effectively I can answer your questions, but here's a few things I know ...
    It sounds like your issue is with the LMX2485, but it might be good to know that the LMX2531 has an itnernal VCO frequency calibration. What this means is that when you program the R0 register, it chooses a VCO band. The consequence is that if you program this device with an invalid signal and later bring it a valid signal with out reprogramming the R0 regsiter, it will not lockcorrectly because the VCO will be stuck in the wrong band.
    The LMX2485 has no VCO frequency calibration and it should track the input. However, I have smetimes seen VCOs that generate high harmonics wen slammed against the rail and sometimes the PLL can lock to the harmonic and get stuck there. Maybe this could be happening to the LMX2485. We do allow the user to view the output of the N divider through the Ftest/LD pin and you can use this to test what frequency the LMX2485 thinks the VCO is at.
    As for jitter, we really have not characterized this. Typically we are fairly tolerant, but if a harmonic or spur is very high, then potentially you could lck to that. The sensitivity for these devices tends to improve over frequency to a point, then degrade. So if you operate near the lower range, it might be possible that the device is more sensitive to the harmonic than the carrier. So if you have matching issues or harmonics above -20 dBc, then maybe this is the issue. Viewing the Ftest/LD when the PLL us unlocked is more likely.
    Regards,Dean