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LMX2492EVM: Simple RF TX/RX setup from two LMX2492EVM boards, but PLL can't lock

Yuke Tian
Prodigy 80 points
Part Number: LMX2492EVM
Other Parts Discussed in Thread: LMX2492

Hello,

I am trying to use two LMX2492EVM to build a simple RF TX and RX system. Attached are the system setup. 

On the first board, only PLL (U1: LMX2492) is used. The VCO (U2:MACO-009265) is not used (R4 is disconnected and no Vtune to U2).  The RX antenna's signal feed back PLL's Fin* signal. The R6, which connect the on-board VCO output, is removed. 

On the second board, only VCO is used (R4 is disconnected).  The Vtune is connected by a wire from the 1st board to the 2nd board.  The TX antenna is transmitting RF signal. 

The idea is simple: 1st board receives RF signal to drive Vtune. The 2nd board receives Vtune to drive VCO to transmit RF signal.  The two boards share the same 5V and ground. The setup is similar to the single LMX2492EVM board, except now the RF TX (by VCO) and RF RX (by PLL) are separated into two boards.  

To our surprise, the PLL can only lock (by monitoring MUXOut LED) around 9.1GHz. And the lock is not solid (LED blinking, instead of solid on).  In the single board system, the PLL can lock from 9.1GHz to about 10GHz. The LED is solid on.

Please help us to understand this behavior.  This is the first step to use the LMX2492 in a real TX/RX setup. The LMX2492EVM uses the on-board trace of Fin to feedback, not through the SMA antenna. 

Thanks a lot for the help.

Yuke

  • 0
    TI__Guru* 84520 points

    Hi Yuke,

    What was Fin* signal power? Are there any other signals picked up by the RX antenna with different frequencies stronger than the VCO TX signal?

  • 0 in reply to Noel Fung
    Prodigy 80 points

    Hello Noel,

    Thanks for the quick response. 

    We don't know how to measure the Fin* signal power.  We used a spectrum analyzer and was able to pick up the VCO TX signal with the correct frequency (agree with PLL setting frequency). So, we assume the RX antenna also pickup the setting frequency.  Are there any measurements we can do to debug these ?

    We appreciate your kind help. 

    -Yuke

  • 0 in reply to Yuke Tian
    TI__Guru* 84520 points

    Hi Yuke,

    Typical Fin sensitivity is around -30dBm at 9G-10GHz. You have to ensure that this requirement is met. In addition, since you used an antenna to pick up radiation signal to Fin, you have to ensure there is no other signal with stronger power go to Fin, otherwise, the weak 9G-10GHz signal will be ignored.

  • 0 in reply to Noel Fung
    Prodigy 80 points

    Hi Noel,

    So you think that the Fin signal, which is picked up from the antenna, is too weak. I am thinking of adding a RF amplifier in the TX board to boost the TX signal from VCO.   Do you think is that will help ?

    Thanks,

    Yuke 

  • 0 in reply to Yuke Tian
    TI__Guru* 84520 points

    Hi Yuke,

    You'd better add a filter as well in order to filter unwanted signal coming to Fin.