Because of the holidays, TI E2E™ design support forum responses will be delayed from Dec. 25 through Jan. 2. Thank you for your patience.

  • State Resolved
  • Locked Locked
  • Replies 4 replies
  • Answers 1 answer
  • Subscribers 21 subscribers
  • Views 114 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LMX2694-EP: RFOUTA and RFOUTB output power differences

Thomas Frei
Prodigy 10 points
Part Number: LMX2694-EP
Other Parts Discussed in Thread: LMX2594

Tool/software:

Hello,

I am using the LMX2694 to clock two identical devices using both differential outputs RFOUTA and RFOUTB. During my testing I noticed that I measure two vastly different voltages at each of the two outputs, with a difference of up to 50%. The output power difference is heavily dependent on the output frequency, especially for values between 4-8GHz.

As a method of verification I used the Evalboard containing the LMX2694 and I get similar results there, although they are not as bad as in my own design. Worst case I measured a difference in the output amplitude of 25%, independent on the output power or output MUX settings. 

I am wondering whether this behaviour is expected or if both outputs should behave nearly identical regarding their output power? Is there any data available regarding the output power across frequency for each output and are similar values expected for all devices? In the datasheet of the LMX2594 is a graph available for a single ended output (Fig. 18), but there is no mention if the same value is expected for all output signals.

Kind regards,

Thomas

  • +1
    TI__Guru* 93670 points

    Hi Thomas,

    This is the data I took in the lab.

    Freq 1G 2G 3G 4G 5G 6G 7G 8G 9G 10G Hz
    RFoutBM 6.18 6.98 8.27 8.85 7.64 6.4 4.5 2.61 0.84 0 dBm
    RFoutAM 6.05 7.09 8.5 9.04 7.46 6.45 5.44 2.74 0.75 -0.3 dBm
    RFoutAP 6.44 7.39 8.5 9.04 8.06 7.82 5.79 3.75 2 1.19 dBm

    The output power should be same from both RFout, as the structure of the output buffers is identical. The variation is likely due to pcb layoiut. 

    On the EVM, output B is single-ended output with the P pin terminated onboard. From the above data, both outputs' M pin output power is very similar, except at 7GHz. RFoutAP was terminated with a 50Ω SMA load during the measurement. 

    However, when I terminate RFoutM pin and measure RFoutAP, I have got different power when the frequency is above 5GHz. Layout effect becomes obvious when frequency gets higher.

    In addition, the pull up resistors may affect the output power. On the EVM, we are using a higher quality resistor to ensure the resistance does not change much vs frequency. We have also used "broadband" capacitors at supply filtering and output AC-coupling. Layout of the pull-up resistors and the filtering capacitors is also important. On the EVM, the two pull-up resistors are closely spaced and share one filtering capacitor. If the layout has these resistors separated but still share one filtering capacitor, then we may get bigger power difference from P and M pin.

  • 0 in reply to Noel Fung
    Prodigy 10 points

    Hello Noel,

    Thank you for your elaborate response. The layout of the outputs on my board is not identical, which might explain the differences in my case. I am using broadband resistors and capacitors, which means that I can exclude the passives as the cause of the problem.

    I have one additional question regarding your measurement results. Do you think the difference at 7GHz is caused by having the 50Ω load at the SMA connector instead of it being directly placed on the PCB? Because aside from this both outputs of the evaluation board look identically routed to me. 

    Another effect I observed with the evaluation board is that the output power mismatch is dependent even on small frequency changes. I observed small jumps in output power when adjusting the divider in the feedback by 1 bit, which resulted to a change in frequency of <100MHz in my case. 

  • 0 in reply to Thomas Frei
    TI__Guru* 93670 points

    Hi Thomas,

    The difference may be due to the termination is at the SMA connector, I only have one board, otherwise I can cross check with another board. 

    The dependence on small frequency changes could be due to the synthesizer is using different VCO core. We have 7 VCO inside the chip, a VCO calibration will determine which VCO core to use. If the required VCO frequency is located at the boundary of two adjacent core, it is possible that f1 is using VCO core1 while f1+50MHz will be using VCO core2. If this happen, there will be an abrupt power change. 

  • 0 in reply to Noel Fung
    Prodigy 10 points

    Hello Noel,

     

    I think I omitted some information in my last message. The jumps in the output power were only observed on one of the two outputs (RFOUTA/RFOUTB), which is why I think this is not due to the used VCO core. This may still be because of the layout differences, maybe I will be able to do some additional testing in the upcoming days. But I would say that your answer with the main reason why I am measuring such a large difference in output power must be the layout difference is correct.