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LMX2492EVM: RF output power

AY
Prodigy 180 points
Part Number: LMX2492EVM
Other Parts Discussed in Thread: LMX2594, LMX2492,

I was measuring the power of RF_out port a 10.5GHz and found that the output power is at the level of approximately -5dBm. According to the datasheet of the VCO, the minimum output power at the RF port is +4dBm and a typical output power at +7dBm. With a resistive power divider on the evaluation board, I assumed that the power at RF_out port should be approximately -2dBm(i.e. +4dBm - 6dB), but I got -5dBm. I am trying to find out where is the extra losses coming from. How good is the resistors used in the divider at 10GHz?

I crossed checked the EVM manual where the RF_out/2 port was measured and the frequency was 4.8GHz. There is no power divider on the board for the RF_out/2 port and the output power from the VCO chip should be +1dBm(min) and +4dBm(typ). Figure 6 in the manual shows that the power is -3.15dBm. Again, why is the measured output power much lower than the level specified in the VCO chip datasheet?

  • 0
    TI__Mastermind 28001 points

    AY,

    Indeed there are losses and also loss due to mismatch.  The attached document has a chapter on the end that discusses how the output power was measured.

    Also on the LMX2594 EVM, we had a few that had some poorly soldered SMA connectors, especially the center pin.  Try another output to ensure it is not this.

    For the resistors, we use a special 50 ohm RF resistor and special RF capacitor on the board.  In any case, you should be able to get much more than -5dBm at 10 GHz.

    Regards,

    Dean8446.High Frequency Open Collector Outputs.pdf

  • 0 in reply to Dean Banerjee
    Prodigy 180 points
    Hi Dean,

    It seems like the LMX2594 has an integrated VCO with open collector configurations. However, the EVM that I am using is for LMX2492. There is a MACOM VCO external to the chip on the evaluation board. So, the attached document might not be applicable for LMX2492.

    Can you explain more for LMX2492EVM?
  • 0 in reply to AY
    TI__Mastermind 28001 points
    AY,

    Oh yes, I mixed up LMX2492 and LMX2594.


    The resistors from in the splitter are not at all specialized for RF nor is the AC coupling capacitor. Nor is the SMA connector. We have done some experiments at high frequency and found that these components can definitely matter.

    It is very conceivable that there is a substantial amount of loss on this board for the output of the VCO. As this board uses an external VCO that is not manufactured by TI, we really didn't spend much focus on trying to get the highest output power as we were not really trying to showcase the VCO, but rather our PLL. In other words, higher output power could be possible.

    On the divided output, we see there is still loss, but much less. This makes sense to me as at lower frequencies, losses tend to be less.

    Regards,
    Dean