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LMH2832EVM-50: The eval board does not have max gain as advertised.

Youngho Suh
Intellectual 490 points
Part Number: LMH2832EVM-50
Other Parts Discussed in Thread: LMH2832

Hi,

I evaluated LMH2832-50 EVM with input signal of -44dBm at 45MHz. With GUI, I set the gain of 30dB but found output power of only -22dBm, only 22dB gain rather than 30dB. It looks like gain control is good from 30dB to -9dB but absolute gain is not there. Is this inherent to EVM that loose 8dB but will have 30dB gain with differential interface? Or something else goes on?

Also, about bias control at GUI, it does not change gain or linearity by changing the bias. Can you let me know what exactly the bias control helps?

Thanks,

  • 0
    TI__Genius 15975 points

    Hi Youngho,

    The 30dB max gain advertised for the LMH2832 is the voltage gain for the part with differential interface, which is different than the power gain (dBm) you are measuring on the LMH2832-50 EVM. Also, the 30dB voltage gain does not account for losses incurred on the EVM, which you would otherwise measure on your setup. Would it be possible for you to attach the measurement setup of the gain evaluation?

    For measuring power gain on the LMH2832-50 EVM, you would expect 4.77-dB loss at the LMH2832 output due to the two 40-ohm output series resistors and output transformer. Also, there is a 1-dB power loss each from the input and output transformers. So, you should really see ~ 6.77-dB power loss from the 30-dB max gain or in-total 23.23 dB power gain (see below). An additional 1.2dB loss could be accounted for the cables at the input and output, which would really explain the 22-dB total power gain from the LMH2832-50 EVM.

    Best Regards,

    Rohit

     

  • 0 in reply to Rohit Bhat
    Intellectual 490 points
    Hi Rohit,
    Thanks for the explanation. My test setup is very simple. -44dBm in from signal generator + LMH2832-50 EVM + spectrum analyzer to measure the gain. I understood why the power gain was lower than I expected. So, mainly losses from balun and output mismatch cause this extra 6-7dB losses from your explanation.

    Another question I have:
    My circuit configuration with this device will be LMH2832 + differential BPF at output. the filter input impedance will be 150 ~200ohm and will not use 40 ohm series resistors and 0.1uF series caps. Then this signal will go to connectors such as QMS-026-06.75-L-D-A. And this signal will be going to ADC eventually.
    If I do like this, I expect I could have full 30dB power gain since no baluns associated and impedance matching is all 150 ~200 ohm. will this be correct?

    I wonder why the EVM output port has 1:1 balun rather than 3:1 to match with 50 ohm? Just curious.

    Thank you.
  • 0 in reply to Rohit Bhat
    Intellectual 490 points
    Hi Rohit,
    I will put low insertion loss LPF after the amplifier (About <1.5dB insertion loss). The filter was designed with input impedance of 150 ohm. Then do I need to put external series resistor with 130ohm as shown in Table 2 of the datasheet? Section 9.3.3 described I may not need resistor if low insertion loss filter was used. can you advise if I need to put two external series 130ohm resistor before the 150ohm differential input filter?

    Thanks
  • 0 in reply to Youngho Suh
    TI__Genius 15975 points

    Hi Youngho,

    It is possible to design a low insertion loss filter after the LMH2832, but I would still add 5-ohms series resistor on each output just in case you run into stability problems. However, I don't think you need to put two external series 130ohm resistor for building your differential 150-ohm filter.

    The device does support full 30-dB voltage gain, but based upon the impedances seen at the input and output the power gain will change. If your input and output to the LMH2832 are referenced to 150-ohms, then you should achieve 30-dB power gain.

    The EVM output has 1:1 transformer instead of 3:1 because it allows us to test the LMH2832 output with heavier loads by reducing the series resistor.

    Best Regards,

    Rohit