ADMV1014 IF filter - must it be reflectionless?

Hello EZs,

I'm designing a PCB using the ADMV1014. I don't require a large IF bandwidth so don't really care about anything above 300 MHz, therefore am putting a low-pass filter on the I and Q differential outputs. 
My current filter choice (Mini-Circuits' LFCG-320+) is reflective in its stopband (500 - 6000 MHz) - what will happen when most of the output power at frequencies above 300 MHz is reflected back to the ADMV's I_P, I_N, Q_P and Q_N pins? Is this okay, or should I get a reflectionless filter?
Cheers
Joe
  • +2
    •  Analog Employees 
    on Dec 2, 2019 6:42 PM 10 months ago

    Hi Joe,

    If your Baseband Bandwidth (I'm assuming you're using IQ Mode) is up to 300MHz then I would be concerned about the 2*RF-8*LO (LO frequency referenced from ADMV1014 LO ports) frequency at 600 MHz. The datasheet spur table suggests that this would be approximately 60dBc lower than the main baseband output. Even so the Mini Circuits filter has a very poor VSWR (~12) at 600MHz and all the power at 600MHz will be reflected back into the part. If you already have these filters then I would strongly recommended adding a 3dB or even better 6dB attenuator in line between the filter the and IQ pins. 

    Thanks,

    Taz

  • Hi Taz,

    Thanks for your detailed reply! 

    I am indeed using IQ mode. What happens to the power reflected back to the ADMV I/Q pins? Is it absorbed internally?

    Cheers

    Joe

  • 0
    •  Analog Employees 
    on Dec 14, 2019 5:21 PM 10 months ago in reply to airdas

    Hi Joe,

    The reflected signal will be absorbed back in to the part. There is a BB amplifier after the mixer so there is less chance of self mixing and degrading the DC offsets.

    However, the reflected signal could degrade the SNR and impact the common mode voltage generation circuitry on the BB amplifiers. 

    Thanks,

    Taz