HMC681A Low Frequency Performance


the datasheet for the HMC681ALP5E says its frequency range of operation is DC-1GHz, but the performance curves shows going from 50 MHz up to 1 GHz only, I would like to know if this component is suitable for operation near 10 MHz, and if at these lower frequencies its characteristics are similar to those at 50 MHz as showed in the datasheet.


  • 0
    •  Analog Employees 
    on Aug 1, 2016 11:15 PM

    Hi ADIguy,

    The HMC681A will operate down toward DC with the limitations described in the post linked below:

    HMC681A DC Blocking Capacitor For RFin

    That post will give you an idea regarding the identical Darlington amplifiers that are contained within the package.

    For the 50-350 MHz EVB tuning we have some S-parameter and OIP3 data below 50 MHz...but that data doesn't tell the entire story since the main elements limiting the low frequency performance are C1, C2, C9, C10, L1, and L2, highlighted below:

    L1 and L2 function as RF chokes so, we need their reactance magnitudes to be high at the frequencies of interest so that they block the RF from the DC bias line. C1, C2, C9, and C10 function as AC coupling caps/DC blocks so, we need their reactance magnitudes to be low at the frequencies of interest so that they pass the RF signal.

    I recommend that you obtain the 50-350 GHz EVB (part # 119171-HMC681ALP5) and measure it in its stock configuration to evaluate performance below 50 MHz. Then change the values of the capacitors and inductors referenced above so that they're more suitable for operation below 50 MHz. That means increase both the capacitance and inductance values so that the capacitors are of adequately low reactance magnitude at 10 MHz (suggest <1 ohm) to pass the signal and the inductors are of adequately high impedance magnitude at 10 MHz (suggest >200 ohms) to block the RF from the DC bias lines.

    In general, you'll want to make sure that those inductors and capacitors provide adequate response across your entire operating frequency range of interest (i.e., you might need to consider component specifications such as SRF).