Testing for a defective LNA

Discussion in 'Wireless & RF Design' started by Theor, Apr 8, 2015.

  1. Theor

    Thread Starter New Member

    Aug 1, 2012
    Hi everyone. I've purchased an LNA from Minicircuits and I don't get any signal from it.
    The LNA is connected on an antenna and its range is 1250-1650mhz.

    Connecting the receiver on the antenna with no LNA, I can receive the usual signals from the FM radio, GSM, GPS and so on, but I just get line noise with the LNA. Powered or not makes no difference, it basically acts as a full-band filter.

    I don't have a signal generator to thoroughly test the L band, but I'm suspecting the unit is defective and I'm not sure how to test it. Could have I inadvertently fried it despite using a regulated 12v power supply? Impedance between VCC and GND is ~6 ohms, hasn't changed between when I received it and now.
  2. seanstevens


    Sep 22, 2009
    Hi Theor,

    I am not an expert on this subject but as I have done a lot of RF work both transmitter and receivers including LNAs, I thought to pass on my opinion seeing as no on else has.

    This is obviously a narrow-ish band LNA which means it has its own filters in which case it should cut out a lot of the type of signals you described as being able to receive without the LNA FM radio, GSM etc. when you use the LNA.

    However, a concerning point is that you have a ~6 Ohm impedance, I presume resistive as measured using an Ohm meter, in which case 12/6 = 2 Amps of current would flow in the circuit! Is this just an LNA or is the LNA a part of an RF amplifier? LNA generally takes a few mAs to operate, have you measured the current consumption while the unit is powered up?

    Another thing is that many LNAs use GASFET active parts (transistor / IC) which are very sensitive to static discharge both from person to the device (handling) as well as atmospheric discharge to the antenna the LNA is connected to which could also damage the active device and generally make it 'deaf' and useless.

    If not done already, I would do a quick current measurement, keep an eye on temperatures as if the impedance is around 6 Ohm, 2 Amps would be flowing through the circuit if your power supply can supply that and that would seriously heat something up.

    Good luck.