SRA-3MH Overloading

Discussion in 'The Projects Forum' started by Twigg, Jul 31, 2015.

  1. Twigg

    Thread Starter New Member

    Jul 25, 2015
    Hi all,

    I'm building a low-side downconverter using an SRA-3MH mixer from MiniCircuits based on someone else's published design. The prior designer claims that they fried a lot of these mixers from too high of an LO power. That made me scratch my head because, while I'm new to all things RF, I thought that mixers were supposed to be designed to work well at a fixed power level. It could just be he didn't attenuate the LO signal well enough, but I thought of another possible explanation I wasn't sure about and want to get some feedback on.

    The schematic can be found on page 49 of this PDF.

    I'm aware there are a bunch of errors, of which I've probably only caught a handful thus far, but what I'm interested in is the SRA3MH mixer and its inputs and outputs.

    So here's my theory. From what I've learned about this mixer, it's essentially an RF switching circuit composed of a Schottky diode bridge gated by the LO input. Since Schottky diodes are ideal fast switches, I figured that virtually all of the LO power is rerouted to the IF output. But at the IF output, you have the input impedance of the op amp (~10^12Ohms), so virtually all of the LO power gets reflected back into the mixer. With nowhere else to go, the excess power gets dissipated in the Schottky diodes until they cook.

    Am I totally off here? If I am correct, then couldn't I just put in a small shunted load before the op amp and take the burden off the mixer itself? I would test it out, but I've only got two mixers and I'm a bit of a wuss.

  2. alfacliff

    Well-Known Member

    Dec 13, 2013
    the mixers do require a load, check in the minicircuits labs literature for info.
  3. Twigg

    Thread Starter New Member

    Jul 25, 2015
    Thanks, very good to know.

    I've been all over the datasheet but there's nothing indicating a load. Would calculating a load resistance value based on the RF and LO powers and a safe current level work or is there more going on?

    For example, say the input RF power was 50mW, the LO power was 20mW, and I wanted an IF current of 10mA. Could I just use a load with a resistance equal to 2 * 70mW / (10mA)^2 = 1.4K?
  4. bertus


    Apr 5, 2008
  5. Twigg

    Thread Starter New Member

    Jul 25, 2015
    This explains a lot. Thanks!