Diode ring mixers matching

Discussion in 'Wireless & RF Design' started by Ángel_1487627185, Feb 8, 2018.

  1. Ángel_1487627185

    Thread Starter New Member

    Feb 20, 2017
    I'm testing upconverting and donwconverting a DSB-SC signal using two diode ring mixers in a fixture that connects both mixers with a matching network.

    The first mixer has 81Mhz at LO, 100kHz at IF and an amplifier at RF. If I put a 50 ohm load at the amplifier output, the DSB signal looks as expected:


    The amplifier has Zo=40+20j and the mixer has Zi=0.5+80j. The mathing network I'm using has a 20p shunt capacitor (matching 40+20j to 50 ohm) followed by an L section (matching 50 ohm to 0.5+80j).

    If I connect both mixers with that network, the signal gets very distorted before and after the network:


    If I remove the low pass filter at the downconverting mixer IF port and replace it by a 50 ohm load, it has a very large impact on the signals:


    The mathing network seems to be working, because if I remove it:


    Someone can help me deciphering what's going on? Is this mathing network ok? If not, how is it done? Are those impedances well calculated?

    Thanks in advance :)