FT114-43 Oscilloscope CMRR

Step 1: Calibrate Oscilloscope

  1. Set Oscilloscope for 50Ω Input
  2. Setup Signal Generator for 50Ω output at 14MHz.
  3. Connect 50Ω Oscilloscope input to Signal generator
  4. Adjust Signal generator output for, example, 1.8Vrms at Oscilloscope
Oscilloscope measuring 1.8Vrms calibration voltage


Step 2: Measure Common Mode Voltage

  1. Place DUT Balun onto CMRR Test Fixture
  2. Connect Signal generator to Fixture P1 and Oscilloscope to P2
  3. Adjust Oscilloscope sensitivity for measurement
Oscilloscope measuring 2.345mVrms common mode voltage


Step 3: Compute CMRR

  1. Compute dB for 1.8Vrms Input and 2.345mVrms common mode voltage
    20 x Log10(1.8V/2.345mV) = 57.7dB
  2. Subtract Fixture 10x current transformer, 20dB
    57.7 - 20 = 37.7dB CMRR


Step 4: Compute Balun Choke Impedance

  1. Compute Common Mode Current
    2.345mV / 50Ω = 0.0467mA
    Remove Fixture 10X current transformer, 0.0467mA x 10 = 0.467mA
  2. Compute Choke Impedance as (V/2) / Current
    (1.8 / 2) / 0.467mA = 1927Ω


Step 5: Measure CMRR at Multiple Frequencies and Plot

  1. Measure at several frequencies
    Frequency
    MHz    		Common Mode
    Vrms
    1 11.05
    1.5 7.02
    1.8 5.8
    2.5 4.3
    3.5 3.44
    5 2.9
    7 2.58
    10 2.39
    14 2.33
    18 2.37
    21 2.42
    24 2.49
    28 2.6
    30 2.67

  2. Plot Data
    gnuplot CMRR equation "(20 * log10(($2/1000)/1.8) + 20)"

    Frequency versus CMRR