Let’s Use ESR III. Tuning/ Matching Microwave Bridge

ESR may be used to measure a wide range of samples. However, it is necessary to determine the most appropriate conditions for each sample. Here we explain how to adjust the microwave-bridge. Before placing the sample in the tube into cavity resonator, open Q-Dip window. Press the MOD button on the screen. Next, place the sample in the cavity. Be sure to set sample in this order.

• Apply about  1 mW microwave power.  A curve as shown below is normally displayed on the screen.
  
• In the REF-OFF condition, change Frequency so  that  the  Q-dip  comes  to  center of the screen. Then, adjust Coupling so that bottom of the Q-dip becomes deeper.
  
• Turn REF-ON. The Q-dip is often distorted.
  
• Adjust Phase so that Q-dip becomes symmetric and the bottom is at the center.
  
• Turn AFC-ON. Q-dip disappears and a central spot or point appears, and the AFC meter becomes active.
  
• Check the position of Detector Current at 0 mW microwave power.
  
•  Increase Power slowly. The Phase and Detector Current indicators gradually change. Slide Phase and Coupling to keep Phase indication at a minimum and to keep Detector Current indication the same as that indicated  at 0 mW.
  
• Gradually increase Power to 200 mW, keeping the Detector Current indication unchanged. This position is called “just-coupling”, where the highest sensitivity measurement may be expected.
  In adjustment, pay attention so that meter does not reach the maximum.
  After attaining “just-coupling”, be sure to reduce the Power to the appropriate level for the actual measurement.
  When exchanging a sample, be sure to reduce the power to less than 1mW.

Depending on the sample, there may be cases when the Q-Dip is not obtainable at stage 2. Also, even if a Q-Dip is obtained, as the microwave power is increased, DET CURRENT moves a lot, so it is unstable even at only a few mW. In such a case, it is possible that the sample has a large microwave loss. So, please consider the sample tube and the volume of sample again by referring to Application Note ER-060004.