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EPR System Performance Tests

The following information describes procedures for testing the performance of the Bruker EPR spectrometer. The Signal to Noise Ratio Test measures the spectrometer's sensitivity. The procedure is especially designed to test as many of the components of the spectrometer as possible with one simple test. Therefore, it provides a good criterion for comparing the sensitivity of different spectrometers. The Cavity Background Signal Test measures the background signal of the cavity.

Signal to Noise Ratio Test

 

  1. Set the spectrometer up with the following parameters:

    Modulation Amplitude8.0Gauss
    Modulation Frequency100kHz
    Receiver Gain5.0 x 10^5 
    Phase0Degrees
    Time Constant1310.72msec
    Centre Field3480Gauss
    Sweep Width50Gauss
    Conversion Time163.84msec
    Resolution of Field Axis1024 
    Microwave Attenuation12dB


  2. Acquire a weak pitch spectrum. If the EPR signal is clipped, turn the receiver gain down and acquire the spectrum again.

  3. Set the microwave attenuator to 0 dB. The noise trace for the signal to noise ratio is performed at a higher power level. This tests the stability of the microwave source and the power supplies as well as other components which contribute to high power noise.

  4. Set the receiver gain to at least twice that used for the weak pitch EPR signal.

  5. Wait several minutes. The cavity must come to thermal equilibrium because the high incident microwave power heats up the cavity. After a few minutes, you should retune the cavity coupling and the microwave frequency.

  6. Set the spectrometer to Time Sweep mode. Enter the Parameters menu and the Measurement mode parameters submenu. Change the Measurement Mode to Time-Sweep. Enter the Field parameters submenu and change the Static Field to 3455 G.

  7. Set offset on signal channel. Sometimes, when running with extremely high gains and high microwave power, you may have to adjust the DC offset of the signal channel. Enter the View Signal mode in the microwave bridge controller menu. Press the space bar to invoke the on-line parameter editor. Adjust the offset (rof) until the noise trace is centred in the display.

  8.  Acquire a noise trace. The amplitude of the noise should be large in order to get a good estimate of the amplitude. If the signal clips, turn down the receiver gain.

  9. Measure the peak to peak height of the weak pitch signal. Plot the spectrum using the Plot display, digital command in the Plot commands submenu. Please note the scale factor for the plot.

  10. Measure the noise level. Plot the spectrum using the Plot command submenu. Please note the scale factor for the plot. Draw two straight parallel lines which encompass most of the noise trace. The peaks in the noise should cross the lines approximately six to ten times across the whole page.

  11.  Calculate the signal to noise ratio. The formula for calculating the signal to noise ratio is:

     
    Formula for the calculation of the signal to noise ratio

     



  12.  Signal Amp. is the height measured in step 9., Noise Amp. is the noise level measured in Step 10., and Weak Pitch Conc. is a calibration factor which takes into account the concentration of spins in the sample. The weak pitch concentration is printed on the side of the weak pitch sample tube. The factor of 2.5 translates the noise level into a RMS (Root Mean Square) noise level. The factor of 4 corrects for the fact that the signal and noise trace were taken at different (by a ratio of 16) incident microwave power levels. The square root of 1.31072 corrects the signal to noise ratio for a time constant of one second. The signal to noise ratio calculated from the equation above should be at least 330 for the spectrometer to meet specification.

     
    Formula for the calculation of signal to noise ratio with values inserted

 

Cavity Background Signal Test

 

  1. Run part of a signal to noise ratio test. Acquire a weak pitch spectrum as described in the previous section. Instead of acquiring the spectrum at 12 dB microwave attenuation, it should be run at 3 dB. You may have to adjust the receiver gain to keep the signal from clipping.

  2. Measure the peak to peak height of the weak pitch signal. Plot the spectrum using the Plot display, digital command in the Plot commands submenu. Please note the scale factor for the plot.

  3. Remove the weak pitch sample. Store it in a safe place. Retune the cavity and bridge after removing the sample.

  4. Set the spectrometer up with the following parameters:

    Modulation Amplitude8Gauss
    Modulation Frequency100kHz
    Receiver GainSame as
    for weak
    pitch
     
    Phase0Degrees
    Time Constant1310.72msec
    Centre Field2600Gauss
    Sweep Width5000Gauss
    Conversion Time163.84msec
    Microwave attenuation3dB

  5. Acquire a background spectrum with the parameters in the table above.

  6. Measure the height difference between the highest and lowest point of the background spectrum. Plot the spectrum using the Plot display, digital command in the Plot commands submenu. Please note the scale factor for the plot.

  7. Calculate the background signal to weak pitch signal ratio. Use the following formula:

     
    formula to calculate the background signal to weak pitch signal ratio

     



    The specifications for an ER 4102ST cavity is that this ratio not exceed ¼.

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