EPR Measurement

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COMING......little bit lazy


Basic Instrumentation for EPR system


Circuit Diagram for EPR system

Measurement Procedure

  1. Frequency Modulation

Refer to circuit diagram. Very naively speaking,this is a frequency sweep process roughly from 9 MHz to 23 MHz. This is because all the resonance frequencies of Rb and K ,you are going to find ,in the range of our holding field reside within this range. We can know roughly where the resonance frequency is corresponding to our holding field. This is basically from theoretical calculations and also based on your personal experience in "EPR" you gathered since years. Therefore we just need to scan a short range rather than looking a signal in that huge range of 15 GHz. Just from our experience in lab. , the following ranges are sufficient as an initial step. BUT NO GUARANTY AT ALL !!! BECAUSE THIS IS NOT SO SMOOTH AND SIMPLE AS ONE CAN WRITE .

  1. 10 --- 12 MHz ( Rb)
  2. 15----18 MHz ( Rb + K)
  3. 19----20 MHz (K)
  4. 21----23 MHz (K)
Here it is assumed that the polarization is not very low. Otherwise , you are going to see some more signals for free. Also field is assumed to be in the range 25 to 27 G.

STEPS

  1. Refer to the circuit diagram and make sure the output from the back of the lockin is NOT CONNECTED to the PI box and the integration gain is OFF.
  2. Check the output from the photodiode with the oscilloscope first to make sure you have more than 120 mV DC. If you are getting ~220/230 mV ....dont worry ...you are still fine. Connect it to the lockin as input.
  3. Make sure the RF amplifier is ON. If you are using the comparatively small one from Kentucky, put the knob at 12 o'clock position.We did measurements at as low as 9 o'clock position. But lets just not make things complicated. But if you are using the big blue old dumb amplifier , you have no option . Just turn it on and hear for the sound. The indicator light does not work.
  4. Open the EPRFM.VI and click on the RUN button.
  5. Set the Start Freq and stop frequency according to your wish .i.e. the range mentioned above.
  6. Set the step size = 0.01 for a quick scan. Or 0.005 is more or less nominal.
  7. Set the lockin time constant = 100 or 200 ms ( should try both)
  8. Set the lockin sensitivity = 200 uV/ 500 uV/ 1mV nominal ( I cant say unless I sit with you at this moment)
  9. Set the FM dev = 20 KHz first . Later need to be adjusted and make it larger depending on the signal. But you can start with 40 or 50 KHz as well
  10. Set the gain = - 5dB ( might need to adjust later) . But just to start with. You can use -2 dB as well.
  11. Lets just not care about the phase of the lock in now.
  12. START SWEEP
  13. Lets say, you have the lineshape signal . PLOT it.
  14. Choose /select the linear regions of both the channels ( X and Y)
  15. FIT with order 3 unless it is really really linear. So its better to FIT order 3 at the beginning.
  16. Once you fit , take a look at the calculated PHASE
  17. SET PARAMETER
  18. click on I KNOW I KNOW
  19. START SWEEP again
  20. This time you should see all the signal is in one channel ( X- usually) and you have the correct phase ( more or less)
  21. Follow steps 13,14,15,16,17 again. In step 15, this time you probably need ( can ) use FIT ORDER 1.
  22. When you are done with the SET PARAMETER and I KNOW I KNOW , record the number of turns for the gain in the PI box. Make sure that the overall gain we are shooting for is -0.5. It is the default value in the VI . But if you find its different, SET IT TO -0.5 and RECALCULATE the number of turns again. This NUMBER OF TURNS in the PI gain is the heart of our measurement. Please pay enough respect to this number and also importance.

NOTE

  1. To have our PI box properly working for the real AFP measurement, always try to maintain a width of 100 KHz or more of the FM sweep spectrum which implies +/- 50 KHz around the resonance point. This is because if our polarization is reasonably high , the shift of the frequency you are going to have in the AFP spectrum is more than 50 KHz and the PI box needs to lock within that !
  2. The parameters that you can adjust for broadening the signal are FM deviation, Sensitivity and the RF gain. Now to be honest, its simply impossible to write "HOW" here.
  3. If you really dont have any idea .....or afraid of messing things up...just let the target expert know
  4. Usually NO SHIFT WORKER DOES EPR.






2. AFP sweep (The real polarization measurement)

Once you are done with the FM sweep and have the number for the turns for the GAIN for the PI box, follow the procedure below to get the AFP sweep done.

  1. Refer to the circuit diagram. Keep everything same as they were.
  2. Connect the lockin output from the back of the lockin to the INPUT of the PI box.
  3. SET the GAIN to the number you've got from the FM sweep vi.
  4. Open EPRAFP.VI
  5. RUN it. SET the DEFAULT parameters. Freq. 78 to 92 KHz. V_rms = 0.8 V.
  6. Set it to MANUAL or AUTO. If AUTO , SET the wait time to 15 sec/20 sec.
  7. POWER ON. You will see the frequency ( GREEN) in the first plot, frequency fluctuation in the second plot and lockin signal in the third plot. Just dont worry about those at this moment.
  8. SET the Integration gain at knob = 1 first in the PI box. Usually between 1 to 2 should be okay. But you have to play around.
  9. TURN the INT GAIN ON in the PI box. Yiu should see in the third plot....the lockin signal approaching 0. +/- 5 mV is okay enough.If not , try to increase the INT GAIN slowly till you feel comfortable.
  10. WAIT for couple of seconds.
  11. START SWEEP. You should hear a "click" and see a drop or rise in the frequency ( "well" or "hat" shape) in the first plot.
  12. If you are doing in MANUALLY, wait for 10 to 15 sec in that flipped state and hit "START SWEEP" again to get back to the original state. If you feel you need more data in that flipped state, wait for another 10 sec.
  13. Repeat the above two steps one more time or two.
  14. Make sure you end up in the original state. Then POWER OFF. SAVE the data
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