EPR Measurement

COMING......little bit lazy

Basic Instrumentation for EPR system

• HP/E4400B ( RF Function Generator)
• SR620 ( Frequency Counter)
• DS345 ( Modulator)
• EG&G7265 ( Lock-in Amplifier)
• PI Feedback Box ( HOME MADE ----UKY)
• RF amplifier
• UDT-10 or ( Photodiodes)
• ( D2 Filters)

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

1. AFP sweep (The real polarization measurement)