- 1 Basic Instrumentation for EPR system
- 2 Circuit Diagram for EPR system
- 3 EPR/NMR Frequency Sweep Calibration Procedure
- 4 Measurement Procedure
- 5 STEPS
- 6 NOTE
- 7 MORE NOTE
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)
Circuit Diagram for EPR system
EPR/NMR Frequency Sweep Calibration Procedure
- Lasers are ON.
- Holding field is stable.
- Oven/Cell temperature is stable.
- D2 light output sufficient(>20mV).
- All cable connections are good. Special attention to: Pumping chamber pickup coil/Lock-in Trigger/D2 diode.
- No data taking.
- No beam.
- Automatic spin flip is Paused .
- Target is in He3 position.
EPR FM Sweep
Use the code
- Disconnect the Input cable on PI Box and switch integration gain to OFF.
- Run EPR FM sweep code.
- Load Default.
- Check RF power (-5dBm ~ -10dBm).
- Check Frequency Sweep Range (~16MHz for spin up pumping, ~20MHz for spin down pumping).
- Check Sweep steps (5~10kHz).
- Check FM deviation (~100kHz/V for high polarization).
- Do FM Sweep and optimize settings.
- Create a snapshot of final sweep and exit the program.
NMR Frequency Sweep (optional)
Use the code
- Make sure the NMR pumping chamber lock-in has trigger from Frequency Sweep.
- Load the settings for automatic spin flip.
- Do a quick sweep to make sure the frequency range is large enough to include the resonance.
- Make sure the phase is right.
- Make sure the sensitivity is large enough so that no saturation will happen.
EPR AFP Measurement
Use the code
- Connect the EPR lock-in output cable to the Input of PI box, turn the integration gain ON.
- Run the program.
- Check the settings of NMR Frequency Sweep and make sure they are the same settings of auto spin flip.
- Make sure that Saving Data button is pushed.
- Turn ON RF.
- Do 2 pairs of automatic flip with 5-10 second wait time.
- If do manual flip, make sure that the total number of flips is even.
- Turn OFF RF.
- Record data directory and cell temperature.
- Make a snapshot and exit.
Use the code
- Open the latest EPR data folder.
- Run the program.
- Input corresponding pumping direction.
- Input recorded average pumping chamber and target chamber temperature.
- Record polarization and calibration constant.
- Create a snapshot.
- Click the Exit button, so that the calibration constant will be automatically saved into registry.
Back to Production (Optional)
- Redo the setup of the automatic spin flip.
- Continue automatic spin flip.
- Ask for beam and continue production.
Submit a Report
- Post a Halog with all information and snapshots collected during the EPR calibration.
- 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 .
- 10 --- 12 MHz ( Rb)
- 15----18 MHz ( Rb + K)
- 19----20 MHz (K)
- 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.
- 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.
- Check the output from the D2 diode 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.
- 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.
- Open the EPRFM.VI and click on the RUN button.
- Set the Start Freq and stop frequency according to your wish .i.e. the range mentioned above.
- Set the step size = 0.01 MHz (10 kHz) for a quick scan. Or 0.005 MHz is more or less nominal.
- Set the lockin time constant = 100 or 200 ms ( should try both )
- Set the lockin sensitivity = 200 uV/ 500 uV/ 1mV nominal ( I cant say unless I sit with you at this moment )
- 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
- Set the gain = - 5dB ( might need to adjust later, but no less than -12 dB otherwise the counter will not register signals ) . But just to start with. You can use -2 dB as well.
- Lets just not care about the phase of the lock in now.
- Set the modulator frequency to 100 Hz and the amplitude to 1.5 Vpp.
- [START SWEEP]
- Lets say, you have the lineshape signal . [PLOT] it.
- Choose /select the linear regions of both the channels ( X and Y )
- [FIT] with order 3 unless it is really really linear. So its better to FIT order 3 at the beginning.
- Once you fit , take a look at the calculated PHASE ( Just take a look, nothing needs to be done )
- [SET PARAMETER]
- click on [I KNOW I KNOW]
- [START SWEEP] again
- This time you should see all the signal is in one channel ( X- usually unless the program is broken ) and you have the correct phase ( more or less )
- Follow steps 13 -- 18 again. In step 16, this time you probably need ( can ) use FIT ORDER 1.
- 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.
- 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 !
- The parameters that you can adjust for broadening the signal are FM deviation and Modulator amplitude. Sensitivity and the RF gain will have some effects but not very large. Now to be honest, its simply impossible to write "HOW" here.
- If you really dont have any idea .....or afraid of messing things up...just let the target expert know
- 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.
- Refer to the circuit diagram. Keep everything same as they were.
- Connect the lockin output from the back of the lockin to the INPUT of the PI box.
- SET the GAIN to the number you've got from the FM sweep vi.
- Open EPRAFP.VI
- RUN it. SET the DEFAULT parameters. Freq. 78 to 92 KHz. V_rms = 0.8 V.
- Set it to MANUAL or AUTO. If AUTO , SET the wait time to 15 sec/20 sec.
- 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.
- 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. ( The magic numbers we found is 1-1.2 for 100ms time constant and 1.5-1.7 for 200ms time constant)
- 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.
- WAIT for couple of seconds
- PRAY TO GOD.
- START SWEEP. You should hear a "click" and see a drop or rise in the frequency ( "well" or "hat" shape) in the first plot.
- 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.
- Repeat the above two steps one more time or two.
- Make sure you end up in the original state. Then POWER OFF. SAVE the data
Couple of things here are worth mentioning. First of all, there are no standard TROUBLESHOOTING procedures in this measurement. Numbers of factors can affect the measurement either alone or together. So there is no way one can write all. Still there might be some common trivial problems that usually occur.
- Say, you start the sweep and the signal can not be locked fully. Try increasing the INT GAIN first. If not so effective, try increase the GAIN a little bit. Try to play around more some time. This will be effective only if you are within the range of the FM signal. You remember the +/- 50 kHz , I mentioned. So if you are really near the cross over point ( the resonance frequency) and you think the number you got for the turns in the GAIN from the FM sweep vi is after you adjusted the width of the FM signal large enough to cover the frequency shift the first plot of the AFP vi. This is important because if your signal is too narrow in the FM sweep, your flipping will result in somewhere else rather than near the zero crossing resonance. SO no matter how much you try to adjust the GAIN / INT GAIN.....you wont get anything. But if you are sure that you are fine within the range , this playing around with the GAIN/ especially with the INT GAIN will be sufficient enough.
- If you see no FLIP when you start a sweep, try couple of times again . But always remember to go back to the original state. Check the connection, check the photodiode signal. If those are fine, you are probably OFF from the resonance. You can tell roughly from your second plot as well. DO THE FM SWEEP AGAIN and GET THE PARAMETERS.
- Sometimes we experienced in the lab that the communication between the VI and the electronics delays the whole process. So if other things look okay, try to restart all the electronics. But this should be the last step.....
- If you have the AFP signals , but very noisy....PLAY AROUND with the SENSITIVITY and TIME CONSTANT of the lock in. But remember , change one thing at a time. The effects are correlated !! I cant say right here whether to decrease or increase...but if you have very huge fluctuations , probably you need to DECREASE the time constant . The safe value for TC is 100ms, 200ms and the safe value for SEN is 200 uV to 1 mV.