EPR Lineshape Measurement - Frequency Modulation Sweep:

Use this configuration when you wish only to find the Electron Paramagnetic Resonance (EPR) frequency, not actually track its position. This method allows the user to observe the lineshape of the EPR transition (Rb D2 line) light emitted from the cell as a function of applied frequency to the EPR coil.

First, construct the circuit described in figure by following these steps:

Figure: Circuit for EPR lineshape measurement

1. Position the PIN diode so that it looks directly at the light beam coming from the cell. If pipe is used for guiding the light from the cell to where the PIN diode is located, you may need to adjust the focusing lenses within the pipe such that most of the light is collected and reaches the PIN diode.

2. Measure PIN diode output, it should be a DC signal with an amplitude of -100 mV to -150 mV. If the signal is less than -100mV, check the light and PIN diode again.

3. Set Lock-in amplifier parameters:
   AC Gain: 50 dB; Input Limit: 10 mV;
   Sensitivity: 1 mV; DR 16;
   Time Constant: 100 ms; Osc: 0.000 Hz;
   

4. Connect PIN diode to input A of the Lock-in amplifier.

5. Set Modulation source DS345 parameters:
   Function: sine wave;
   Sweep/modulate: LIN SWP.
   

6. Connect DS345 'output' to PI circuit 'mod in', DS345 'sync output' to Lock-in Amplifier 'ref in'.

7. Set PI circuit Integration off, make sure the input is disconnected.

8. Set Wavetek 80 parameters:
   Modulation Mode: VCO;
   Output: sine wave;
   Operating mode: FUNC.
   

   Be sure that the VCO indicator is lit. (The VCO is used because it is very sensitive to small changes in voltage. At a carrier frequency of 10MHz, a 1mV input will produce a 400Hz shift in the output frequency.)

9. Connect PI circuit output to Wavetek 80 VCO IN through an 100:1 attenuator. Set the 100:1 attenuator switch to ``FMS'' (no attenuation).

10. Connect Wavetek 80 sync out to SR 620 Counter. Connect Wavetek 80 output to the cable leading to the EPR coils. Make sure that the Wavetek 80 output is not in standby mode.

11. Bypass the RF capacitor by setting the switch to 'EPR' on RF capacitor box front panel.

Now that the circuit is in place, open the LabView program called ``FMSweep.vi'' (in windows-diagram).

1. Input the start frequency for your sweep in the upper left of the screen. At the start of pumping, the resonance frequency should be around the number given by this formula:

   EPR base resonance = (466000 * holding field in Gauss)

   Thus, your sweep should begin below and end above this number. However the resonance frequency is different for different Rb polarization directions in the pumping cell. Typically with 25 Gauss holding field the starting sweep frequency is 11.1 MHz for right-handed light, or 11.5 MHz for left-handed light.

2. Input the voltage at which you will run the EPR coils. Typically, this is 8.0 Vpp, but it can go as high as 16.0 Vpp.

3. Next, set the step to be 1KHz and the number of steps to be 500. Keep in mind that the lineshape you are looking for is about 500kHz in width.
4. Set the number of points per sweep, as well as the sweep duration. Typically we take 2 points per sweep and 500ms for each point. The maximum speed should not exceed two points per second.

5. The program will automatically generate the data file to store the data, with a filename related to date and time the program is running. However, you can change the filename and path through the dialog bar located at the right bottom corner.

6. After all of the necessary data has been entered, the sweep may begin. To start the program, simply click on the white arrow in the upper left corner of the screen. You should be able to read the current Lock-in signal amplitude and the EPR coil frequency from the screen.

7. Under normal conditions, in the plot of Lock-in signal vs. EPR coil frequency showed by the program, you should find a dip followed by a peak, or vice versa.

8. You might need to adjust the Lock-in Amplifier phase. You can use ``auto-phase'' in reference channel menu or you can adjust it manually as follows:

   When the program goes at the maximum of the peak (or mininum of the dip), check the Lock-in Amplifier phase by reading the X and Y signals from Lock-in Amplifier front panel. Since the LabView program only reads X channel, the phase should be adjusted such that X channel has a big signal while Y channel is roughly zero (or only consists noise). If not, adjust the reference channel phase through the front panel menu of the Lock-in amplifier. You don't need to stop the program to do it, it is easier to adjust phase by checking the output of the running program simutaneously. After the phase is optimized, run the program again.

   Manual adjustment is useful when the signal is small compared to the background (cable resonance, noise etc.), where ``auto-phase'' will track the phase of the background.

9. An EPR signal of amplitude higher than 25 $\mu$V extracted by FM Sweep will be good enough to do the EPR polarization measurement.

10. Once the measurement is done, disconnect the output of Wavetek 80 to the cable leading to RF coil.