 Hello, we will continue to discuss the various components of an EPR spectrometer. We have seen the source of microwave which is usually a Klyström or a Gann oscillator. Why do we need to use my waveguides, how it restricts us to work only at a fixed frequency mode, we have also seen role of microwave cavity and how it allows us to place the sample at the maximum magnetic field with the minimum of electric field. The next important component in the spectrometer is the magnetic field and since the experiment involves keeping the frequency constant and vary the magnetic field, magnetic field has to be varied continuously and it is easily done by using an electromagnet and you know the electromagnet is possible if I have a coil and it pass current through this with I then current flows through this and it can form a magnetic field, solid nerd in fact. Now for EPR experiment we use a special type of coil, coil and Helmholtz coil, so let us just start from here coil goes here and then there is a gap and another part of coil comes here and then here we apply current here I goes here and here. So the magnetic field will form here and this is you can guess that this is the place where sample is kept there, but this is not good enough because the magnetic field strength will not be very high to increase that we insert a magnetic soft iron core here. So the magnetic field lines are forced to stay inside this is a soft iron core and coil is wound around this core and to complete the magnetic field lines these are you can delete this space this is a yoke, so the double yoke arrangement is used this is all made up of soft iron core. So here the magnetic field lines will have this sort of arrangement let us say similarly here this magnetic field lines are completed. So this gives very homogeneous magnetic field and the center of this two pole pieces there a current can be varied if you then the magnetic field also vary here what is required is that the magnetic field that is seen by the sample has to be very homogeneous why because many appear spectra come with very very narrow very closely space nine and narrow lines there if the field is not homogeneous or if homogeneity inhomogeneity is more than this width or even the separation here then this lines cannot be resolved a typical requirement is that this inhomogeneity within the sample you know the sample is kept in cavity. So let us say within the cavity dimension the inhomogeneity should be less than 10 milligrams this is the inhomogeneity and this is necessary in the presence of a magnetic field which is of the order of let us say 3 kilo gas which is our X band frequency which is the typical requirement of the homogeneity of the magnetic field there now how to measure the magnetic field very precisely usually one uses a gauss meter which is made up of Hall effect. So this Hall effect gauss meter the probe is kept somewhere here and the magnetic field can be measured now this measurement is usually good enough for common not very precise measurement where one does not require this type of precision one need this type of precision one uses another technique which is based on this NMR technique NMR gauss meter there is nothing but a very small NMR spectrometer is made and NMR sample is kept very near let us see here this is the NMR probe and this is the NMR machine small NMR machine. So here the resonance condition for NMR is just new if proton NMR is used there new will be G of proton bore magneton and B of this magnetic field here. So one looks at the resonance frequency of the sample of proton that is kept here and then frequency can be measured very accurately and with high degree of precision from this and knowing this one can find out this with a high precision that way the magnetic field can be measured. The next important component of the spectrometer is of course the detector. So detector job is to measure the micro power that is coming out of the cavity and which will carry the information about the absorption of microwave by the sample. So what it does is to convert the microwave power, electrical current or voltages so that it can be further amplified and recorded. Now the detector that is used in EPS spectroscopy is usually a detector silicon diode something like this this is a diode so diode is kept inside. So when radiation form this on this one this looks at the electric field of the microwave and the current is produced here which is related to the intensity of the microwave. Here I can put a voltmeter and put a resistance and current meter this one the resistance will be varied. So by adjusting the variable resistance I can vary the current. So this is a simple detector. Now the trouble is that this being a diode it is not a linear device. Current that is generated here because of the micro power is a non-linear function of the power that is falling on the diode and it looks like this diode current versus the power. So it is sort of like this type of. So at a very low level of micro power the current is very small and then at the power increases it shoots up. So the sensitivity therefore very much depends on where I am working. Now go back to our discussion of the microwave cavity where I said that the EPR absorption gives a small disturbance in the cavity and that causes some extra reflection of microwave from the cavity. So if the extra reflection is a measure of let us say reflected power given by this magnitude here then the diode current will change by very very imperceptible amount here to here very small amount. Instead the same disturbance if it is taking place somewhere here then you see that there will be this much change in the diode current. So here it becomes much more sensitive for the same change of reflected power from the cavity if the diode is working here it has become insensitive when it works here it is much more sensitive. So it is very important therefore to decide where we want to work obviously we want to work here. Now to do that I therefore have to adjust the operating position of the diode somewhere in this region here. Now this is easily done by giving a bias power to the diode that is you allow a constant microwave power to fall on the diode all the time and that you can decide where we are. Now this can be done two ways microwave power enters here and using this iris tuning arrangement here I match it very well and I said ideally a matching should be such that no power gets reflected from here. Instead of that suppose you sacrifice some amount of sensitivity and mismatch it deliberately let microwave power gets out of this and we bring this operating condition somewhere here that is the way one can do and then EPR spectrum will be adding more to the reflection and then that will be the EPR signal. But then you see we are sacrificing some amount of sensitivity by deliberately mismatching the cavity matching. So another way to do would be that we work at the critical matching so that no reflection takes place instead we find some other path to bring microwave to the diode which will see later in our EPR spectrometer diagram which is called the biasing the diode using a different path of the microwave power that falls on the diode. Now here how much bias power I can give the way it looks like the more the merrier type of arrangement that if you keep on increasing the sensitivity seems to be increasing but that does not happen like all electron device a diode also produces noise at this noise depends on the bias power. The bias power if you keep on increasing noise also increases and that is shown here quantitatively in this slide here. So this is the crystal current so increasing crystal current is possible by increasing the microwave power falling on the diode. So L stands for the loss for converting the microwave to the voltage so as the crystal current increases the loss decreases that is what I said that if you keep on increasing here the efficiency of conversion from microwave power to voltage increases but same time the noise which is less thermal noise which is related to some sort of parameter called temperature that keeps increasing here in the linear direction. So increasing crystal current or increasing bias increases the noise component. So you cannot keep on increasing the bias in this fashion here you have to have a certain compromise I get an optimum sensitivity and optimum noise. So with this we have covered all the major components of an EPS spectrometer. Before I conclude I like to introduce a unit of measuring this various characteristics of this microwave components and a microwave power and this unit is called unit of let us say attenuation or amplification which is called DB DB stands for decibel what it is before I say what it is it is all microwave components when they carry microwave they lose certain amount of radiation that is the loss this one here. So radiation goes through this tube this hollow portion here but idea is that it whatever goes in must come out nothing should be lost here but that is not possible no matter how good it is how good the conductivity is which decides the quality of trans loss there will be certain loss always present there. Now how much the microwave power is getting lost that is given by this unit DB now how do you define let us say some gadget is here power is going in power is coming out. So here that DB if it is a power is very lost of course P out will be less than P in that is some losses taking place DB in loss in DB is given as 10 log to the base 10 P in by P out. So this is a logarithmic scale so if let us say P in is P out into there is half the power is coming out here whatever reason then if you put it here then this will be loss in DB will be 10 log base 10 to 2 if you do this log 2 then 10 this will be about 3.0 something come. So this 3 DB loss is taking place here when half the microwave goes in there because of the logarithmic nature and also the base 10 very easy to mentally come out some quick number let us say that something like this that suppose the loss is a 20 DB then this 10 is there so this this ratio must be 2 so 20 DB here that means this ratio will be 100. So 100th of power is coming here that is 20 DB suppose instead of loss there is some device which amplifies the signal power goes in and comes out is higher than that the same way I can define the amplification factor in terms of DB that will be here this is higher than that again let us say if the 20 DB means the ratio will be 100 that is 1 milliwatt goes in 100 milliwatt comes out the amplification is 20 DB. So this is therefore a ratio measurement for any device there will be sudden loss because of its own intrinsic loss of this one and then there could be property of the device could also be such that in one direction micro power goes in and other direction it does not come out we will see some example later this is called non reciprocating device one of them is called the isolator. So this is the micro isolator and we drew it this way so the power goes in here it comes out here with very little loss so I call insertion loss let us say of the you know let us say 0.1 DB that means you can figure out from here that almost 99 percent micro radiation come out here also. But in other direction here micro goes in and what comes out there this could be attrition could be let us say about 20 DB that means if the radiation start enter here only about 100th of that will come out here. So this is the isolator it is a unidirectional device where micro power is allowed to go through in one direction. Again I am saying that these are the relative or ratio measurement which gives some idea about the characteristics of that. But sometimes we also like to mention the absolute power of the micro radiation and of course milli watt is the unit. Sometimes it is a milli watt one uses an equivalent definition of micro power in terms of DBM this is called DBM this is the absolute value of the micro power and in a very similar fashion this is defined as DBM is equal to small m here not a subscript DBM not subscript 10 log base 10 power by 1 milli watt. So here the reference power is kept at 1 milli watt. So you see that if it is power that is falling on the whatever system you can think of is 1 milli watt then this becomes 1 log 10 equals 0. So 0 DBM is 1 milli watt of power but if P is let us say 10 milli watt then in DBM will be this by this is 10 log 10 is 1 10. So 10 DBM power is equivalent to 10 milli watt of power. Similarly other round if power is equal to 20 DBM what will be the actual power this will be 20. So this ratio has to be 100 so 100 milli watt of power corresponds to 20 DBM this corresponds to 100. With this we end this discussion.