Lecture 1 - experimental

Question 1

• Phase sensitive detection compares the modulated signal with a … … having the … frequency as the… … and is only sensitive to signals which have the … frequency and phase as the … …
• The signal channel produces a DC signal proportional to the … of the … … …
• Results in absorption signal being the same modulation frequency and can … … … at this modulation frequency therefore … …
• Field modulation and lock in detection means absorption spectrum (black) shape becomes 1^st derivative (blue) absorption shape which has much … …

Answer 1

• Phase sensitive detection compares the modulated signal with a reference signal having the same frequency as the field modulation and is only sensitive to signals which have the same frequency and phase as the field modulation
• The signal channel produces a DC signal proportional to the amplitude of the modulated EPR signal
• Results in absorption signal being the same modulation frequency and can lock in detect at this modulation frequency therefore eliminating noise
• Field modulation and lock in detection means absorption spectrum (black) shape becomes 1^st derivative (blue) absorption shape which has much higher sensitivity

Question 2

• Magnetic field adjusted very precisely by varying current in … of … Field controller utilizes … … to measure the field and the feedback loop to ensure that the field is set precisely, and the field sweeps are linear
• Quality factor and coupling: transmission line coupled to cavity … … … … is transmitted from transmission line to cavity as possible

Answer 2

• Magnetic field adjusted very precisely by varying current in windings of magnet. Field controller utilizes Hall probe to measure the field and the feedback loop to ensure that the field is set precisely, and the field sweeps are linear
• Quality factor and coupling: transmission line coupled to cavity ensures as much energy is transmitted from transmission line to cavity as possible

Question 3

• Microwave bridge (1) houses the µw source (1.1) and the detector (1.2). An attenuator (1.3) is used to vary the µw power incident on the cavity and the circulator (1.4) ensured that only the µw power reflected back from the resonatorsreaches the detector (1.2) which converts themicrowave power to an electrical current.It is important that the detector operates at the input power level withoptimum sensitivity. Therefore, a reference signal (1.5) is applied whose phase mustbe adjusted to the phase of the signal.
• The microwave source (2) is tuned to the resonant frequency of the cavity resonator (2) housing the sample. When critically coupled no power is reflected e.g. impedance of cavity and waveguide are matched. At EPR resonance some of the µw- energy is absorbed by the sample, and the impedance of the cavity changes such that it is no longer critically coupled, and some of the µw-power is reflected to the detector
• Tuned so no power reflected at moment it starts absorbing, get change in signal which can be measured easily as is a small change detected about 0 as no power is reflected
• Gives small back-ground noise, improves S/N
• Resonator with magnet around it works by sample sitting at maximum in microwave magnetic field then reflected
• Microwaves from source go in to resonator and circulator which allows them to go to detector (1 direction) which protects the source from reflecting

Answer 3

• Microwave bridge (1) houses the µw source (1.1) and the detector (1.2). An attenuator (1.3) is used to vary the µw power incident on the cavity and the circulator (1.4) ensured that only the µw power reflected back from the resonatorsreaches the detector (1.2) which converts themicrowave power to an electrical current.It is important that the detector operates at the input power level withoptimum sensitivity. Therefore, a reference signal (1.5) is applied whose phase mustbe adjusted to the phase of the signal.
• The microwave source (2) is tuned to the resonant frequency of the cavity resonator (2) housing the sample. When critically coupled no power is reflected e.g. impedance of cavity and waveguide are matched. At EPR resonance some of the µw- energy is absorbed by the sample, and the impedance of the cavity changes such that it is no longer critically coupled, and some of the µw-power is reflected to the detector
• Tuned so no power reflected at moment it starts absorbing, get change in signal which can be measured easily as is a small change detected about 0 as no power is reflected
• Gives small back-ground noise, improves S/N
• Resonator with magnet around it works by sample sitting at maximum in microwave magnetic field then reflected
• Microwaves from source go in to resonator and circulator which allows them to go to detector (1 direction) which protects the source from reflecting