Automatic frequency controller

Fig. 21. Diagram of EPR spectrometer with automatic frequency control (AFC).

Klystrons. The most commonly used radiation source is a klystron these tubes are available at discrete frequencies between 2.5 and 220 GHz. Many klystrons can be tuned over a range up to 3 % of the nominal frequency by a control that varies the physical dimensions of a resonant cavity inside the tube. Finer adjustment of the frequency is achieved by varying the voltage applied to the resonator and reflector electrodes. Thermal stability is obtained by immersion of the entire tube in an oil bath, or by water or air cooling. A feedback circuit provides automatic frequency control (AFC) to continuously correct the output frequency to the resonance frequency of the cavity. The power output of the klystrons used in EPR spectrometers is generally about 300-700 mW. The most widely used frequency for EPR spectrometers is 9.5 GHz, which is called X-band. 

To fine-tune the cavity, the spectrometer is put in the operate mode. Adjust the microwave frequency, the iris position (resonator parameter), and the reference arm current ( bias ) so that the analog indicators for the automatic frequency control ( AFC ) and the diode always stay at the center as the microwave power is increased from minimum (e.g., 50 dB, 2 fiW) to maximum (e.g., 0 dB, 200 mW). This indicates that at all power levels, the majority of microwave power is stored in the resonator and very little is reflected. Adjust the signal phase to let the diode indicator reach the maximum, and then decrease the bias if necessary to put diode back to center again. 

AFC (automatic frequency control) A circuit that automatically keeps an oscillator on frequency by comparing the output of the oscillator with a standard frequency source or signal. 

Automatic frequency control (AFC) A system designed to maintain the correct operating frequency of a receiver. Any drift in tuning results in the production of a control voltage, which is used to adjust the frequency of a local oscillator so as to minimize the tuning error. 

AFC area frequency coordinator automatic frequency control... 

Automatic frequency control Eliminates the need for constant adjustment of the power supply after initial setup. 

Continuous Control automatic and control loops, database storage, scan frequencies, complex control schemes (cascade, ratio, predictive)... 

Due to the reduced Q factor of the quartz crystals in liquids, and therefore decreased phase slope, the requirements of the circuit with respect to phase (frequency dependence, noise, temperature dependence), to amplification linearity, and to temperature constancy are much higher. One electrode of the quartz crystal should be grounded to minimize parasitic effects and to allow operation of quartz arrays in conductive liquids. The increased damping of the oscillator should be overcome by automatic level control. The control variable in the amplitude control loop can be used as an independent measurement value. It also allows for calibration of/osc with respect to/s [36]. 

The Lever oscillator [39], Fig. 16, allows the application of series resonance configurations with one-side quartz electrode grounding. Since the effect of parasitic capacitance is minimized and simple shielding is possible, this circuit configuration is especially suited for under-liquid QCM. Besides the series resonance frequency, the series resonance resistance Rs can be measured. For this purpose the Lever oscillator allows a largely transistor current gain-independent measurement of the resistance. An automatic level control provides a signal proportional to Rs. 

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