28-08-2014, 01:25 PM
My understanding is as follows.
To maintain oscillations, there must be a 360° phase-difference between anode and grid. (180° phase-difference due to internal capacitance anode-to-grid + 180° phase difference anode-to-grid on account of a predominately inductive load in the L/C cct. in the anode = 360° net phase difference = +ve feedback). In the case of a triode, that internal capacitance is furnished by the grid-anode capacitance of the valve. In a tetrode or pentode, although that capacitance will be less, a capacitance from anode to grid will still be required: an external 'real' capacitor may be necessary. In either case, some of the r.f. current flowing in the crystal will flow through that capacitor. The r.f. load on the oscillator is usually taken from the anode, so if that load increases, the r.f. current through the crystal increases also. If that load becomes excessive, apart from freq. stability and waveform distortion problems, the excess current through the crystal could cause it to fracture. For those reasons, it is usual to insert a buffer 'amplifier' between the crystal osc. and the r.f. load.
Al.
To maintain oscillations, there must be a 360° phase-difference between anode and grid. (180° phase-difference due to internal capacitance anode-to-grid + 180° phase difference anode-to-grid on account of a predominately inductive load in the L/C cct. in the anode = 360° net phase difference = +ve feedback). In the case of a triode, that internal capacitance is furnished by the grid-anode capacitance of the valve. In a tetrode or pentode, although that capacitance will be less, a capacitance from anode to grid will still be required: an external 'real' capacitor may be necessary. In either case, some of the r.f. current flowing in the crystal will flow through that capacitor. The r.f. load on the oscillator is usually taken from the anode, so if that load increases, the r.f. current through the crystal increases also. If that load becomes excessive, apart from freq. stability and waveform distortion problems, the excess current through the crystal could cause it to fracture. For those reasons, it is usual to insert a buffer 'amplifier' between the crystal osc. and the r.f. load.
Al.
