The above circuit illustrates several low noise techniques as applied to a low phase noise crystal oscillator.
The circuit is only suitable for fundamental crystals as there are no mode suppression components. The oscillator transistor Q104 is cutoff most of the time except for a short period at the peak of the crystal current waveform. Q103 regulates the dc collector current of the oscillator transistor Q103. The dc collector current of Q104 and hence the crystal current is set by the value of R116.
Darlington transistor Q101 buffers the output of the filter (R101, R102, C101, E102) which low pass filters the power supply noise above 0.16Hz. The oscillator supply voltage is produced by low pass filtering the average output of 4x LM329 buried zener references (Z101-Z103). Q105 buffers the low pass filter output and Q106 compensates for the base emitter voltage drop of Q105.
The collector base voltage of the oscillator transistor Q104 is relatively high to reduce its collector base capacitance and its voltage sensitivity. This, together with the low dc gain from Q104 base to collector, keeps Q104's contribution to the oscillator flicker phase noise low. Common base buffer transistor Q102 has a low dc gain (0.5x) from base to collector, its emitter current is set by LED101, and R108 so that the buffer flicker phase noise is low.
The low noise oscillator power supply ensures that the modulation of oscillator transistor collector base capacitance by power supply noise and hence the power supply induced oscillator phase noise is low.
For further examples see:
additional Crystal Oscillators