A low noise four phase crystal oscillator
Posted on Feb 6, 2014 10022
A 10 MHz crystal is typically 4 ohms at its series resonance and 40 kiloohms at its parallell resonance. To get low sideband noise the power dissipated in the crystal has to be reasonably large, the design used here dissipates 0. 2mW in the crystal, 7mA and 30mV across the crystal. An oscillator can be seen as an amplifier which has feedback throug
h a filter. The amplifier of a series resonance oscillator must have both a low input impedance and a low output impedance, preferrably below 4 ohms to preserve the selectivity associated with the high Q of the crystal. The thermal noise in the losses of the crystal is k * T0 = -174 dBm/Hz. With a power dissipation of - 7 dBm the flat noise floor is thus 167 dB below the carrier if we compare the power dissipated in the losses of the quarts to the thermal noise generated by the same losses. The flat noise floor at the collector side may be a good deal better than this even if the amplifier impedance is matched to, or lower than the resistive part of the crystal impedance. This is because the voltage gain for the thermal noise produced within the crystal drops quickly with the frequency separation since the reactive part of the crystal impedance comes in series with the noisy 4 ohm resistive part. The noise generated by the transistor itself will be attenuated by the large emitter impedance presented to the emitter by the crystal at frequency separations of a few kHz or more so the flat noise floor may be well below 167 dBc/Hz. The flat noise floor can be made much lower than one would expect from comparing noise power to signal power using the noise figure and feed impedance at the resonance frequency of the crystal. A termination resistor from emitter to ground may improve the noise close to the carrier but it may degrade...