Introduction to Quartz Frequency Standards - Noise in Crystal Oscillators

Although the causes of noise in crystal oscillators are not fully understood, several causes of short-term instabilities have been identified. Temperature fluctuations can cause short-term instabilities via thermal-transient effects (see the section below concerning dynamic f vs. T effects), and via activity dips at the oven set point in OCXOs. Other causes include Johnson noise in the crystal unit, random vibration (see the section below concerning acceleration effects in crystal oscillators), noise in the oscillator circuitry (both the active and passive components can be significant noise sources), and fluctuations at various interfaces on the resonator (e.g., in the number of molecules adsorbed on the resonator's surface).

In a properly designed oscillator, the resonator is the primary noise source close to the carrier and the oscillator circuitry is the primary source far from the carrier. The noise close to the carrier (i.e., within the bandwidth of the resonator) has a strong inverse relationship with resonator Q, such that L(f) µ 1/Q⁴. In the time domain, s_y(T) » (2 X 10^-7)/Q at the noise floor. In the frequency domain, the noise floor is limited by Johnson noise, the noise power of which is kT= -174 dBm/Hz at 290 K. A higher signal (i.e., a higher resonator drive current) will improve the noise floor but not the close-in noise. In fact, for reasons that are not understood fully, above a certain point, higher drive levels usually degrade the close-in noise. For example, the maximum "safe" drive level is about 100 mA for a 5-MHz fifth overtone AT-cut resonator with Q » 2.5 million. The safe drive current can be substantially higher for high-frequency SC-cut resonators. For example, L(f) = -180 dBc/Hz has been achieved with 100-MHz fifth overtone SC-cut resonators at drive currents - 10 mA. However' such a noise capability is useful only in a vibration-free laboratory environment. If there is even a slight amount of vibration at the offset frequencies of interest, the vibration-induced noise will dominate the quiescent noise of the oscillator (see the section below concerning acceleration effects in crystal oscillators).

When low noise is required in the microwave (or higher) frequency range, SAW oscillators and dielectric resonator oscillators (DROs) are sometimes used. When compared with multiplied-up (bulk-acoustic-wave) quartz oscillators, these oscillators can provide lower noise far from the carrier at the expense of poorer noise close to the carrier, poorer aging, and poorer temperature stability. SAW oscillators and DROs can provide lower noise far from the carrier because these devices can be operated at higher drive levels, thereby providing higher signal-to-noise ratios, and because the devices operate at higher frequencies, thereby reducing the "20 log N" losses due to frequency multiplication by L(f) = -180 dBc/Hz noise floors have been achieved with state-of-the-art SAW oscillators [20]. Of course, as is the case for high-frequency bulk-wave oscillators, such noise floors are realizable only in environments that are free of vibrations at the offset frequencies of interest. Figures 17 and 18 show comparisons of state-of-the-art 5 MHz and 100 MHz BAW oscillators and a 500 MHz SAW oscillator, multiplied to 10 GHz. Figure 17 shows the comparison in a quiet environment, and Figure 18 shows it in a vibrating environment.

Figure 17. Low-noise SAW and BAW multiplied to 10 GHz (in a nonvibrating environment).

Figure 18. Low-noise SAW and BAW multiplied to 10 GHz (in a vibrating environment).