Microwave Journal
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Ensuring Peak Oscillator Performance in Real World Systems

May 13, 2016

All telecom, navigation, control and measurement systems require reference frequency oscillators, and the stability of this oscillator often sets the limits of system performance. Frequency stability vs. temperature and time (long- and short-term) and phase noise are the most important characteristics of reference sources.

Table 1

Figure 1

Figure 1 Thermal model of an oven-controlled crystal oscillator.

High stability, oven-controlled, low noise crystal oscillators are particularly important, given their critical applications: 5 to 10 MHz crystal oscillators are used in the phase-locked loops of atomic frequency standards. These oscillators determine the phase noise at ≥ 0.1 Hz and the Allan deviation from 0.1 to 10 s. 50 to 100 MHz crystal oscillators are found in modern frequency synthesizers. The noise level of this crystal oscillator determines the synthesizer’s noise, so the crystal oscillator phase noise is specified from 10 Hz to 1 MHz offset from the carrier. 5 to 40 MHz high stability, low noise crystal oscillators are being used in synchronization modules for time base maintenance.

The requirements for reference frequency sources are constantly tightened: smaller size, reduced power consumption, lower phase noise and improved frequency stability vs. time and temperature. To ensure the best performance, this article addresses the design of ultra-low noise oscillators and system factors that can degrade oscillator performance (see Table 1), as the oscillator and system designs are both important to maximizing performance. Neglecting system considerations may cause an excellent oscillator design to underperform when it is installed in the system.

HIGH STABILITY, LOW NOISE OSCILLATORS

Individual oscillators were designed to cover 5 to 10 MHz and 80 to100 MHz with significantly improved phase noise and Allan deviation. The size of each oscillator was reduced while maintaining high frequency stability vs. temperature. Reducing an oscillator’s size makes it more difficult to maintain or improve frequency stability over temperature. Using thermal simulation (see Figure 1) and optimizing the structural circuitry, the frequency stability of the single, oven-controlled crystal oscillator design was close to that of a double, oven-controlled oscillator.1,2

Table 2

Figure 2

Figure 2 Improved phase noise (a) and Allan deviation (b) of the 10 MHz OCXO.

5 to 10 MHz Oscillator Design

The design of low noise oscillators is impossible without a good crystal, and the demands for better phase noise and Allan deviation require significant improvements in crystal technology. Several approaches havebeen developed to increase performance and yields:

  • Special technology for treating the surface of blank crystals
  • A measurement system to test crystals at different power dissipations
  • Studies of power dissipated in crystals vs. close-to-the carrier phase noise (see Table 2).

The data in Table 2 shows that the type 1 crystal is least sensitive to dissipated power.

In addition to the crystal, the circuitry of the 5 to 10 MHz oscillator affects the phase noise and Allan deviation. To improve phase noise close to the carrier (0.1 to 10 Hz offset), the system’s quality factor must be maximized. The oscillator circuit must preserve the maximum loaded Q-factor of the crystal resonator. In addition, providing “pure” voltage to the crystal oscillator and matching circuits improves phase noise close to the carrier. The oscillator’s printed circuit board (PCB) layout requires careful attention. The oscillator’s current may cause interference in the near and far zone phase noise and affect the Allan deviation for 1 to 10 s. The matching between the oscillator — particularly the crystal circuit — and buffer amplifier, as well as the noise figure of the amplifier, will influence the far zone noises (1 kHz to 1 MHz offset). Optimization of the circuit design and layout can yield 10 to 20 dB improvement in phase noise in the far zone.

Incorporating these crystal and circuit measures achieves considerable improvement in the phase noise and Allan deviation (see Figure 2). Table 3 summarizes the performance of two 10 MHz oscillators designed with these approaches.

80 to 100 MHz Oscillator Design

What is true for the 5 to 10 MHz oscillator designs applies to the 80 to 100 MHz oscillators. However, the 80 to 100 MHz oscillators have additional considerations. The linearity of the buffer amplifiers, through which the signal is fed to the output, affects the phase noise “floor” (10 kHz to 1 MHz offset), making the choice and optimization of the transistors in these amplifiers important. Feedback from unwanted loops causes self-excitation and “humps” in the phase noise curves, which affects the phase noise of a 100 MHz oscillator. So the amplifier layout should minimize unwanted capacitive and inductive couplings. A 5 V, 100 MHz oscillator incorporating these design considerations achieves the phase noise performance shown in Figure 3. Its performance is summarized in Table 3.

Table 3

Figure 3

Figure 3 Improved phase noise for a 100 MHz, 5 V oscillator.

SYSTEM CONSIDERATIONS

When integrated into a system, factors other than the oscillator may hurt performance. Parasitic resistance, digital control, air flow and vibration can degrade phase noise, Allan deviation and frequency stability.

Parasitic Resistance

Figure 4 shows the oscillator frequency control circuit, which has elements both inside and outside the oscillator. From the figure it follows that

Ucontrout = Ucontr + Unoise,
where U designates voltage.

If Ucontrout is fixed, and Unoise changes, Ucontr will also change, directly affecting the phase noise, Allan deviation and frequency stability vs. temperature.

Figure 4

Figure 4 Oscillator frequency control circuit.

Figure 5

Figure 5 Parasitic resistance created by a common ground trace on the PCB (a). Improved layout by eliminating the common trace (b).

The ground wires of the control and reference sources, high frequency output cascade and oven ground are connected to the ground lead inside the oscillator. In precision oscillators, these grounds are separate and only connected on the board at the oscillator ground pin. This minimizes the common resistance these currents flow through (Rn in Figure 4), as the oven current varies linearly with ambient temperature and will contribute noise. This grounding concept is illustrated with the PCB layouts shown in Figure 5. In Figure 5a, the common ground trace creates the parasitic resistance, Rn. To remove it, separate ground tracks are created on the PCB, shown in Figure 5b.

Figure 6

Figure 6 Oscillator frequency stability vs. temperature with (a) and without parasitic resistance (b).

Figure 7

Figure 7 Oscillator phase noise with (a) and without (b) parasitic resistance.

Over the operating temperature of a 5 V, oven-controlled crystal oscillator, the typical change in current is ∆I = 1 A. The oscillator can be adjusted by about 1 × 10-6 as the control voltage changes over 5 V, making K = 2 ×10-10/mV. With a parasitic resistance Rn = 0.01 Ω, a variable capacitance diode voltage change over temperature will make

Figure 8

Figure 8 Distorted phase noise from an incorrectly built DAC-controlled oscillator.

Unoise = ∆IRn or 10 mV.

The frequency drift (dF/F) vs. temperature is determined by

dF/F = KUnoise or 2 ×10-9.

Using a 12 V oscillator — meaning a lower oscillator current change for a given temperature range — with a smaller adjustment range and lower parasitic resistance results in better frequency stability with temperature; however, dF/F will still be in the range of 10-10.

Figure 6 shows how parasitic resistance can degrade frequency stability vs. temperature. Parasitic resistance may also increase phase noise, from as low as 0.1 Hz offset (see Figure 7).

Digital Control

Using a digital-to-analog converter (DAC) to control oscillator frequency can degrade the oscillator’s phase noise (see Figure 8) and Allan deviation. This is due to the discrete voltage steps of the DAC and interference between the oscillator frequency and DAC clock. The latter can cause interference components on the phase noise from 1 kHz to 1 MHz.

To understand the effect a DAC step has on Allan deviation, consider an oscillator with Allan deviation of 6 × 10-13 per 1 s. To avoid any effect on Allan deviation, the frequency step must not exceed  Fs = 2 to 3 × 10-13. If the oscillator adjustment is ∆F = 1 ×10-6, then the total steps make

Figure 9

Figure 9 Oscillator frequency stability vs. temperature in still (a) and moving air (b).

S = ∆F/Fs, which requires a 22-bit DAC.

This example shows that controlling oscillator frequency at normal Allan deviations of 5 to 6 × 10-13 is difficult. To effectively use a DAC for frequency control, the DAC should have a high number of bits, the capability to filter the switching steps and a control algorithm with the most infrequent switching between bits. The DAC ground should be separate, as previously discussed.

To eliminate possible interference from an internal or external DAC clock, a controlled oscillator frequency shall be brought to DAC input, using a frequency divider, if needed.  

Air Flow

The temperature of the oscillator body will vary with any interaction with air flows. The intensity and speed of the air will determine the heat removed from the oscillator, which will trigger the thermostat. For precision, oven-controlled oscillators installed in equipment containing fans, the air flow should not directly hit the oscillator. Vibration from the fan may also increase the phase noise. If these factors are not well controlled, the following adverse effects can occur:

  • An intensive air flow is equivalent to extending the oscillator’s operating temperature range to negative values, which reduces frequency stability
  • The oscillator’s oven may be unable to maintain the desired operating temperature due to insufficient power
  • The Allan deviation in the range of 5 to 100 seconds will deteriorate because of air flow variation and attendant temperature fluctuations at the oscillator body
  • The equipment design will be more complicated to accommodate the increased power consumed by the oscillator power supply.

A comparison of the frequency drift of an oscillator in still and moving air is shown in Figure 9

Vibration

The phase noise of an oscillator will be degraded by vibration. The phase noise from random vibration may be modeled using the g-sensitivity of the oscillator.3

L(f ) = 20 log[(|Γ||A|F0)/(2F)]

where |Γ| is the g-sensitivity of the oscillator,

|A| = (2PSD)1/2

where PSD is the power spectral density, F0 is the oscillator operating frequency and F is the frequency offset from the carrier.

Knowing the level of vibration during operation is not always possible. Nonetheless, the oscillator should be located on the PCB away from points of possible resonance or close to sources of vibration, such as fans, transformers and motors. If the direction of the vibration is known, the oscillator manufacturer can advise which oscillator axis has the minimum g-sensitivity, as it will vary among oscillator designs.

Conclusion

Not paying attention to the above factors may lead to a considerable degradation in the performance of precise, low noise, oven-controlled oscillators. Since the performance of the oscillator often determines system performance, degrading oscillator performance will impair the system.

References

  1. A.G. Nikonov, A.V. Kotyukov, A.S. Kamochkin and N.I. Dyakonova, “Recent  Achievements  in Performance of Low Profile Ultra Precision Single Oven Quartz Oscillators,” EFTF2012.
  2. Y. Vorokhovsky, A. Nikonov, A. Kotyukov and A. Kamochkin, “Efficiency of Circuitry and Design Optimization in Development of Precision Ovenized Quartz Oscillators,” PTTI2014.
  3. John R. Vig, “Quartz Crystal Resonators and Oscillators for Frequency Control and Timing Applications - A Tutorial,” April 2012.