Ultra-Stable Low-Power OCXOs as CSAC Alternative

Dr. Igor Abramzon, Xtal Ball Technologies , Israel

Introduction.

  High precision mobile systems of navigation, telecommunication, instrumentation, synchronization demand today usage of high stability frequency/time reference which in great degree determines their hold-over in GPS-denied conditions. In case of the portable equipment with battery supply, utmost stability of the reference should be accompanied by its smallest sizes and minimal power consumption.
  For a long while the chip scale atomic clocks (CSAC) ensuring outstanding junction of highest accuracy with very low consumption and compact packaging have remained unsurpassed solution for the properties being inaccessible for other frequency control (FC) techniques.
  Meantime, CSAC domination can round out nowadays due to emergence of a new frequency references based on the internally heated crystal resonator (IHR) technology. These unique devices incorporate best performances of CSACs and low-noise OCXOs offering extraordinary agglutination of “atomic” stability, very low phase-noise, miniature sizes and extremely low power consumption.
  In the review below Xtal Ball presents basic performances of its novel FC crystal products in comparison with modern CSAC models.

Construction of modern IHR oscillators

  A design of the IHR oscillator (IHRO), a pacemaker of a primary crystal clock, is based on the internally heated resonator structure where the crystal plate is integrated in the evacuated TO-8 crystal holder together with the whole miniature oven control system (Fig.1).

Figure 9
Fig. 1. Schematic drawing of IHR internal structure

  The IHR unit is mounted in the unheated PC board bearing the sustaining and other oscillator circuitry. The whole assembly can be sealed in compact steel cases or used unpackaged as DIP14 compatible module (fig. 2).

Fig.2. External view of ultra-stable IHR oscillators:
XBO14 compatible, XBO20 and XBC25 hermetical models.

  Excellent thermal isolation of the internal heating system from environment in the TO-8 vacuum volume ensures very low power consumption of the IHROs in comparison with conventional OCXOs. Meantime, achievement of utmost frequency stability of the IHRO is rather complicated task requiring elimination of numerous adverse factors, such as temperature sensitivity of the unheated circuitry, impact of temperature gradients in crystal plate and of mechanical stresses in plate mounting structure, changes of vacuum conditions in the TO-8 holder, etc.
  Solution of these intricate technical problems took years of intensive researches that has been rewarded by appearance of novel IHRO products possessing extraordinary performances.

Basic characteristics of novel IHR oscillators (IHRO)

  Long-term frequency stability (aging) is crucial parameter of the oscillators and clocks, which actually determines holdover of the system operating in autonomous conditions.
  Typical frequency vs. time behavior of the novel 10 MHz IHROs is depicted in fig. 3. From the aging curves, the frequency drift doesn’t exceed 1 ppb after first month of operation and 4 ppb after nine months.
  Calculated dependence of the aging rate (df/f)/dt on time (fig. 3b) indicates below 0.03 ppb/day values after first month operation with slowdowns to 0.02 ppb/day after 50 days operation.

Fig. 3. Aging behavior of novel IHR oscillators:
a) frequency vs. time drift; b) aging vs. time rate

  Another important parameter retrace defines frequency shift of the reference caused by temporary break of its operation. It was found experimentally, this frequency shift correlates with aging rate of the oscillator. That explains negligible values of retrace observed at the ultra-stable IHROs having below 0.1 ppb/day aging – less than 0.5 ppb after up to 7 days operation halt.
  Temperature instability is one of main factor degrading precision of the references operating at wide ambient temperature changes. The ambitious task to upgrade this parameter up to the level of the atomic clocks and utra-stable OCXOs has been decided with essential renovation of the IHR structure, improvement of the vacuum condition in the IHR volume and application of sophisticated temperature compensation circuitry. The achieved results are: ±0.3 ppb over (-10 +60)°C and ±0.5 ppb over (-40 +80)°C temperature range (fig. 4).

Fig. 3. Frequency vs. temperature of the new IHRO models.

  Power consumption of the IHROs, in difference from the atomic clocks, depends directly on the internal temperature of the IHR to be set a few degrees above the upper ambient temperature. Table 1 depicts typical consumption of the crystal oscillators at different ambient temperature ranges.


Table 1: Temperature range and power consumption specifications
Ambient temperature range,°C 0 +50 -10 +60 -30 +70 -40 +85
Power consumption at 25°C, mW 45 55 70 85

  Out of the data, the novel IHROs provide the record low consumption among all kind precision references. For the narrow temperature ranges the oscillators consume only about 45 mW at 25°C, that makes them an ideal choice for a plenty of modern battery powered equipment.
  The goal of reduction of short-term instability (Allan deviation) and phase-noise of the new IHROs down the level of the low-noise OCXOs has been attained by substantial reduction of thermal fluctuations in the internal heating structure along with minimization of flicker noise in the crystal resonator and the oscillator circuitry. Obtained excellent results, to 5E-13/1s Allan deviation and almost -120 dBc/Hz @ 1 Hz phase-noise (fig. 5, 6), should open to these new devices the low-noise application niche previously occupied by top rank conventional OCXOs. 4Fig.5. Typical Allan deviation of the 10 MHz IHROs

Fig.5. Typical Allan deviation of the 10 MHz IHROs
Fig. 6. Typical phase-noise pattern of the 10 MHz IHROs.

The IHRO vs. CSAC comparison

  The reviewed performances of the new IHROs are summarized in table 2 in comparison with the most popular CSAC models (www.microchip.com).
  Out of the data, the XBO20 & XBC25 and SA.45S & SA.65S models exhibit similar long-term and temperature stability, while the IHROs provide radically lower short-term instability and phase- noise level at smaller sizes and lower power consumption.

Table 2. Main performances of the top IHROs and CSAC models (operation at 10 MHz)
Performances XBO20 & XBC25 SA.65 SA.45S
Operation frequency, MHz 8-100
Operation temperature range, °C -40 +80 -40 +80 -10 +70
Temperature stability around 25°C, ppb ±0.5 ±0.3 ±1
Aging (typical), ppb/month 0.9
Allan deviation, τ=1s 4E-13 3E-11** 3E-10
Power consumption at 25°C, mW <90 45* 120
Package volume, ccm <9 17
*for (0+50)℃ operational range; ** at 1000 s average time.

Conclusion

  The new generation of the IHROs incorporate best properties of CSACs and top rank conventional OCXOs. Possessing “atomic” frequency stability, excellent short-term stability and phase-noise at extremely low power consumption and miniature sizes these unique crystal references are devoid of obvious weaknesses and seem perfect solution for different high-end modern equipment, especially with battery supply.
  The extraordinary set of utmost performances along with moderate cost gives to the new IHROs evident advantages over CSACs at various high precision battery powered applications, such as top-end portable instrumentation, navigation or seismological ocean bottom nodes.