HIGH STABILITY 10 MHz OCXO

A.R.ELECTRONIQUE - EUROPEAN FREQUENCY AND TIME FORUM -
14-16 MARCH 2000 -
TORINO - ITALY

E. GIRARDET, P. CAPPELLE
A.R.ELECTRONIQUE F-25048 BESANÇON

B. WOLCOFF
A.R.ELECTRONIQUE F-78500 SARTROUVILLE

During the last few years, different papers have been presented, showing either 10 MHz ultra stable oscillators, stability class of a few 10^-11, short term of some 10^-13, or miniaturized, SMD versions, with a class of stability of a few 10^-8.

A.R.Electronique has investigated in an alternative way which takes place between these products and consists in providing high stability, very low phase noise in low size enclosures, combined with a very efficient performance versus cost ratio.

This concept is based on the use of A.R.Electronique designed, SC cut resonators (HC-53/U or TO-8, 4 points enclosure), associated with a very low noise electronic. The thermal regulation is done with a single oven, low size. This principle has been derivated in two versions :

The miniature JUMBOSTAR-FB family offers a class of stability of a few 10^-9, short term lower than 8.10^-13 / 1s, low phase noise, in a 27 x 36 x 16 mm package.

The PULSAR-SL family reaches short term stability better than 5.10^-13/ 1s, very low phase noise, associated with temperature stability of a few 10^-10, in a very low profile case 41 x 51 x 20 mm.

The same concept has been used for ruggerized versions devoted to airborne applications (civil / military), with low g sensitivity resonators. These oscillators give very fast warm-up and low phase noise under vibration.

The main electrical characteristics and frequency stability will be given and placed in regard with other actual products.

Special developments and characterizations for particular applications will be also presented.

Placed between the miniature, SMD or not, OCXO’s with limited characteristics and very high stability, high volume, expensive OCXO’s, the last generation of A.R.Electronique oscillators have been designed for the optimization of three criterias : stability-size-cost.

They are destinated for the new applications in the telecommunication fields, digital broadcast, TTNC, GPS receivers, instrumentation, where the continuous evolution of performances (transmission rate speed, precision of localization…) requires the use of high stability frequency sources, together with miniature size (low profile) and reasonable price.

This last oscillator’s design has been declined in two families offering complementary classes of stability (JUMBOSTAR-FB, PULSAR-SL). Specific versions for hardened environmental conditions (civil and military avionic) have been also developped. This concept was also characterized, as preliminary investigation, under specific space environment (vacuum).

In order to precise the technical context of this study, the table [1] shows the main electrical and mechanical characteristics of these oscillators, in comparison with the related existing families.

The technical requirements have directly involved some technological choices concerning the crystal resonator, the oscillator’s design, as well as the mechanical structure and the thermal control.

The 10 MHz nominal frequency is well adapted for the majority of the expected needs. Based on SC cut crystals, 3^rd overtone, this frequency range allows to reach a best compromise for stability rate, size and manufacturing costs.

The geometrical blanks design is optimized in order to reduce the complex lapping phases.

The housing in HC-53/U or low profile, T08 enclosure, 4 points mounting is well adapted for the reduction of the oven volume, and consequently of the whole oscillator’s size.

The crystal manufacturing process includes stabilization cycles as well as computer characterized pre-aging tests, according with adapted high temperature profiles that have been optimized during this study.

The levels of achieved characteristics for this type of crystal are shown in the following chapters.

The 10 MHz range resonators can be used within a larger frequency scale, through a multiplication or a division of the oscillator’s frequency. However, it will induce a phase noise degradation on the final frequency (20 log N factor for multiplication, noise contribution due to the logic IC’s for division).

For very special requirements, the need of a direct, lower frequency resonator can be planed (typically 5 MHz, SC cut, 3^rd O.T.). The fundamental concepts of these oscillators can be used, with some restrictions due to the crystal enclosure size (minimum oscillator’s height of 25mm).

The high level of expected characteristics for phase noise, short term stability (Allan Variance) requires the use of low noise components with optimized bias points. This approach eliminates the use of full integrated, ASIC oscillators.

The oscillator is a colpitts types slightly modified. The peripheral functions (insulation and amplifier stages, voltage regulators) have been specially designed, in order to reduce both phase noise and frequency stability contributions : supply voltage sensitivity, load sensitivity, as well as long term stability, thermal behavior.

The mechanical design is involved by the expecting size and costs requirements. The internal structure is centered around the oven, which is optimized for the crystal resonator’s thermal regulation. For the JUMBOSTAR-FB family, the sensitive electronic functions are pseudo-temperature regulated. This regulation is more precise for the PULSAR-SL versions, where the size criterions are not so severe. A special effort has been made for minimizing the number of the mechanical components.

These mechanical structures have been also declined for hardened versions, adapted to avionic requirements (larger temperature ranges, vibrations, shock). In that case, the optimization of the mechanical behavior under vibration modifies the thermal insulation of the oven, which degrades the consumption. The start-up speed, which is an important requirement for these products, will require a significative increasement of the oven power after turn-on.

The final thermal regulation of the oven is function of the mechanical structure as well as the associated electronic circuitry. The single oven solution has been choosen : a double oven concept is greatly unfavourable for global size, adjustment processes and reliability.

The control circuit is fully integrated (ASIC), except the temperature sensor and the power heating elements. Beyond the obtention of high regulation rates for low temperature slopes (<1°C/min.), the thermal design has taken into account the response to fast temperature steps : these kinds of OCXO’s are normally working under very low temperature gradient. However, they have to keep their stability under accidental disturbances (typically, delocked PLL after the opening of a rack system).

For the derived avionic versions, the internal temperature gradient effect (ITGE) has to be characterized. It corresponds to the overall frequency shift due to the variation of the OCXO’s geometrical orientation.

The table [2] shows the intrinsec thermal behavior of the different oscillator’s families.

These characterizations are made by measurement of the b mode frequency of the SC cut resonators (linear slope of –30ppm/°C).

For evaluation, some probe tests have been realized under vaccum (pressure < 10-5 torr), for space applications.

The most significative measurements are now shown and discussed. For general datas, please refer to table [1].

The aging is measured 2 times per day over 20 to 90 days (automatic bench). Fig [1] is a typical curve.

Statistical results on a batch of 100p are shown on fig [2] (after 21 days) and [3] (after 60 days).

For very precise requirements, this characteristic is performed with a complete temperature cycle (+25, -20, +60, +25°C, 0,5°C/min, 400 min) (fig [4]).

The similar test have been applied as preliminary experimentation under vacuum, without any change with the standard version (fig [5]).

Fig [6] shows the frequency response to a temperature step of +/-35°C (temp. slope of 1°C/sec.).

Fig (7] characterizes the I.T.G.E. test. Both temperature and g sensitivity effects are simultaneously taken into account.

The thermal design combined with the intrinsec SC cut characteristics offer a very short stabilization time for both frequency and consumption (fig [8], [9]).

The retrace behavior is mainly dependant of the quartz resonator, as well as the test profile. Fig [10], [11] give statistical results on 20 pieces, 40 and 400 min after turn on (24 hours turn off at +25°C).

This parameter has been particularly optimized during the study. Fig [12] shows a typical curve (PULSAR-SL).

The JUMBOSTAR-FB is slightly degraded (see table [1]), because of some electronic simplifications due to the size.

The Allan Variance is measured in the time domain by comparison with a reference USO (fig [13]).

This measurement can be also derivated from the phase noise curve, after frequency versus time domain conversion.

Initially designed for specific applications, these families of compact, high stability, low noise oscillators are now completely qualified. Their modular structures are well adapted for complementary developments where size and high performance criteria are critical.

REFERENCES

[3] Identification and measurement of different types of noises present in a frequency source.