Ultra-Stable Low-Power OCXOs as CSAC Alternative

Dr. Igor Abramzon, Xtal Ball Technologies , Israel

Introduction.

High precision modern equipment for navigation, communication and instrumentation demands ultra-stable frequency reference enabling accurate holdover in GPS-denied conditions. At the battery supply applications, minimal power consumption of the reference is another mandatory requirement.
For a long time the chip scale atomic clocks (CSAC) have proffered unsurpassed junction of highest frequency stability with less than 120 mW consumption however, at serious lack of the short-term stability.
Xtal Ball presents now alternative concept of ultra-stable frequency/time reference built on the internally heated resonator (IHR) technology. These unique devices incorporate the best properties of CSACs and high-end conventional OCXOs offering extraordinary agglutination of “atomic” stability, low phase- noise, miniature sizes and extremely low power consumption.

Construction of the IHR oscillators

Design of these new oscillators is founded on the IHR structure (Fig.1), where the crystal plate is integrated inside evacuated TO-8 holder with the internal heating system fixing the plate’s temperature at the operation point.

Fig. 1. Schematic drawing of IHR internal structure

The IHR unit is mounted on an unheated motherboard bearing besides the rest oscillator circuitry. The assembled structure can be hermetically sealed in a standard steel case or used as the DIP14 compatible unpackaged module (Fig. 2).

Fig.2. External view of ultra-stable IHR references:
XBO14 compatible, XBO20 and XBC25 hermetically sealed models.

Very low power consumption of these devices results from excellent vacuum thermal isolation of their internal heated structure from environment. Meantime, achievement of extra-high frequency stability has demanded long intense researches and introduction of crucial innovations in the IHRO construction and technology.

Basic characteristics of novel IHR oscillators

Long-term stability (aging) is a major parameter of the clocks, to a great extend determining holdover of the system operating in autonomous conditions.
Modern IHROs exhibit aging rate below 1 ppb per month reached after 20 days operation and below 0.02 ppb/day after 60 days operation (Fig. 3). Such extraordinary results exceed the best stability of conventional OCXOs and don’t yield that to the low power atomic clocks.

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

Temperature instability is another crucial parameter providing precision of the reference under severe climate conditions. The IHRO models packaged in steel cases exhibit below ±0.5 ppb frequency deviation over (-40 +85)°C range (Fig. 4), while temperature stability of the DIP14 unpackaged models is about 2 ppb.

Fig. 4. Frequency vs. temperature of the new IHRO models.

Power consumption of the IHROs display obvious dependence on upper temperature of the operation temperature range (table 1) and falls to the record 45 mW for (O +50)°C range.

Table 1: Consumption of IHROs vs. ambient temperature ranges

Ambient temperature range,°C	0 +50	-10 +60	-30 +70	-40 +85
Power consumption at 25°C, mW	45	55	70	85

Short-term instability (Allan deviation) and phase-noise of the novel IHROs (fig. 5, 6) are practically even with these parameters of the low-noise conventional OCXOs that was achieved after radical reduction of thermal and flicker noise in the IHR structure and the circuitry.

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 contest

Considered performances of the new IHRO references are compared in table 2 with two most stable CSAC models (www.microchip.com).

Table 2. Comparison of the new IHROs and CSACs

Performances	XBO20 & XBC25	SA.65	SA.45S
Operation frequency, MHz	10
Operation temperature range, °C	-40 +85	-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	1E-12	3E-11**	3E-10
Power consumption at 25°C, mW	<100 45*	120
Package volume, ccm	<9	17

*for (0+50)℃ operational range; ** at 1000 s average time.

As it follows from the data, both the crystal and atomic references perform close long-term and temperature stability. At the same time, the IHROs provide drastically lower short-term instability and phase-noise level at noticeably smaller sizes and power consumption.
Possessing “atomic” frequency stability at very low Allan deviation, phase-noise and power consumption the new crystal references seem ideal alternative to CSACs and high-end conventional OCXOs at a plenty of modern applications, such as high-end portable instrumentation, communication, navigation or ocean seismic geological exploration.