Application Note
EX-380/385 Series
4 Pin Dip Evacuated Miniature Crystal Oscillator
(EMXO)
TM
Supply Voltage:
3.3 Vdc,
5 Vdc or 12 Vdc available
Aging: <1x10
-9
/day, <1x10
-7
/year, <1x10
-6
/10 years
Temperature Stability: to ±7.5x10
-8
over -20
o
C to +70
o
C
Acceleration Sensitivity: 1x10
-9
/g, Total Gamma
Uses SC Family 3
rd
Overtone Crystal
Low Power Consumption: <0.35 watts @ +25
o
C
Frequencies: 10 to 20 MHz (other frequencies in development)
Fast Warm-up: 1 to 2 minutes
Patented Technique*
Notes: 1. We acknowledge the support of
the U.S. Army for work on
resonators associated with this
product under contract #1X66001-
97-C-8635.
*U.S. Patent 5,917,272.
SONET / SDH, DWDM, FDM, ATM, 3G
Telecom Transmission and Switching Equipment
Wireless Communication Equipment
Military Airborne and Mobile systems
OCXO (Oven Controlled Crystal
s
Oscillators) are used when frequency vs.
temperature requirements are too stringent to
be met by a basic XO (Crystal Oscillator) or
TCXO (Temperature Compensated Crystal
Oscillator). With an OCXO, the temperature
of the crystal and critical circuits are kept
constant as the temperature outside the
oscillator varies. Controlling the temperature
inside the oscillator with an oven maintains
this constant temperature. In an OCXO, the
changes in the ambient temperature are
sensed and then fed back to an oven control
that continually maintains a constant optimum
temperature inside the oscillator enclosure.
An OCXO can improve the crystal's inherent
stability by more than 5000 times. The
oven control system is not perfect, the open
loop gain is not infinite, there are internal
temperature gradients inside the oven
(oscillator) and, in a conventional oven, the
circuitry outside the oven shell is subjected to
ambient temperature changes that can "pull"
the frequency.
The improved temperature stability
performance of a conventional OCXO over an
XO or TCXO comes at a steep price. OCXO
power consumption, for instance, is greater
by a factor of over 200. There is also a size
consideration. In an ordinary OCXO, a
crystal is enclosed in a metal case which is
then placed inside an oven shell together with
temperature sensitive circuitry, and then
Vectron International • 267 Lowell Road, Hudson, NH 03051 • Tel: 1-88-VECTRON-1 •Web: www.vectron.com
surrounded by thermal insulation. All
this, plus any additional circuitry are then
placed in a metal housing making for a bulky
package, which becomes very difficult to
miniaturize.
To overcome these obstacles, the EX-380
Series EMXO (Evacuated Miniature Oven
Controlled Crystal Oscillator) was specifically
developed to achieve OCXO performance
while significantly lowering power
consumption and reducing package size.
These characteristics go hand in hand since
reducing package size makes it easier to
improve power consumption. First, the
volume of the package was made as small as
possible to reduce the volume that the oven
needs to heat. Secondly, the most effective
insulation needed to be used. In the EX-380
Series, Vectron has done both. The package
was designed to single DIP dimensions of
0.82" x 0.52" x 0.3", and the oscillator uses a
vacuum as the insulation medium - a dramatic
improvement over conventional polyfoam or
fiber based insulation material. Another great
contributor to Vectron efforts to reduce size
s
and still provide "oven oscillator" performance
was to eliminate the use of large packaged
crystals which, up until now, had to be used to
achieve good aging. Instead, a way to use an
open crystal blank was found and made
practical. Vectron has succeeded in resolving
outgassing and contamination issues which
could degrade performance. To do this
required manufacturing the oscillator with a
high internal vacuum level, with low internal
outgassing, to provide the needed thermal
insulation. A high level of cleanliness was
needed to prevent contamination of the open
(un-encased) crystal blank and to ensure
exceptional long term crystal aging.
+Vcc
Voltage
Regulator
Oven
Control
Circuitry
Power
Transistor
&
Resistor
Thermistor
+Vcc
Precision
VCXO
+/-1.0ppm
Output
Buffer
Stage
(HCMOS)
Output
Crystal
Blank
10MHz
3rd OT
Upper
Substrate
Lower
Substrate
Figure 1. EX-380 Functional Block Diagram
The key design feature of this package
utilized the concept of integrating the precision
crystal in blank form in combination with
hybrid microelectronics circuitry. In doing this,
obtaining good aging perf ormance was
paramount. For this reason, a cold welded
package was chosen rather than a more
traditional resistance welded package. Cold
weld sealing provided a true metallurgical
bond between ductile metal surfaces without
added heat from the sealing process. Under
the high tonnage pressure introduced through
the indentation of the welding die, a plasticity
flow of material takes place on the mating
surfaces. The end result is a hermetically
sealed enclosure without contamination from
weld splashes, dust and vapors. And, most
important, cold weld sealed enclosures
achieve a high level of vacuum integrity.
Vectron International • 267 Lowell Road, Hudson, NH 03051 • Tel: 1-88-VECTRON-1 •Web: www.vectron.com
Mechanically, the hybrid circuit and crystal
assembly are suspended directly over a
highly insulating structure to minimize heat
energy loss through conduction. In addition,
the entire assembly is thermally insulated to
the enclosure by vacuum at a pressure level
of 10-6 torr. Based on the steady state
thermal conduction calculation, this package
design resulted in a thermal resistance of
>300 degrees C/watt.
The EMXO manufacturing process is
interesting and a process flow diagram is
shown below in figure 2.
(1) Substrate
Fabrication
(8) CW
Header Attach
oven for curing. After the cure process, the
hybrid is cleaned to remove organic and non-
organic contaminants. Wires are bonded on
the hybrid circuit as interconnects. The hybrid
circuits are then attached to the cold-weld
package with adhesive. Finally, blank crystals
are mounted onto the clips and tuned to the
nominal frequency needed, by an evaporation
process, to a typical accuracy of 1 ppm. The
units are then cold weld sealed. The oven
itself is heated by direct thermal conduction
applied to a heat conductive substrate.
This "Open Blank" EMXO concept is shown
and compared to conventional OCXO
construction, in Figure 3 below.
1. Evacuated
Crystal in
Metal Case
3. Can & Cover
& Polyfoam
Insulation
(2) TF
Resistor Trim
(7) Snap
Substrate
(3) Crystal
Clip Attach
(6) Wire
Bond
(11) Electrical
T
est
(14) Oven
T
emp Set
(4) Component
Mount
(5) Plasma
Clean
(12)Xtal Blank
Mount
(13) Vacuum
Cure
(20) Cold
Weld
(21) Leak
T
est
(9) Epoxy
Cure
(16) Epoxy
Cure
(17) UV /
Ozone Clean
(24) Pack
& Ship
(10) Wire
Bond I/O
(15)Mount F
.S
Component
(18)Vacuum
Bake-out
2. Electronics
on PC board
(19) Freq
Fine-Tune
(22) Final
T
emp T
est
4. Evacuated oscil-
lator (EMXO) using:
*Open Crystal Blank
*Thick Film Microcir-
cuit
*Insulation via a
Vacuum
*All in 4 pin DIP
(23) Cover
Marking
Figure 3, Open Blank EMXO concept
In figure 3 above, 1,2 and 3 reflect
conventional OCXO construction while 4 is the
EMXO. It shows the major elements of the
SDIL EMXO. Only one substrate is used and
all the elements are heated. The oscillator is
e s s e n t i a l l y a C M O S g a t e t yp e wi t h a n
additional varactor diode and LC trap for
overtone select. The resonator is a 3rd
Overtone, AT or doubly rotated cut as required
by the application, both of which offer superior
aging performance when compared to a
traditional fundamental resonator.
Figure 2. EMXO Process Flow Diagram
Substrates are fabricated with thick film
screen printing techniques with each
deposition layer subjected to three different
process stages - print, dry and fire. Crystal
clips are attached to the gold conductor trace
on a substrate with high thermal conductivity.
All active and passive components are
mounted on the substrate using a conductive
adhesive and then moved to a convection
Vectron International • 267 Lowell Road, Hudson, NH 03051 • Tel: 1-88-VECTRON-1 •Web: www.vectron.com
It is anticipated that the EX-380 Series will
find applications where performance in severe
mechanical environments is equally important
to electrical performance. An additional focus
for the EX-380 series, therefore, was to
provide robust construction to withstand high
shock and vibration and to yield good
G-sensitivity performance. For example, when
the physical orientation of an oscillator is
changed, there is a small frequency change
(typically not more than several parts in 10
-9
for a 90-degree rotation) due to change in
stress on the crystal blank resulting from the
gravitational effect upon the crystal supports.
This characteristic is known as "tip-over" and
is expressed in 10
-9
/g where one g represents
one-half of a 180-degree orientation change.
To minimize this change and also to enhance
performance under shock and vibration, the
crystal blank is mounted in a symmetrical
mounting structure, instead of the more
traditional 2 or 3 points. This helps to achieve
a high shock and vibration endurance level,
low
g-sensitivity performance and
symmetrical heat transfer. Also, when a
crystal oscillator is operating and subjected to
vibration, spurious frequencies are generated,
offset from the frequency of oscillation by the
frequency of vibration. These are commonly
referred to as "vibration induced side-bands"
and these side-bands behave similarly to
phase noise. The amplitude of these spurious
outputs is related to the amplitude of vibration,
the mechanical design of the crystal supports,
and the mechanical design of the oscillator
assembly, including the crystal mounting. Here
a l s o t h e s ym m e t r i c a l c r ys t a l m o u n t i n g
structure helps to reduce unwanted noise.
Both AT and doubly rotated crystals can be
used in the EX-380 Series. Even though
many types of doubly rotated crystals
produce lower amplitude spurii under vibration
than the AT, this characteristic is primarily
determined by the mechanical design of the
crystal and oscillator rather than the specific
crystal cut. In many applications, Vectron uses
doubly rotated resonators in the oscillator to
provide lower close in phase noise, better
aging rates and reduced acceleration
sensitivity. In less critical applications, less
expensive AT cut crystals are used.
The following figures represent the typical
actual test data on qual samples for various
characteristics.
Figure 4
Frequency vs Temperature
Characteristics EX-380
SERIES @ 10 MHz
(Typical)
Vectron International • 267 Lowell Road, Hudson, NH 03051 • Tel: 1-88-VECTRON-1 •Web: www.vectron.com
Figure 5
Warm-up (Restabilization)
Characteristic at +25º
C
EX-380 @ 10.00 MHz
Figure 6
This is a typical phase noise
plot for the EX-380 series at
10 MHz. Significant phase
noise improvement, both
close in and at the noise
floor, can be obtained on
special order.
Vectron International • 267 Lowell Road, Hudson, NH 03051 • Tel: 1-88-VECTRON-1 •Web: www.vectron.com
