Q&A

Yes, this industrial temperature range (-40 ºC to + 85 ºC ) is optional on most crystals. Depending on the crystal package, temperature ranges of up to -55 ºC to +105 ºC may be available. 
 
Contact ARGO's engineering department to discuss your specific requirements.
A "ceramic resonator" is a resonator fabricated from a piezoelectric ceramic material, of which there are several. These materials are not naturally piezoelectric, the property is induced in them during manufacture. Some of the materials used for this purpose are quite remarkable and represent major engineering accomplishments. 
 
To our knowledge, however, none of these devices can compete with quartz in terms of frequency stability, particularly over a range of temperatures, nor can their operating frequencies be held to any real accuracy. These types of resonators exhibit "Q" values much lower than those manufactured from quartz. 
 
These devices, I believe, tend to age far more than do quartz crystals. Still, if a very low cost, loosely specified device is suitable for an application, piezoelectric ceramics have much to commend them.
Before beginning a design or purchase of a crystal , there are system parameters which must be considered. Below are questions which need to be determined by your system. These parameters will determine the crystal specifications. 
 
(1) On what crystal frequency do you wish to operate?
 
(2) How much can the frequency be off at room temperature(25℃)?
 
(3) What is the temperature range over which the crystal will operate?
 
(4) How much can the crystal change frequency over the temperature range?
 
(5) Is the crystal to be operated at Series or Parallel resonant?
 
(6) If operated at parallel,what is the parallel capacitance in picofarads(pF)?
 
(7) Is pullability important?
 
(8) What holder type or can size do you require?

The HC49U crystal utilizes a circular AT cut crystal blank. Due to their small size, most surface mount crystals use a rectangular AT strip cut crystal blank. ("Strip" refers to the rectangular shape of the blank.)

While both are AT cut blanks, several differences exist due to their different geometry. In general, given the same frequency and overtone, an AT strip cut will have a lower Shunt Capacitance (CO) and Motional Capacitance (C1) than the AT cut. Because pullability is a function of the capacitance ratio CO/C1, AT strip crystals have less pullability.

Quartz crystals are not linear devices and they do not behave in a linear way. But, it is theoretically possible to achieve symmetrical pulling.

Pullability is the amount by which the frequency of a crystal will change when the circuit condition is switched from series to parallel resonance.

Pullability is also used to describe the frequency change that occurs when the load capacitance is switched from one value to another.

"Activity dips" are sudden increases in the resistance of a crystal that may perturb the frequency. 

I would not make the change without a thorough investigation. Some of the SMD (surface mount device) crystal units are comparable to conventional crystal units, others are not.

In general, SMD's have higher resistance, differing values of shunt and motional capacitance, and are more sensitive to drive level. The pullability characteristics of these devices may differ significantly from a conventional crystal.

I would recommend a fairly exhaustive qualification sequence before making the switch.

Drive Level - HC49U crystals are typically rated 1.0mW max, while the HC49S and most SMD crystals are rated at only 0.5mW or 0.1mW maximum. See question 11 for more information on exceeding the maximum drive level of a crystal.

Pullability -some applications require tuning the frequency of the oscillator, either by mechanically or electrically changing the value of load capacitance.

The HC49S and surface mount crystals have less pullability than the HC49U. It is important to verify whether the HC49S or the surface mount crystal will have sufficient pull range for the application. Generally, an HC49S has one half the pullability as an HC49U.

ESR - The Equivalent Series Resistance (ESR) is generally higher with the HC49S and surface mount crystals, and can cause a problem if the oscillator circuit does not have sufficient loop gain.

Exceeding the maximum drive level (power dissipation) of a crystal can lead to an increased rate of aging, Drive Level Dependency (DLD) problems, increased number and intensity of activity dips that can stop oscillation and, at very high drive levels, breakage of the crystal blank.
Load capacitance is defined as being the total capacitance present in an oscillator circuit as measured or calculated across the pins of the crystal socket. Load capacitance has the effect of increasing the frequency of a crystal unit.
Use this formula to approximate the value of capacitors needed:
 
             CL=((C1 x C2) / (C1 + C2)) + C stray
 
C stray is the stray capacitance in the circuit, typically 2-5pF. 
 
If the oscillation frequency is high, the capacitor values should be increased to lower the frequency. If the frequency is low, the capacitor values should be decreased, thus raising the oscillation frequency. 
 
When CL =20pF, C1 and C2 will be approximately 27-33pF each, depending on the amount of stray capacitance.