Crystals Matching Test

Matching test is to ensure the crystal component specification and its circuit carried out as follows:
1. Frequency accuracy
2. Negative resistance
3. Drive level
4. Startup time

The above tests are designed based on the actual needs of different applications or users. There will be some additions or deductions due to the change of its condition. It is recommended these tests be carried out at the design stage.

Matching of frequency accuracy:

The matching of frequency accuracy can take 2 ways:

Keeping the specifications of the crystal component and adjusting the external capacitance of the circuit.

Maintaining the same external capacitance of the circuit, change the specifications of the crystal component.

  these two methods can let the frequency accuracy of a circuit achieve to the qualification level. 

(generally require: △ fl ≤ 5ppm.)

External capacitance of some integrated circuits is given from the specification booklets. In this case, an external capacitance is regarded as the recommended capacitance. Then determine the specification of the crystal component, which means “matching”.

In the process of “matching”, it notes that crystal component may not afford to vibrate (under-estimated negative resistance), if the load capacitance of the circuit is too huge. If the load capacitance is too small or neglected, it may appear the phenomenon that the crystal cannot afford to vibrate or crystal vibrates at the “harmonic” frequency due to inadequately loop phase shift. Thus, the load capacitance of the circuit should be as small as possible in the absence of leading work at the harmonic frequency if we need to maintain a reliable and fast “start-up” oscillator.

Use the frequency signal as an emission source, and then place the probe to get closer to the crystal component (but not contact its surface). Analyze its frequency-related parameters from the frequency spectrum analyzer (instrument), this method can avoid the irregular data from the parameters of the probe.   

Negative resistance matching:

Test and measure negative resistance of oscillated circuit:

Connect the adjustable resistor in series with crystal component on the circuitry. Regulate the adjustable resistance to 0 ohm so that it can make the circuit have a stable oscillation. And then gradually increase the variable resistance of the adjustable resistor on the circuitry to stop its oscillation. Report this resistance with reference to symbol “vr”.  

The negative resistance │-r │ = rs + vr

(rs: equivalent series resistance, e.s.r. Of the crystal component), requires

 │-r │> (5 ~ 10) × rs.

The size of the negative resistance will directly affect the stability of the oscillator’s performance.

If the negative resistance is not capable to meet the requirements of the above conditions, an external capacitor must be adjusted to a small value. Then testify the actual negative resistance again, until it achieves the criteria of our needs. The specification of the crystal component should be changed while the external capacitance is altered. This setting can ensure the matching characteristics of the frequency accuracy.

Determine the excitation power of the crystal component:

Crystal components operates in the condition with a sufficient excitation power to ensure their short-term and long-term stability. We can calculate the excitation power of the crystal component referred to the follow paragraphs: 

Start-up time for xtal:

The features of some circuits depends greatly on the start-up time

Start-up time is indispensable in different varieties of circuit applications, such as communications. Extend the start-up time will cause the circuit to be not functional and instable.

A large “load capacitance” will produce a long stable activation time. Thus, shorten the start-up time of its vibration can be achieved by decreasing its values of load capacitance.  

Frequently q/a:

1. How to choose a crystal chip for your product design?

For the high-tech and reliable system design, crystal selection is indispensable at the first stage, especially its design for hibernating the system (other words to say, “induce the low voltage supply in order to reduce the power loss”). Since the low supplied voltage reduces the excitation power of its own crystal, resulting in slow start-up and non-vibration of its crystal. This phenomenon does not particularly disclose in the “reset” stage, since there are enough power disturbances to easily induce the oscillation. At the “hiberated” stage, the disturbance of this circuit is much smaller than its perturbance. Therefore, cause the vibration of this chip will easily perform in this condition. In the oscillated circuit, the crystal component cannot be too “excitable” (easily vibrate to reach a high harmonics level), but also not be less incentive (not easily vibrate from its normal stage). “crystal” selection must be at least considered with the following factors:

Resonant frequency

Load capacitance

Excitation power

Characteristic in its operating temperature

Stability

2. How can you choose capacitors c1 & c2?

Since each crystal component has its own characteristics, the best answer to find out the appropriate external component value should refer to the information from the manufacturer.

C1, c2 values have better as small as possible in its allowable operation range. With the small cap value (c1 or c2) in its device, it can maintain the performance stability and shorten the start-up time.