1. Negative resistance (-R)

Since a crystal oscillator acts as a resistor that makes it difficult for current to flow through it, it is necessary to have power (oscillation starting power) to cancel the resistance component of the crystal with an IC in the oscillator circuit, etc. to start the oscillation. This oscillation starting force (oscillation power) is called negative resistance (-R), and the unit is Ω.
The larger the negative resistance, the easier it is to oscillate.
(I think it's easier to understand if you think that the resistance R is originally a negative component, and by adding a negative sign in front of it, the overall force becomes a positive force.)

In order to find the value of the negative resistance, the crystal oscillator is actually mounted in the circuit, specifically caused to oscillate, and the oscillation starting power of the circuit (IC) is calculated. For that purpose, the equivalent series resistance value (ESR) and parallel capacitance value (Co) of the crystal unit are required.

Since complicated calculations are required, the only way is to ask the crystal manufacturer to actually measure and calculate.

2. Oscillation margin

Oscillation margin is a magnification that indicates how much margin the circuit (IC) has to oscillate when the ESR value of the crystal unit is the worst of the standard values. The magnification is expressed by the ratio of the resistance value when the crystal is mounted on the circuit and the negative resistance (-R).

The resistance value when the crystal is mounted in the circuit is called the load equivalent resistance (Re), and its value is usually 1.2 to 1.3 times the ESR value. (This means that when a crystal is put into a circuit, its resistance increases.)

In order for the crystal oscillator to oscillate normally, the value of the negative resistance (-R) must be at least several times the equivalent resistance under load (Re). In other words, the oscillation starting power must be several times greater than the resistance component of the crystal.

The higher the magnification, the more power the circuit (IC) has and the more room there is for oscillation. In general, it is recommended that the oscillation margin be 5 times or more.

In actual circuit matching, calculate the oscillation margin with the circuit received, and if the margin is low, consider a method of increasing the negative resistance (or increasing the oscillation margin).
However, since the C value, R value, load capacitance (CL) value, drive level, etc. of the circuit are interrelated, it is necessary to comprehensively consider and determine the optimum conditions through trial and error.

3. Drive level

When designing the circuit, you must also pay attention to the drive level (crystal oscillator current).
If an excessive current flows through the crystal (the drive level is too large), spurious emissions are likely to occur, and oscillation may become unstable. Also note that if the drive level is too high, abnormal jumps in frequency and series resistance may occur in the temperature characteristics.

The drive level is adjusted by adding a damping resistor (Rd) to the circuit and varying its value. Increase the value of Rd to lower the drive level.

Also, reducing C1 and C2 will also lower the drive level.
However, if Rd is too large, the negative resistance (-R) will decrease, and changing the values of C1 and C2 will change the load capacitance and negative resistance. Depending on the frequency band, the values of Rd shown in the table below are recommended. (Reference example)

It is necessary to adjust Rd while checking the drive level and negative resistance value.

* If the value of negative resistance is small, reduce the value of Rd. (May even drop to zero)
*If sufficient negative resistance cannot be obtained even if Rd is zero, decrease the values of C1 and C2 to increase the negative resistance.
* If you cannot obtain sufficient negative resistance even by lowering the values of C1 and C2, there is a way to tighten the ESR standard. This makes it possible to improve the oscillation margin. (Please note that tightening the ESR standard will affect the unit price.)
*If sufficient margin is still not obtained, it may be necessary to review the circuit, such as changing the IC.

4. Circuit matching (circuit analysis)

In order to oscillate under optimum conditions, the values of C and Rd on the circuit board, the load capacitance of the crystal, the drive level, the negative resistance (-R), and the oscillation margin are Since various factors are intertwined such as whether or not, future troubles can be minimized by examining the optimum circuit constants, oscillation conditions, etc. through circuit matching and designing the circuit according to the results.

When requesting circuit matching (circuit analysis) from Kyushu Dentsu, please check the following items.

* When requesting circuit matching, the power supply must be clearly stated, and a board and circuit diagram are required.
*It is necessary to specify the external capacitor value and damping resistance value in the circuit diagram.
*Please make an analysis request after confirming normal operation.
*The cost of circuit matching may be charged depending on the purchase conditions.

Please allow 1-2 weeks for analysis. After the analysis is completed, we will promptly return the items we have entrusted to you.