Oscillation Circuit Evaluation Methods
What is "Negative Resistance"(3) <Improvement Method>
The following formula shows the oscillation conditions of the crystal unit.
|-R| > RL (|-R| = negative resistance, RL = load resonance resistance）
If the value of |-R| in the circuit constant does not satisfy the target value of negative resistance value with sufficient margin, NDK tries to improve the negative resistance by following attempts.
(1) Reduce the capacitance of external capacitors (Cin, Cout)
＜Theoretical formula of negative resistance＞
-R [Ω]= -gm / (ω2 × Cin × Cout) (in case of Rd = 0Ω)
(gm: transconductance, ω : 2π × frequency, Cin, Cout : Capacitance of external capacitor)
The relationship between the negative resistance and capacitor when Cin and Cout are changed is shown below.
(*) Graph measured under the condition : Cin= Cout
(2) Reduce the value of the damping resistance (Rd)
The relationship between the negative resistance and damping resistance when the damping resistance is changed is shown below.