The Negative Resistance Test Method in Oscillator Crystal Circuit

What is negative resistance crystal oscillator? It refers to the passive component crystal keeping stable output frequency and working in the oscillator crystal circuit, usually refer to the two functions of crystal ( output&input ) towards the circuit.

Negative resistance is not the electrical parameter of a electronics crystal, but it is an important characteristic parameter in the oscillator crystal circuit. The negative resistance will be affected by the external capacitance at both ends of the crystal oscillator and the stray capacitance of the whole circuit board during the crystal oscillator is matched. The starting vibration of the crystal oscillator is caused by the noise in the circuit. The principle is that with the increase of resonance, and gradually achieves a stable frequency and a certain oscillation amplitude, named resonance.

Noise is a chaotic oscillation frequency, while the frequency of quartz crystal is a stable frequency. The final result is that the frequency of quartz crystal will stand out from many complicated frequencies. Therefore, the frequency stability of the crystal oscillator is the key, avoid to frequency hopping.  The larger value of the negative impedance, the crystal oscillator easily vibrates. General electronic products required a negative resistance is more than 3 ~ 5 times of natural crystals.

Negative resistance test method:

1）add a Variable Resistor “R” in the oscillation circuit.

2) Starting R=0, and then raising R value until the Oscilloscope has no waveform output

3)Recording R value.

4) compare the resistance under normal temperature, required >5 times, matching test complete