In principle, it is a non-gate circuit, which can be regarded as an amplifier with a particularly large gain. The crystal has the smallest impedance to the sine wave with a specific frequency f, and only allows the sine wave with a specific frequency in the square wave signal to pass through. The inverter converts sine waves into square waves. Connecting a resistor can be regarded as a feedback resistor. Its function is to make the static working point of frequency converter G 1 always at the turning point of voltage transmission characteristics and ensure the amplification of small signals. For CMOS circuits, the value of R is usually 1 ~ 5m, while for TTL circuits, it is only about1k.. The feedback coefficient depends on the ratio of C 1 and C2, and C 1 can also fine-tune the oscillation frequency.