fet low distortion crystal oscillator

Description: This is a design circuit for a low distortion crystal oscillator. The circuit generates a sine wave with low phase noise and distortion, allowing for the use of a crystal with less than 1mV dissipated across it. The crystal serves to filter the signal current. If the load impedance is low, the JFET will drive the impedance effectively. When the load is approximately 50 ohms, it is advisable to use an emitter follower in conjunction with a voltage step-down transformer or a matching network for additional buffering. The value of C3 determines the output voltage; increasing the value of C3 will yield a lower output voltage, while decreasing it will produce a higher output voltage. If an overtone crystal is employed, a choke should replace the 1K emitter resistor, resonating with C2 at a frequency slightly above the fundamental frequency for third overtone crystals. When using high-Q overtone crystals, the value of C3 should be lower, as these crystals require driving at much lower levels than fundamental crystals. Additionally, the output level should be minimized. If the crystal's rated power or current is known, the drive level can be measured. To measure the drive level, a 100-ohm resistor can be temporarily connected across C3 to measure the signal level at the source of the FET. The crystal current can be determined by the voltage divided by 100.

The low distortion crystal oscillator circuit is designed to produce a stable sine wave output with minimal phase noise and distortion, making it suitable for precision applications. The circuit typically utilizes a JFET (Junction Field Effect Transistor) for its high input impedance characteristics, which is essential for minimizing loading effects on the crystal. The crystal oscillator operates efficiently at low power levels, ensuring that the crystal does not dissipate more than 1mV, thus preserving its integrity and performance.

The selection of C3 is crucial as it directly influences the output voltage. For applications requiring a lower output voltage, a higher capacitance value for C3 is recommended, while a lower capacitance value will increase the output voltage. This flexibility allows the circuit to be tailored to specific application requirements.

In scenarios where an overtone crystal is utilized, the circuit design must account for the specific resonant characteristics of the crystal. The replacement of the 1K emitter resistor with a choke is necessary to achieve resonance with C2, which enhances the circuit's performance at frequencies above the fundamental frequency, particularly for third overtone crystals. High-Q overtone crystals necessitate careful attention to the circuit's drive levels, as they must be driven at lower levels compared to fundamental crystals to avoid distortion and potential damage.

To accurately measure the drive level, a temporary connection of a 100-ohm resistor across C3 provides a means to assess the signal level at the FET source. This measurement is critical for evaluating the crystal current, which is determined by the voltage across the resistor divided by its resistance. Such measurements are essential for ensuring that the oscillator operates within the specified parameters and maintains optimal performance.

Overall, this low distortion crystal oscillator circuit exemplifies a meticulous approach to design, balancing performance with component selection and configuration to achieve desired operational characteristics.This is a design circuit for Low Distortion Crystal Oscillator circuit. This circuit generate a sine wave that has low phase noise and distortion. This circuit can be used to perform a crystal with less than 1mV dissipated in crystal. The crystal is used to filter the signal current. This is the figure of the circuit; If the impedance loads is low , the JFET will drive the impedance. When the loads is about 50ohm, it will better if an emitter follower combined with a voltage step-down transformer or matching network for further buffering. The value of C3 determined the output voltage, if the lower output voltage is required, the C3 ²s value should be increased and decrease the value of C3 when the larger output voltage is needed.

If overtone crystal is used, a choke should replace the 1K emitter resistor. This choke must be resonates with C2 at a frequency slightly above the fundamental frequency for thirdovertonecrystals. When uses the high-Q overtone crystal, the value of C3 should be lower because the High-Qovertonecrystals should be driven at much lower levels than fundamental crystals.

Besides that, the output level should be set as low as possible. If the crystal`srated power or current is known, the drive level can be measured. To measure drive level temporarily connect a 100 ohm across C3 and measure the signal level on the source of the FET. The crystal current is determined by V/100.