Microminiature thermostatted crystal oscillator with low phase noise and topological adaptation to load impedance

Abstract

The utility model relates to the field of radio electronics, in particular to small-sized precision quartz oscillators (SPCO), intended for use in systems with stringent requirements for signal spectral purity and frequency stability under space-constrained conditions. The technical result of the utility model is to minimize the influence of an active load on the quartz crystal quality factor and to reduce phase noise by 6-10 dB when operating with a high-impedance load. The quartz crystal oscillator comprises an oscillator on a transistor (14), a quartz resonator (4), a varicap (6), and an output circuit (19) at the "cold" terminal of the resonator. Inside the housing, the quartz crystal oscillator contains a correction capacitor (17), connected to the emitter of the transistor (14) and brought out to an external contact of the housing. To adapt to a high-impedance load, which causes an increase in frequency, the said contact is grounded via an external jumper (18).This connects the capacitor (17) to the emitter circuit, providing a discrete reduction in frequency to the nominal value. 3 z.p. f-ly, 1 fig.

Description

[0001] Technical field

[0002] The utility model relates to the field of radio electronics, in particular to small-sized precision quartz oscillators (SPCSO), intended for use in systems with strict requirements for spectral signal purity and frequency stability under conditions of limited dimensions (for example, a 7x5 mm case).

[0003] Technology Level

[0004] The closest technical solution (prototype) is the design of a quartz crystal oscillator described in the author's application for utility model 2026102641 005419 dated 02.02.2026, which implements highly efficient signal pickup from the "cold" terminal of a quartz crystal resonator through a capacitive divider.

[0005] Prototype drawback: the lack of active buffer stages (due to space constraints in the microminiature package) creates a technical contradiction. When optimizing the circuit for a high-impedance load (over 1 kOhm) to improve phase noise by 6-10 dB, connecting a standard low-impedance load (50 Ohm) causes a sudden frequency shift of 2-4 ppm. This shift is comparable to the full tuning range of the varicap (±5 ppm), which deprives the OCXO of the adjustment margin to compensate for temperature drift and natural aging.

[0006] The essence of the utility model

[0007] The design of the proposed microminiature thermostatted quartz oscillator is illustrated by the circuit diagram (Fig. 1). The oscillator is implemented on a printed circuit board and housed within a shielded metal housing (shown by the dotted line in the diagram).

[0008] The device is based on an oscillator built around a bipolar NPN transistor (14) using a capacitive three-point circuit. The transistor's collector (14) is connected to the power supply via a resistor (13), and the emitter is connected to the common bus (ground) via an inductor (15), which creates an AC load for the cascade. Capacitors (9, 10, 11) form a capacitive feedback circuit, necessary for maintaining stable oscillation. The circuit's power supply is filtered by a capacitor (16).

[0009] The frequency-setting circuit is connected to the base of the transistor (14) and contains a series-connected quartz crystal (4) and a varicap (6). The varicap (6) is supplied with a control voltage, V_TUNE, through an RC filter circuit (7, 8), providing smooth frequency adjustment to compensate for temperature variations within a narrow range (about + / - 5 ppm).

[0010] Output interface 19 (RF_OUT) is implemented on the second ("cold") terminal of the quartz crystal (4). The signal is collected via a decoupling capacitor (1) and matching elements (2, 3). This topology minimizes the influence of the active load on the quartz crystal's Q factor and achieves a 6-10 dB reduction in phase noise when driving high-impedance loads (more than 1 kOhm).

[0011] To adapt the generator to various load types, a discrete-analog compensation circuit is included in the circuit. A correction capacitor (17) (with a value in the range of 0.5-5.0 pF) is located inside the housing. Its first terminal is connected to a low-impedance node in the circuit—the emitter of the transistor (14). The second terminal of the capacitor (17) is routed to an additional external contact terminal on the housing.

[0012] The device operates in different modes as follows:

[0013] 1. Low-impedance load mode (50 ohms): the external switching element (18) on the motherboard is open. The generator operates at the calculated center frequency, adjusted for the influence of a 50 ohm load.

[0014] 2. High-impedance load mode (1 kOhm or more): When disconnecting a 50 Ohm load and switching to a high-impedance consumer (e.g., a PLL input), the oscillation frequency spontaneously increases by 2-4 ppm. To return it to the nominal value, the external contact is grounded through the switching element (18) (by soldering the jumper). A capacitor (17) is connected in parallel to the emitter circuit, providing the required negative frequency shift. Grounding the external contact in this mode ensures shielding of the emitter node from external interference.

[0015] 3. Precision Alignment Mode: If ultra-precise (down to fractions of a ppm) frequency alignment with the grid of a specific device is required, an additional discrete SMD capacitor is installed in the gap of the switching element (18). The series connection of the internal (17) and external capacitors allows for smooth variation of the total inserted capacitance, compensating for the individual parasitic capacitance of the consumer's wiring.

[0016] 4. Aging compensation: when operating in 1 kOhm mode, in the event of a natural decrease in the quartz frequency over a long period of time (10-15 years), breaking the previously sealed jumper (18) allows you to discretely raise the frequency, returning the varicap (6) to the center of its control characteristic.

[0017] The choice of the transistor emitter (14) as the connection point of the correction circuit is due to its low impedance, which ensures the stability of phase noise and insensitivity to the length of the outer track of the element (18).

[0018] Description of drawings

[0019] Fig. 1 shows the schematic diagram of the generator.

[0020] Keywords: 1 - decoupling capacitor; 2, 3 - matching elements; 4 - quartz resonator; 6 - varicap; 7, 8 - RC filter circuit; 9, 10, 11 - feedback capacitors; 13 - collector resistor; 14 - bipolar transistor; 15 - emitter inductance; 16 - power supply filter; 17 - correction capacitor; 18 - switching jumper 19 (RF_OUT) - output contact.

Claims

1. A quartz crystal oscillator containing an auto-oscillator on a transistor (14), a quartz crystal resonator (4), a varicap (6) and an output circuit (19) on the “cold” terminal of the resonator, characterized in that it is equipped with a correction capacitor (17) placed inside the housing and connected between the emitter of the transistor (14) and an additional external contact terminal of the housing, designed with the possibility of connection to a common bus via an external switching element (18) to compensate for a change in frequency when the load impedance changes.

2. The generator according to paragraph 1, characterized in that the switching element (18) is made in the form of a section of a printed conductor on a board external to the housing, intended for discrete closure by a soldered connection.

3. A generator according to paragraph 1, characterized in that the connection of the additional external contact terminal of the housing to the common bus is made through an additional reactive element installed in the gap of the switching element (18).

4. The generator according to paragraph 1, characterized in that the capacitance of the capacitor (17) is selected from the condition of ensuring a negative frequency shift equal to the magnitude of the frequency shift when switching from a load of 50 Ohm to a load of more than 1 kOhm.

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