Low output impedance resonant oscillator buffer amplifier

Abstract

FIELD: radio electronics.SUBSTANCE: invention aims at a tenfold reduction in the influence of the load on the generation frequency, the ability to operate with a supply voltage of 1.0 V and significant suppression of additive phase noise. For this purpose, a buffer amplifier is proposed, which contains a power source, decoupling capacitor (1), antiparasitic resistor (2), current-setting resistor (3), resistor (4), inductance (5), transistor (6), correcting inductance (7), blocking capacitor (8), capacitor (9), decoupling capacitor (10), wherein the parameters of the LC circuit are selected from the condition of ensuring a reduced characteristic resistance.EFFECT: tenfold reduction in the influence of the load on the generation frequency, ability to operate with a supply voltage of 1.0 V and significant suppression of additive phase noise.1 cl, 4 dwg

Description

[0001] Technical field

[0002] The invention relates to radio electronics, in particular to devices for amplifying and generating high-frequency signals, and can be used in quartz, LC and SAW reference frequency generators for measuring and telecommunications equipment.

[0003] Technology Level

[0004] Resonant amplifiers are known to reduce the influence of external load by using a buffer stage with the signal being sampled through an inductor tap (partial connection). The disadvantage of such solutions is their high sensitivity to parasitic mounting parameters and the complexity of their design at microwave frequencies, which reduces frequency stability when load parameters vary over a wide temperature range.

[0005] The closest analog (prototype) is resonant amplifiers with signal pickup via a partial capacitive circuit (from the common point of two series-connected capacitors connected in parallel to the circuit inductance). In these devices, the characteristic impedance of the circuit is selected to match the load power (e.g., 50 ohms) or to ensure maximum output signal level.

[0006] The main drawback of the prototype is that changing the external load impedance (especially its reactive component) directly changes the equivalent capacitance of the resonant circuit. This causes a significant shift in the oscillation frequency (a "pull-in" effect), since the external load is effectively part of the frequency-setting circuit. Therefore, existing solutions do not provide sufficient isolation of the resonant node while maintaining high gain efficiency and the ability to operate at low supply voltages.

[0007] Technical result

[0008] The technical result is a tenfold reduction in the deviation of the generation frequency when changing the load impedance (to the level of a few ppb), ensuring stable operation at an ultra-low supply voltage (from 1.0 V) and effective suppression of higher harmonic components.

[0009] An additional advantage of the proposed solution is the reduction in the contribution of additive phase noise from the active element. This is achieved by operating the stage with a gain close to unity, which increases the input signal level relative to the transistor's inherent noise and improves the overall Q-factor of the output resonant node.

[0010] Essence of the invention

[0011] The objective of the invention is to create a buffer amplifier that ensures precision stability of the generation frequency with significant changes in the load impedance, simplification of the circuit and maintaining operability under conditions of ultra-low supply voltage.

[0012] The technical result is achieved by the fact that in a buffer amplifier containing an active element (microwave transistor) and a collector resonant LC circuit, the output signal is taken directly from the full inclusion unit of the circuit through a separating element.

[0013] Unlike the prototypes, where the circuit parameters are optimized for power matching, in the proposed solution the parameters of the active element and the LC circuit are selected to ensure an extremely low characteristic resistance of the circuit while simultaneously introducing a correcting inductance into the emitter circuit of the active element.

[0014] The novelty lies in the combination of the following factors:

[0015] 1. Direct signal reading from the high-capacitance collector unit allows the circuit to be used as a "hard" voltage source, less sensitive to load shunting.

[0016] 2. The emitter inductor creates frequency-dependent negative feedback (NFB), which vectorially compensates for phase shifts introduced by the load into the collector circuit. This prevents changes in load impedance from being transformed into the transistor's input capacitance, thereby decoupling the master oscillator frequency from the buffer output.

[0017] 3. Using an emitter inductor instead of resistive bias circuits eliminates DC voltage drop, ensuring stable operation with supply voltages as low as 1.0 V, which is unattainable with classic circuits with deep negative feedback.

[0018] Thus, a tenfold reduction in the influence of the load on frequency is achieved (down to the ppb level) while maintaining a simple single-stage amplifier structure.

[0019] Description of drawings

[0020] • Fig. 1 - Schematic diagram of the buffer amplifier (basic version).

[0021] • Fig. 2 - Schematic diagram of a variant with an active quartz filter and an electronic tuning circuit in the emitter.

[0022] • Fig. 3 - Qualitative view of the amplitude-frequency characteristic (AFC) of the buffer amplifier according to the variant of Fig. 2.

[0023] • Fig. 4 - Comparative curves of spectral density of phase noise.

[0024] Implementation of the invention

[0025] The buffer amplifier, the circuit diagram of which is shown in Fig. 1, is implemented based on an active element (6). It is preferable to use a microwave bipolar transistor as the active element (6), but field-effect transistors (MOSFET, GaAs, GaN) or composite transistor structures can also be used.

[0026] Schematic construction:

[0027] The input signal from the resonant oscillator is fed to the base (gate) of the active element (6) through a series circuit consisting of a decoupling capacitor (1) and an antiparasitic resistor (2). Resistor (2) prevents self-oscillation at frequencies significantly above the operating band. The operating point (bias) of the active element (6) is set by a current-setting resistor (3), connected between the collector (drain) and the base (gate), which ensures automatic regulation of the DC mode.

[0028] The emitter (source) of the active element (6) is connected to the common bus ("ground") via a correction element (7), which has an inductive impedance at the operating frequency. The term "inductance" (7) in the context of the present invention refers to both a discrete component (inductor, chip inductor) and a distributed structure (microstrip line section, short-circuited loop) or an active circuit (gyrator) simulating inductance.

[0029] The output resonant circuit is formed by a parallel connection of an inductor (5) and a capacitor (9), connected between the collector (drain) and the common bus. The stage is powered through a circuit containing a resistor (4) and a blocking capacitor (8), forming a low-pass filter for power supply decoupling. The output signal is collected directly from the full-switch on point of the collector LC circuit via a blocking capacitor (10).

[0030] Device operation and technical effect:

[0031] When supply voltage (from 1.0 V) is applied through resistor (4) and inductor (5), the active element (6) enters operating mode. Tuning the collector circuit to the operating frequency is accomplished using the principle of "weighting" the resonant node.

[0032] When maintaining the resonant frequency unchanged, the value of the capacitor (9) is selected to be increased by 5-10 times, and the value of the inductance (5) is proportionally reduced relative to the values ​​characteristic of resonant amplifiers matched in power.

[0033] This change in parameters leads to a significant reduction in the characteristic impedance of the circuit. As a result, the equivalent resistance of the collector node becomes comparable to or lower than the standard load impedance (50 ohms), which switches the cascade to operating mode with a voltage gain close to unity. Operation in unity-gain mode allows a signal of increased amplitude to be fed to the buffer input. This leads to an increase in the ratio of the useful signal level to the intrinsic noise of the active element (6), which dramatically reduces the additive phase noise of the device. At the same time, the high capacitance of the capacitor (9) makes the resonant frequency of the circuit little affected by parasitic capacitances introduced by the external load through the capacitor (10).The correcting inductance (7) in the emitter circuit at the operating frequency creates frequency-dependent negative feedback, which vectorially compensates for phase shifts arising from load impedance fluctuations. This ensures deep decoupling between the amplifier output and the main resonant oscillator, eliminating frequency pulling.

[0034] In one embodiment (Fig. 2), the buffer amplifier is equipped with a precision filtering and frequency adjustment circuit. A series circuit consisting of a resistor (16), a quartz resonator (15) and a capacitor (14) is connected in parallel with the emitter inductor (7). To ensure the possibility of electronic frequency tuning, an additional branch is connected in parallel with the capacitor (14), consisting of a series-connected capacitor (13) and a varicap (12), the anode of which is connected to the common bus. The control voltage is supplied to the cathode of the varicap (12) through a high-resistance resistor (11). In this configuration, the inductor (7) is selected with an increased value, which creates deep negative feedback and minimizes the gain of the transistor (6) outside the passband. At the series resonance frequency of the quartz resonator (15), the feedback is weakened, which ensures the nominal output signal level only in a narrow frequency band.

[0035] Resistor (16) controls the Q factor and bandwidth of the resulting crystal filter. Decreasing the value of resistor (16) narrows the bandwidth, the value of which is selected taking into account the frequency temperature response (FTR) of the resonator (15). Capacitors (13, 14), together with the varicap (12), provide tuning of the filter's center frequency in the range of 5-7% to synchronize with the master oscillator.

[0036] To achieve maximum stability under changing temperature conditions, the quartz resonator (15), made in an SMD case (sizes from 1612 to 5032), is placed in a microthermostat (microoven), which maintains a constant operating temperature of the crystal.

[0037] Figures 3 and 4 show graphical dependencies confirming the achievement of the technical result. The sharp narrowing of the bandwidth and the suppression of additive noise are due to the dynamic change in the depth of negative feedback in the emitter circuit, determined by the resonant properties of the quartz resonator (15).

[0038] Example of specific implementation:

[0039] For a frequency of 50 MHz, the parameters are selected such that the reactive energy stored in the capacitor (9) is many times greater than the energy transferred to the load. With a 50 ohm load impedance variation of + / - 10%, the frequency deviation does not exceed 5 ppb, confirming the achievement of the stated technical result.

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