哈特莱振荡器

哈特莱振荡器（Hartley oscillator），又称赫特利振荡器，电感三点式振荡器，是一种由电容和电感的调谐电路（即LC振荡器）决定振荡频率的电子振荡器电路。该电路是美国工程师雷夫·哈特莱于1915年发明的。Hartley振荡器的特点是调谐电路由一个电容器与串联的两个电感（或单抽头电感）并联，振荡所需的反馈信号取自两电感连接的中心。

## 操作

<image>

Hartley振荡器的突出特点是谐振电路包含两个串联线圈（或者通常用抽头线圈）与电容器并联，在相对高阻态的LC槽电路和线圈间，相对低电压/高电流点之间有一个放大器。

1905年的原始版本使用一个三极管作为屏极接地（阴极跟随器）组态中的放大器件，含有三节电池，以及独立可调线圈。右图显示的简化电路使用一个JFET（共漏极组态）、一个LC谐振电路（这里的单绕组是抽头的）和一个单电池。该电路说明了Hartley振荡器的操作：Circuit operation

- JFET的源极（若使用BJT则是发射极；三极管则是阴极）的输出与它的栅极（基极）的信号同相位，电压与它的输入（整个槽电路两端的电压）粗略相等，但电流被放大了，即它是作为电流缓冲器或Sallen–Key拓扑。
- 然后这个低阻态输出送入到线圈抽头中，有效地进入一个可以提高电压的自耦变压器，需要比较高的电流（相对于线圈顶部的电流来说）。
- with the capacitor-coil resonance, all frequencies other than the tuned frequency will tend to be absorbed (the tank will appear as nearly 0Ω near DC due to the inductor's low reactance at low frequencies, and low again at very high frequencies due to the capacitor); they will also shift the phase of the feedback from the 0° needed for oscillation at all but the tuned frequency.

Variations on the simple circuit often include ways to automatically reduce the amplifier gain to maintain a constant output voltage at a level below overload; the simple circuit above will limit the output voltage due to the gate conducting on positive peaks, effectively damping oscillations but not before significant distortion (spurious harmonics) may result. Changing the tapped coil to two separate coils, as in the original patent schematic, still results in a working oscillator but now that the two coils are not magnetically coupled the inductance, and so frequency, calculation has to be modified (see below), and the explanation of the voltage increase mechanism is more complicated than the autotransformer scenario.

A quite different implementation using a tapped coil in an LC tank feedback arrangement, still called a Hartley oscillator (or sometimes "the" Hartley Oscillator circuit) is to employ a common-grid (or common-gate or common-base) amplifier stage, which is still non-inverting but provides voltage gain instead of current gain; the coil tapping is still connected to the cathode (or source or emitter), but this is now the (low impedance) input to the amplifier; the split tank circuit is now dropping the impedance from the relatively high output impedance of the plate (or drain or collector).

<image>

The Hartley oscillator is the dual of the Colpitts oscillator which uses a voltage divider made of two capacitors rather than two inductors. Although there is no requirement for there to be mutual coupling between the two coil segments, the circuit is usually implemented using a tapped coil, with the feedback taken from the tap, as shown here. The optimal tapping point (or ratio of coil inductances) depends on the amplifying device used, which may be a bipolar junction transistor, FET, triode, or amplifier of almost any type (non-inverting in this case, although variations of the circuit with an earthed centre-point and feedback from an inverting amplifier or the collector/drain of a transistor are also common), but a junction FET (shown) or triode is often employed as a good degree of amplitude stability (and thus distortion reduction) can be achieved with a simple grid leak resistor-capacitor combination in series with the gate or grid (see the Scott circuit below) thanks to diode conduction on signal peaks building up enough negative bias to limit amplification.

<image>

振荡频率约为槽电路的谐振频率。若槽路电容器的电容为 C 而抽头电感的总电感是 L，则

$$f = {1 \over 2 \pi \sqrt {LC}} \,$$

若使用了电感为 L1 和 L2 两个非耦合的线圈

$$L = L_1 + L_2 \,$$

然而，如果两个线圈磁耦合，总电感会因互感 k 而增大

$$L = L_1 + L_2 + k \sqrt{L_1 L_2} \,$$

由于线圈中的寄生电容和晶体管的负载，实际振荡频率将略低于上述。

Hartley振荡器的优点包括：

- 频率可以使用一个可变电容器进行调整，电容器的一边可接地
- 输出幅度保持恒定的频率范围内
- 需要一个抽头线圈或两个固定电感，以及很少的其他组件
- Easy to create an accurate fixed-frequency crystal oscillator variation by replacing the capacitor with a (parallel-resonant) quartz crystal or replacing the top half of the tank circuit with a crystal and grid-leak resistor (as in the Tri-tet oscillator).

缺点包括：

- 如果从放大器，而不是直接从LC电路（除非采用振幅稳定电路），则会得到谐波丰富的输出。

## 外部链接

- Hartley oscillator, Integrated Publishing