Stable, 18-MHz oscillator features automatic level control, clean-sine-wave output

A recent Design Idea described a method for designing simple, high-frequency LC oscillators with few passive components (Reference 1). However, for best results, practical hardware design of a stable oscillator requires more parts and greater complexity. Figure 1 shows a stable, 18-MHz oscillator with automatically leveled output amplitude control and an output buffer that delivers a sine wave with low harmonic content (Reference 2). In addition, this Design Idea replaces the original JFET oscillator with an inexpensive dual-gate MOSFET: an Infineon Technologies BF998, available from DigiKey and other sources.

The heart of the circuit comprises a Hartley oscillator, Q₁. To minimize loading, a 10-kΩ resistor couples the output from Q₁'s source to the high-input-impedance gate of source follower JFET Q₂. In turn, Q₂ drives Q₃, a BJT (bipolar-junction-transistor) emitter follower, which in turn drives BJT amplifier Q₄. Toroidal-core transformer T₁ couples Q₄'s output to a 50Ω load, delivering 2.61V p-p or 12.3 dBm. A Spice-circuit simulation predicts a second-harmonic amplitude of 35 dB below the fundamental. The second harmonic exceeds the amplitudes of all higher order harmonics, and an oscilloscope measurement displays a clean-looking sine wave across the 50Ω load.

To provide a good termination for the amplifier and still obtain 7.3 dBm (1.47 V p-p)—for example, to drive a diode-ring mixer—you can insert a 50Ω, 5-dB pad between output transformer T₁ and the load. Potentiometer R₂ adjusts the RF output level, and, for increased stability, you can replace R₂ with a fixed resistive divider built with low-temperature-coefficient, metal-film, fixed resistors. Part of the signal at Q₄'s collector drives the gate of JFET source follower Q₅ through C₇ and R₉. Diode D₁ rectifies the signal, which, after filtering, feeds operational amplifier IC₁'s inverting input. Resistor R₁ and low-temperature-coefficient potentiometer R₂ divide the 12V supply to produce a dc reference voltage for IC₁'s noninverting input and set the output signal's level. After filtering, IC₁'s dc output drives Q₁'s Gate 2 to set the device's gain and thus control RF output.

Connected to the center tap of coil L₁, trimmer capacitor C₁₈ allows fine adjustment of the oscillator's frequency. If decreased frequency stability is acceptable, you can replace C₁₈ with a low-cost ceramic trimmer. Piston-type trimmers are rather expensive and less available than ceramic trimmers, but a typical ceramic trimmer exhibits a temperature coefficient that's at least an order of magnitude worse than a piston trimmer's. To operate the oscillator at a frequency other than 18 MHz, multiply the inductance of L₁ and the capacitances of C₁₂, C₁₃, C₁₆, C₁₇, and C₁₈ by 18/f_OSC2, where f_OSC2 is the new frequency in megahertz. Adjust the tap for Q₁'s source connection so that it remains at about 20% of the winding's total number of turns as counted from the inductor's grounded end.

A well-regulated external dc source (not shown) provides 12, –12, and 8V to the circuit. To maintain high stability and remain within Q₁'s 12V maximum drain-to-source-voltage rating, the 8V supply powers only the oscillator. Using the specified components and at a constant ambient temperature of 22°C, after an initial 10-minute warmup period, the oscillator's frequency drifts at an average rate of –2 to –3 Hz per minute over one hour.