Design of an Automatic Level Controlled Sine Wave 18MHz Stable Oscillator

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This latest Design Idea shows a method for designing a simple high-frequency LC oscillator with a small number of passive components . However, actual hardware design of a stable oscillator requires more components and is more complex for best results. Figure 1 shows an 18MHz stable oscillator with automatic level output amplitude control and buffering to provide a sine wave output with low harmonic content (Reference 2). In addition, this Design Idea replaces the original JFET oscillator with an inexpensive BF998 dual-gate MOSFET from Infineon Technologies, available from DigiKey and other companies.

The heart of the circuit consists of a Hartly oscillator, Q1. To reduce loading, a 10kΩ resistor couples the source output of Q1 to the high input impedance gate of source follower JFET Q2. Q2 then drives BJT emitter follower Q3, which in turn drives BJT amplifier Q4. A toroidal core transformer, T1, couples the output of Q4 to a 50Ω load to provide 2.61Vp-p or 12.3 dBm. Spice circuit simulations predict a second harmonic with an amplitude 35dB below the fundamental. The second harmonic amplitude exceeds all higher harmonic amplitudes, and oscilloscope measurements show a clean-looking sine wave across the 50Ω load.

To provide a good termination for the amplifier and still get 7.3 dBm (1.47 Vp-p), for example, to drive a diode ring mixer, you can insert a 50Ω 5dB spacer between the output transformer T1 and the load. Resistor R2 is used to adjust the RF output level, and for improved stability, you can also replace R2 with a fixed resistor divider consisting of low temperature coefficient metal film fixed resistors. Part of the signal at the collector of Q4 drives the gate of the JFET source follower Q5 through C7 and R9. Diode D1 rectifies the signal, which is filtered and input to the inverting input of op amp IC1. Resistor R1 and low temperature coefficient potentiometer R2 shunt the 12V supply to provide a DC reference voltage to the positive input of IC1 and set the level of the input signal. After filtering, the DC output of IC1 drives the gate 2 of Q1 to set the gain of the device and control the RF output.


The frequency of the oscillator can be precisely adjusted using trimmer capacitor C18 connected to the center tap of coil L1. If the reduction in frequency stability is acceptable, C18 can be replaced with a low-cost ceramic trimmer. Piston-type trimmers are quite expensive and not as readily available as ceramic trimmers, but typical ceramic trimmers exhibit temperature coefficients at least an order of magnitude worse than piston-type trimmers. To operate the oscillator at frequencies other than 18 MHz, increase the inductance of L1 along with C12, C13, C16, C17, and C18 by a factor of 18/fOSC2. Where fOSC2 is the new frequency in MHz. Adjust the tap on the source connection of Q1 to keep it at approximately 20% of the total number of turns from the ground end of the inductor.

You can replace the series capacitor bank consisting of C12 and C13 with a 13pF capacitor, and C14 and C15 with a 2.5pF capacitor. If you want to redesign the circuit for a different output frequency, adjust the capacitance values ​​of C14 and C15, or their individual replacements, to get enough capacitance to ensure reliable startup under all expected operating conditions. Note, however, that using two capacitors, C16 and C17, helps reduce startup drift, as does using temperature-stable (NP0 characteristics) ceramic dielectric capacitors for C12 to C17. Buffer amplifiers Q2 to Q4 need to be modified to operate above approximately 25 MHz.

A well-regulated external DC power supply (not shown) provides 12V, -12V, and 8V to the circuit. To maintain high stability and keep Q1's maximum drain-source voltage specification of 12V, the oscillator can only use an 8V supply. At a constant ambient temperature of 22°C and using specified components, the average drift rate of the oscillator frequency is -2 to -3 Hz per minute over 1 hour after an initial 10-minute warm-up.

Reference address:Design of an Automatic Level Controlled Sine Wave 18MHz Stable Oscillator

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