Universal, very low distortion, HF broadband crystal oscillator

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+12dBm max, 160m - 10m very low distortion (all harmonics at least -40dBc) XO
requiring only the crystal to be changed for covering different frequencies.

This is one of my best designs so far. Using common components, a superior oscillator can be built, useful as a test oscillator for the lab, as a driver for HF transmitters and receiver mixers etc. I wanted a universal oscillator that could oscillate at all HF bands without changing any components apart from the resonator. This would save cost and pain, for having one circuit made for each band. Also, This oscillator can oscillate at any frequency between bands as well, not only inside the ham bands. Moreover, the oscillator is cheap and easy to build and it has very low distortion (harmonics) at all bands. Combining these characteristics is very difficult and I have not seen such a design posted elsewhere. I have been testing this circuit for quite a long on FFT and I have done quite a few refinements to conclude to this final version.

All coils (LPF and chokes) are molded types +/-10% tolerance or better. The 200nH coil, is composed from two 100nH in series. All these three coils in the LPF, should be aligned in a 3D perpendicular way, if no shielding is used between them, to minimize mutual inductance. That is, the first coil should be perpendicular to the second (X-Y axis) and the second coil must start after the end of the first. The third coil should be perpendicular to the second coil and starting from it's end (Y-Z axis), but also perpendicular to the first coil (X-Z axis). The LPF capacitors are silver mica types 1% tolerance. The 82pF and 150pF capacitors near the JFET, must be NP0 types.

The 22k trimmer, is used to set the output level of the oscillator, for each new crystal that is inserted. The size of the trimmer must be small and it must be kept close to the JFET. Set this trimmer for +12dBm at 50 ohm maximum output level, if you want the harmonics to be kept low (at least -40dBc). In most frequencies you can get more output, but the harmonics and the intermodulation products are increased considerably at higher levels.

Both crystals and ceramic resonators/filters can be used in the oscillator with similar performance. The two BFR96S transistors are getting extremely hot (especially the final one), so leave some space for them, for passive air cooling.

If you want constant output level and frequency changing by the flip of a switch, install each crystal and it's corresponding trimmer, onto each position of a stereo rotary switch and set this trimmer to the desired output level for that crystal.

If you want constant output level for each crystal inserted but without using many trimmers, try an ALC circuit. ALC has been tried with success, by replacing the 22k trimmer with a vt43n1 LDR and pointing a light source onto it (LED or better a low current incadescent bulb) driven from the output of the oscillator through a directional coupler and after some amplification and rectification.

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