by SV3ORA
<20KHz to >65MHz sinusoidal oscillator
by SV3ORA
I have build this oscillator and it worked at first time. It is a VFO 30KHz-30MHz oscillator out of an expensive old signal generator and I found it to extend to more frequencies. I replaced the 100pF variable capacitor with a 700pF and the frequency range extended down to 20KHz. Also not so many band switching coils were needed using this capacitor. I used just 7 coils to cover 20KHz to 30MHz. I have managed to make this oscillator work up to about 65MHz using other air wound coils. Just try almost every coil to see the range. I am pretty sure it can go higher in frequency if using VHF building techniques and appropriate coils but I did not test it as it was of no interest to me at the time. The oscillator can extend much lower too if you use bigger capacitor or greater inductance coil. At 6.5KHz the sinusoidal was highly distorted but I used a small transformer primary as a coil and I did not manage to optimize the circuit for such low frequencies. The sinusoidal signal has good shape and it is very amplitude steady throughout all the bands. The voltage at point +V must be +7V only, for low sinusoidal distortion. Set the 2K2 potentiometer at the LM317L regulator at about 1.2K to obtain this voltage. Then the regulator will maintain this output voltage as long as it is fed by an input voltage greater than 8.5V. The circuit current consumption is about 20mA. The VFO shows some frequency drifting but I have been informed that this can be minimized using rigid construction good quality ceramic coils and silver plated variable capacitor. I believe that temperature stabilizing would worth it too. If you put an audio signal at point MOD you can modulate the oscillator. D1 and D2 are 1N4148
You can use this oscillator in almost all the RF constructions. As a local oscillator for very broadband receivers, as an oscillator for very broadband transmitters, as a sinusoidal function generator, as a wideband VCO (by replacing the variable capacitor with a varicap, e.g. BB212) for a PLL controlled oscillator etc.