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The Colpits oscillator is claimed to have very low sideband noise. In a study made by Tasa, YU1LM, http://yu1lm.qrpradio.com/HF-VHF%20universal%20oscillator-YU1LM.pdf the sideband noise was computed with Ansoft Harmonica to be -170dBc/Hz at frequency separations of 100 Hz and above.
Much more complicated designs that I made for this study are only a little better, 179 dBc/Hz @10 kHz and I thought it would be interesting to build a Colpitts oscillator as described in Tasas article and to measure the sideband noise.
This is the schematic diagram:
I have removed the output filter and increased the series resistor at the output to 47 ohms. The power level at the fundamental is 11.5 dBm. and the sideband noise at 10 kHz is -151 dBc/Hz. (Phase noise plus AM noise.) This is not quite as good as the simulations in Tasas paper indicate.
The reason is a common design problem in oscillators and frequency doublers and that is the high impedance at audio frequencies on the base of the transistor.
By adding two components, a 10 uF tantalum electrolytic capacitor and a small 1 mH inductor the noise can be reduced. For example like this :
The sideband noise of this modified Colpitts oscillator is -163 dBc/Hz. That is the sum of phase and amplitude noise so the phase noise alone is likely to be about -166 dBc/Hz and that is not far from the simulation.
Rather than using the schematic I used it would be possible to simply decouple the base of the transistor with a series link comprised of the inductor and the tantalum electrolyte to ground. That wouls also provide a very low impedance on the base at audio frequencies.
There is one problem with the modification and that is the parallel resonator that the inductor makes with the 470 pF capacitor. The inductor has to be very large in order to put this resonance at a low frequency for the impedance at resonance becomes very high. The resistor in parallel with the inductor will degrade the Q to a point where oscillations do not start at 300 kHz. I have investigated oscillators that use the series resonance because they can be used easily with overtone crystals in the VHF region. For fundamental frequency oscillators in the HF region there is another alternative and that is to use the parallel resonance with e.g. a Colpitts oscillator. The oscillating transistor will then see a high impedance on the resonance but low impedances at the side. The amplifier would see the impedance at the base/gate and produce less noise when the impedance is lower.
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