With the help of Prof. Pozzi I was able to observe clear indication of series resonance using my .003mH coil. At 9.1Mhz this coil and a 1pF capacitor were used. The resonant bandwidth of +-5% Vmax was very narrow with Vmax falling at +- 200kHz. Colpitts Oscillator – I would like to be able to build an oscillator for this project. I chose the Colpitts because it seemed fairly simple.
I was unable to get this circuit to oscillate on the first build. I trimmed the leads on all components to reduce unwanted impedances. I finally was able to observe a 2.1MHz sinusoidal oscillation in the above configuration. I also observed a 8Mhz signal and as high as 13Mhz by adjusting the values of C2 & C3. It is important to note that this circuit is very temperamental. At times it was difficult to initiate oscillation. Prof. Clark pointed out that stray impedances on the breadboard are problematic at high frequencies. He says I would get better results on a circuit board.
When the Colpitts worked, the oscillations were stable and I measured 400mVPP. The next step would be to amplify this signal. I attempted to use a 741 in an inverting configuration with a gain of 2. Unfortunately when the Colpitts was connected to the 741 it stopped working entirely at which point I gave up for the night.
3/17/08
Colpitts testing again - Was able build stable, functioning oscillator. I must’ve misplaced a capacitor during the last trial because the Colpitts worked perfectly this time. I attempted to slow down the oscillator using larger capacitors but was unable to attain kHz oscillations. Not sure why. Also, I added a capacitor in parallel with C3 (see above) with resulted in approximately 75% increase in amplitude at the same frequency. The observed oscillation amplitude was 2Vpp-3.5Vpp with a source of 10VDC-30VDC.
Colpitts Oscillator with loosely wound air-core coil – I connected one end of the transmission coil to the node between L1 and C2. The other end was free. With fo of 8.54MHz and Afo of 2Vpp I observed 1Vpp on the secondary coil. I was unable to observe resonance at this frequency since the capacitance required to match the 0.004mH inductor was 9E-11F which was unavailable to me (and might not exist).
Conclusion – I need to slow the Colpitts to a frequency that is more useful.
