1- a Second Mechanically-Tuned Oscillator Was
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I-30V VaRAcroR +I5 TUNING VOLTAGE 0 0 0 01 I 1-5 Pb' I :' looh I J308 I 0 01 I \I - I I 0.01 I ., 1- I MVIO9 b lool IOOpH 100 m m fig. 6. Ceramic resonator circuit with AC voltage source fig. 5. Colpitts oscillator using varactor tuning and a and capacitive load. 3.58-MHz resonator. A second mechanically-tuned oscillator was con- structed and is shown in fig. 4. One of the problems associated with an oscillator using a ceramic resonator r-------7 and, to a certain extent, a crystal resonator, is that -2 -'-\cP when a large amount of external capacitance is added I I I , I the amount of feedback is reduced and oscillation can I I m become unsteady (stops oscillating). The oscillator I I I shown in fig. 4 represents an attempt to alleviate this I I -- /-. I problem. A dual 500-pF capacitor was used as the I I -- I tuning element across a 10.7-MHz resonator. The gain I /-. I I I of the FET amplifier was varied by increasing the ,, I I capacitance from the source to ground using half of L J the dual capacitor. As the capacitance across the resonator is increased, the gain of the amplifier also increases and the oscillations are stable over the entire fig. 7. Ceramic resonator oscillator. 500 pF range of the capacitor. The tuning range of this oscillator was 325 kHz at 10.7 MHz. A varactor-tuned Colpitts oscillator as shown in fig. 5 was constructed to evaluate the ability of the ceramic shown in series with an AC voltage source and an ex- resonator to serve as a VCO. A 4-MHz resonator was ternal capacitance which represents the varactor. tested, and the results agreed substantially with Since the impedance of the voltage source is zero, the theory. Using a pair of hyper-abrupt varactor diodes, external varactor capacitance is essentially in parallel it was possible to obtain nearly a 100 kHz frequency with the internal capacitance, Co. The voltage at point shift, which agrees with the theoretical calculation of A is exactly out of phase with the voltage at point B 120 kHz. With a large variation in capacitance an at the parallel resonant frequency of the inductor, L1, unsteady oscillatory condition was evident. This is and Co + C2 in series with C1. Only one resonant because the feedback capacitors were much smaller circuit is shown in fig. 6. The usual series resonant than the maximum capacitance of the varactors. It is circuit of L1 and C1 is not a factor here. If there were mandatory that the fixed capacitances across the a resistance between point A to ground, in parallel with ceramic resonator be kept to an absolute minimum. C3, there would be a second point at which the phase In the development of an oscillator that provides a of the voltage at A would be the same as the voltage maximum frequency shift using a ceramic resonator at point B, and that would occur at the series resonant as the tuned circuit, consider the circuit shown in fig. frequency determined by L1, and C1. 6. The equivalent circuit of a ceramic resonator is Figure 7 shows the equivalent circuit of the ceramic June 1985 21 .