J

10-2

Feedback oscillator circuits: (A) Armstrong, (B) Colpitts, and (C) Hartley.

1 - to '¿0-MHz crystui oscillator 245

The resonator of the oscillator could bo either an inductor-capacitor (LC) tuned circuit, as shown in the examples of Fig. 10 2, or a piezoelectric crystal resonator. The latter are single-channel resonators, but are more stable than LC-tuned circuits,

1- to 20-MHz crystal oscillator

Figure 10-3 shows a simple, nearly universal signal generator that cart accommodate crystal frequencies between 1 and 20 MHz. This oscillator operates in the fundamental mode so the marked frequency of the crystal is the actual frequency that it operates on (as opposed to overtone crystals that operate on harmonics of the fundamental frequency). When specifying the crystal, if you are given a choice, ask for a crystal that operates into 32 pF of capacitance. Otherwise, the actual operating frequency might be a little different than the specified frequency.

The amplifier device is a simple bipolar NPN transistor. The transistor selected is not critical, but the 2N3904 and 2N2222 devices have been tried on numerous occasions in this circuit. Whatever transistor is selected, it must operate as an oscillator in the frequency range of interest. If you want to use a similarly rated PNP transistor (e.g., 2N3906), then simply reverse the polarity of the dc power supply. The transistor uses the simplest form of dc bias network: Ri is directly connected between the transistor's collector and its base. The outjmt. signal is taken through a dc-blocking capacitor (C5) acrosa the emitter resistor (R>). A bypass capacitor (0 01 to 0.1 jj,F) is connected between the collector and ground. This capacitor sets the collector at ground potential for ac potentials and keeps it at the rated dc power-supply potential (9 to 12 Vdc),

The feedback network consists of capacitors Cj ;uid C-. The ratio of the values, C1/C2, is selected to achieve a reasonable tradeoff of output level and stability. These capacitors should be either silver mica or (preferably) NPO disk ceramic capacitors. Use of these types of capacitor will prevent frequency drift of the oscillator as a result of temperature-dependent changes in the capacitance of Ci and C3. For the same reason, capacitor Ci should also be a silver mica or NPO disk ceramic.

The circuit as shown in Fig. 10-3 operates at a single frequency because it is crystal controlled. Although it's iiice to think that the crystal always operates 011 the specified frequency, it's likely that differences in circuit capacitance will make it oscillate at a different frequency. If the exact frequency is important then replace CJt with a variable capacitor. Alternatively, connect a 50-pF variable capacitor in parallel across the crystal.

The circuit of Fig, 10-3 is popular with people who have to troubleshoot FM radio receivers. Figure 10-4 shows a signal generator that can be used to align and test common FM radio receivers. The basic circuit is the same as Fig, 10-3, but the cj-ys-tal is replaced with a SPDT switch and a pair of crystals. The 10.7-MHz crystal is used to test the FM !F amplifier stages, so it is on the standard FM IF frequency. The 9-MHz crystal is used to test the front-end and tuning diai accuracy of the receiver. A 9-MHz crystal will output sufficient harmonics at 90,99, and 108 MHz to check the calibration of the low end, middle, and high end of the dial, respectively. Notice that this signal generator does not produce an audio tone in the output of the receiver but

9 to

12 Vdt

12 Vdt

10-3 Colpitis crystal oscillator.

10-4 FM alignment oscillator. Use 10.7 MHz for IF alignment and 9-MHz to align local oscillator at 90,99, or 106 MHz.

rather it is indicated by "quieting" of the receiver and an increase in the strength meter reading,

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