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MC13150 Arkusz danych(PDF) 10 Page - Motorola, Inc |
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MC13150 Arkusz danych(HTML) 10 Page - Motorola, Inc |
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10 / 20 page  MC13150 10 MOTOROLA ANALOG IC DEVICE DATA Local Oscillators HF & VHF Applications In the application schematic, an external sourced local oscillator is utilized in which the base is biased via a 51 Ω resistor to VCC. However, the on–chip grounded collector transistor may be used for HF and VHF local oscillators with higher order overtone crystals. Figure 16 shows a 5th overtone oscillator at 83.616 MHz. The circuit uses a Butler overtone oscillator configuration. The amplifier is an emitter follower. The crystal is driven from the emitter and is coupled to the high impedance base through a capacitive tap network. Operation at the desired overtone frequency is ensured by the parallel resonant circuit formed by the variable inductor and the tap capacitors and parasitic capacitances of the on–chip transistor and PC board. The variable inductor specified in the schematic could be replaced with a high tolerance, high Q ceramic or air wound surface mount component if the other components have tight enough tolerances. A variable inductor provides an adjustment for gain and frequency of the resonant tank ensuring lock up and start–up of the crystal oscillator. The overtone crystal is chosen with ESR of typically 80 Ω and 120 Ω maximum; if the resistive loss in the crystal is too high the performance of oscillator may be impacted by lower gain margins. A series LC network to ac ground (which is VCC) is comprised of the inductance of the base lead of the on–chip transistor and PC board traces and tap capacitors. Parasitic oscillations often occur in the 200 to 800 MHz range. A small resistor is placed in series with the base (Pin 28) to cancel the negative resistance associated with this undesired mode of oscillation. Since the base input impedance is so large, a small resistor in the range of 27 to 68 Ω has very little effect on the desired Butler mode of oscillation. The crystal parallel capacitance, Co, provides a feedback path that is low enough in reactance at frequencies of 5th overtones or higher to cause trouble. Co has little effect near resonance because of the low impedance of the crystal motional arm (Rm–Lm–Cm). As the tunable inductor, which forms the resonant tank with the tap capacitors, is tuned off the crystal resonant frequency, it may be difficult to tell if the oscillation is under crystal control. Frequency jumps may occur as the inductor is tuned. In order to eliminate this behavior an inductor, Lo, is placed in parallel with the crystal. Lo is chosen to resonant with the crystal parallel capacitance, Co, at the desired operation frequency. The inductor provides a feedback path at frequencies well below resonance; however, the parallel tank network of the tap capacitors and tunable inductor prevent oscillation at these frequencies. Figure 16. MC13150FTB Overtone Oscillator fRF = 83.16 MHz; fLO = 83.616 MHz 5th Overtone Crystal Oscillator MC13150 Mixer VEE 28 33 (4) 0.135 µH 39 p 27 k 10 n 1.0 µH VCC (3) 5th OT XTAL + 29 31 1.0 µ 39 p |
Podobny numer części - MC13150 |
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Podobny opis - MC13150 |
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