MC13150FTA Просмотр технического описания (PDF) - Motorola => Freescale
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MC13150FTA Datasheet PDF : 20 Pages
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MC13150
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
28
29
VEE 31
(4)
0.135 µH
33
1.0 µH 39 p
39 p
(3)
27 k 5th OT
XTAL
10 n
+
1.0 µ
VCC
10
MOTOROLA ANALOG IC DEVICE DATA
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