MC13150FTA Просмотр технического описания (PDF) - LANSDALE Semiconductor Inc.

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MC13150FTA

Narrowband FM Coilless Detector IF Subsystem

LANSDALE Semiconductor Inc. 

ML13150

LANSDALE Semiconductor, Inc.

Legacy Applications Information

LOCAL OSCILLATORS

HF & VHF APPLICATIONS

In the application schematic, an external sourced local oscilla-

tor 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 con-

figuration. 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 par-

asitic 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

tolerance. 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 resis-

tive 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 com-

prised of the inductance of the base lead of 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 over-

tones or higher to cause trouble. Co has little effect near reso-

nance 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 induc-

tor, Lo, is placed in parallel with the crystal. Lo is chosen to

resonant with the crystal parallel capacitance, Co, at the

desired operation frequency. This inductor provides a feed-

back path at frequencies well below resonance; however, the

parallel tank network of the tap capacitors and tunable induc-

tor prevent oscillation at these frequencies.

Figure 16. ML13150 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

Page 10 of 20

www.lansdale.com

Issue A

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