MC13158FTB View Datasheet(PDF) - LANSDALE Semiconductor Inc.

Part Name

Description

Manufacturer

MC13158FTB

Wideband FM IF Subsystem For Dect and Digital Applications

LANSDALE Semiconductor Inc. 

ML13158

Legacy Applications Information

LANSDALE Semiconductor, Inc.

LOCAL OSCILLATORS

VHF APPLICATIONS

The on–chip grounded collector transistor may be used for HF and

VHF local oscillator with higher order overtone crystals. The local

oscillator in the application circuit (Figure 12) shows a 5th overtone

oscillator at 122.7 MHz. This circuit uses a Butler overtone oscilla-

tor 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 capacitor 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 reso-

nant 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 per-

formance of oscillator may be impacted by lower gain margins.

A series LC network to 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 capaci-

tors 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 behav-

ior an inductor, Lo, is placed in parallel with the crystal. Lo is cho-

sen to be resonant with he crystal parallel capacitance, Co, at the

desired operation frequency. The inductor provides a feedback path

at frequencies well below resonance; however, the parallel tank net-

work of the tap capacitors and tunable inductor prevent oscillation

at these frequencies.

Page 15 of 23

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