OSCILLATOR

The Colpitts type oscillator is internally biased with a regu-

lated current source which assures proper operation over a

wide supply range The collector base and emitter termi-

nals are at pins 4 1 and 2 respectively The crystal which

is used in the parallel resonant mode may be replaced with

an appropriate inductor if the application does not require

the stability of a crystal oscillator In this case the resonant

frequency will be determined by the inductor in parallel with

the series combination of C1 and C2

TLH5586 – 4

MIXER

The mixer is double balanced to reduce spurious responses

The upper pairs are switched by the oscillator while the RF

input is applied to the lower pair (pin 16) R43 sets the mixer

input impedance at 36 kX The mixer output impedance of

18 kX will properly match the input impedance of a ceramic

filter which is used as a bandpass filter coupling the mixer

output to the IF limiting amplifier

IF LIMITER

The IF amplifier consists of six differential gain stages with

the input impedance set by R2 at 18 kX to properly termi-

nate the ceramic filter driving the IF The IF alone (without

m

V The system bandwidth is limited to about 5 MHz due to

high impedances in the IF which are necessary to meet low

power requirements The IF output is connected to the ex-

ternal quad coil at pin 8 via an internal 10 pF capacitor

FM DEMOD AUDIO OUT

A conventional quadrature detector is used to demodulate

the FM signal The Q of the quad coil which is determined

by the external resistor placed across it has multiple effects

on the audio output Increasing the Q increases output level

but because of nonlinearities in the tank phase characteris-

tic

also increases distortion (see Typical Performance

Characteristics) For proper operation the voltage swing on

pin 8 should be adequate to drive the upper rank of the

multiplier into switching (about 100 mVrms) This voltage

level is dependent on the internal 10 pF capacitor and the

phasis the audio output at pin 9 is buffered by an emitter

follower

OP AMP

The op amp inverting input (pin 10) which is internally refer-

enced to 07V receives dc bias from the output at pin 11

through the external feedback network Because of the low

DC bias maximum swing on the op amp output with 10%

distortion is 500 mVrms This can be increased when oper-

ating on supplies over 23V by adding a resistor from the op

amp input to ground which raises the quiescent DC at the

output allowing more swing (see figure below for selection

of added resistor) The op amp is normally utilized as either

a bandpass filter to extract a specific frequency from the

audio output such as a ring or dial tone or as a high pass

filter to detect noise due to no input at the mixer The latter

condition will generate a signal at the op amp output which

when applied to pin 12 can mute the external audio amp

therefore

R2

R3

e

14

TLH5586 – 5

Increasing OP Amp Swing

SQUELCH TRIGGER CIRCUIT

The squelch trigger circuit is configured such that a low bias

on the input (pin 12) will force pin 13 high (200 mV below

supply) where it can support at leasta1mA load and pin

14 to be a low impedance typically 15X to ground Con-

necting pin 14 to a high impedance ground reference point

in the audio path between pin 9 and the audio amp will mute

the audio output Pulling pin 12 above mute threshold

(065V) will force pin 13 to an impedance of about 60 kX to

ground and pin 14 will be an open circuit There is 100 mV

of hysterisis at pin 12 which effectively prevents jitter

5