LM3661AM NSC [National Semiconductor], LM3661AM Datasheet - Page 5

• Manufacturer Part Number: LM3661AM
• Description: LM3361A Low Voltage/Power Narrow Band FM IF System
• Manufacturer: NSC [National Semiconductor]

Applications Information
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
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 3 6 k
1 8 k
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 1 8 k
nate the ceramic filter driving the IF The IF alone (without
mixer) has a
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-
V The system bandwidth is limited to about 5 MHz due to
will properly match the input impedance of a ceramic
b
3 dB limiting sensitivity of approximately 50
The mixer output impedance of
so Ct
and f
O
e
e
(C1)(C2) (C1
159 L(Ct)
to properly termi-
(See Internal Schematic)
a
TL H 5586– 4
C2)
5
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
tank R
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 0 7V receives dc bias from the output at pin 11
through the external feedback network Because of the low
D C bias maximum swing on the op amp output with 10%
distortion is 500 mVrms This can be increased when oper-
ating on supplies over 2 3V by adding a resistor from the op
amp input to ground which raises the quiescent D C 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
For max swing V
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 least a 1 mA load and pin
14 to be a low impedance typically 15
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
(0 65V) will force pin 13 to an impedance of about 60 k
ground and pin 14 will be an open circuit There is 100 mV
of hysterisis at pin 12 which effectively prevents jitter
p
voltage divider network After detection and de-em-
so (V
therefore
Increasing OP Amp Swing
OUT
S
b
e
V
BE
(V
R2
R3
S
) 2
b
e
V
e
BE
0 7 1
) 2 (from internal circuit)
V
S
b
1 4
a
V
BE
R2
R3
to ground Con-
b
1
TL H 5586 – 5
to