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ML3372EP

Low Powe Narrowband FM IF

LANSDALE Semiconductor Inc. 

ML3371, ML3372

LANSDALE Semiconductor, Inc.

CIRCUIT DESCRIPTION

The ML3371 and ML3372 are low power narrowband FM

receivers with an operating frequency of up to 60 MHz. Its low

voltage design provides low power drain, excellent sensitivity,

and good image rejection in narrowband voice and data link

applications.

This part combines a mixer, an IF (intermediate frequency)

limiter with a logarithmic response signal strength indicator, a

quadrature detector, an active filter and a squelch trigger cir-

cuit. In a typical application, the mixer amplifier converts an

RF input signal to a 455 kHz IF signal. Passing through an

external bandpass filter, the IF signal is fed into a limiting

amplifier and detection circuit where the audio signal is recov-

ered. A conventional quadrature detector is used.

The absence of an input signal is indicated by the presence of

noise above the desired audio frequencies. This “noise band” is

monitored by an active filter and a detector. A squelch switch is

used to mute the audio when noise or a tone is present. The

input signal level is monitored by a meter drive circuit which

detects the amount of IF signal in the limiting amplifier.

LEGACY APPLICATIONS INFORMATION

The oscillator is an internally biased Colpitts type with the

collector, base, and emitter connections at Pins 4, 1 and 2

respectively. This oscillator can be run under crystal control.

For fundamental mode crystals use crystal characterized paral-

lel resonant for 32 pF load. For higher frequencies, use 3rd

overtone series mode type crystals. The coil (L2) and resistor

RD (R13) are needed to ensure proper and stable operation at

the LO frequency (see Figure 13, 45 MHz application circuit).

The mixer is doubly balanced to reduce spurious radiation.

Conversion gain stated in the AC Electrical Characteristic sta-

ble is typically 20 dB. This power gain measurement was made

under stable conditions using a 50 Ω source at the input and an

external load provided by a 455 kHz ceramic filter at the mixer

output which is connected to the VCC (Pin 4) and IF input

(Pin 5). The filter impedance closely matches the1.8 kΩ inter-

nal load resistance at Pin 3 (mixer output). Since the input

impedance at Pin 16 is strongly influenced by a 3.3 kΩ inter-

nal biasing resistor and has a low capacitance, the useful gain

is actually much higher than shown by the standard power gain

measurement. The Smith Chart plot in Figure 17 shows the

measured mixer input impedance versus input frequency with

the mixer input matched to a 50Ω source impedance at the

given frequencies. In order to assure stable operation under

matched conditions, it is necessary to provide a shunt resistor

to ground. Figures 11, 12 and 13 show the input networks used

to derive the mixer input impedance data.

Following the mixer, a ceramic bandpass filter is recom-

mended for IF filtering (i.e. 455 kHz types having a bandwidth

of ±2.0 kHz to ±15 kHz with an input and output impedance

from 1.5 kΩ to 2.0 kΩ). The 6 stage limiting IF amplifier has

approximately 92 dB of gain. The MC3371 and MC3372 are

different in the limiter and quadrature detector circuits. The

MC3371 has a 1.8 kΩ and a 51 kΩ resistor providing internal

dc biasing and the output of the limiter is internally connected,

both directly and through a 10 pF capacitor to the quadrature

detector; whereas, in the MC3372 these components are not

provided internally. Thus, in the MC3371, no external compo-

nents are necessary to match the 455 kHz ceramic filter, while

in the MC3372, external 1.8 kΩ and 51 kΩ biasing resistors

are needed between Pins 5 and 7, respectively (see Figures 12

and 13).

In the MC3371, a parallel LCR quadrature tank circuit is

connected externally from Pin 8 to VCC (similar to the

MC3361). In the MC3372, a quadrature capacitor is needed

externally from Pin 7 to Pin 8 and a parallel LC or a ceramic

discriminator with a damping resistor is also needed from Pin

8 to VCC (similar to the MC3357). The above external quadra-

ture circuitry provides 90° phase shift at the IF center frequen-

cy and enables recovered audio.

The damping resistor determines the peak separation of the

detector and is somewhat critical. As the resistor is decreased, the

separation and the bandwidth is increased but the recovered

audio is decreased. Receiver sensitivity is dependent on the value

of this resistor and the bandwidth ofthe 455 kHz ceramic filter.

On the chip the composite recovered audio, consisting of

carrier component and modulating signal, is passed through a

low pass filter amplifier to reduce the carrier component and

then is fed to Pin 9 which has an output impedance of 450Ω.

The signal still requires further filtering to eliminate the carrier

component, deemphasis, volume control, and further amplifi-

cation before driving a loudspeaker. The relative level of the

composite recovered audio signal at Pin 9 should be consid-

ered for proper interaction with an audio post amplifier and a

given load element. The MC13060 is recommended as a low

power audio amplifier.

The meter output indicates the strength of the IF level and

the output current is proportional to the logarithm of the IF

input signal amplitude. A maximum source current of 60 µA is

available and can be used to drive a meter and to detect a carri-

er presence. This is referred to as a Received Strength Signal

Indicator (RSSI). The output at Pin 13 provides a current

source. Thus, a resistor to ground yields a voltage proportional

to the input carrier signal level. The value of this resistor is

estimated by (VCC(Vdc) – 1.0 V)/60 µA; so for VCC = 4.0

Vdc, the resistor is approximately 50 kΩ and provides a maxi-

mum voltage swing of about 3.0 V.

A simple inverting op amp has an output at Pin 11 and the

inverting input at Pin 10. The noninverting input is connected

to 2.5 V. The op amp may be used as a noise triggered squelch

or as an active noise filter. The bandpass filter is designed with

external impedance elements to discriminate between frequen-

cies. With an external AM detector, the filtered audio signal is

checked for a tone signal or for the presence of noise above the

normal audio band. This information is applied to Pin 12.

Page 11 of 19

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