MC3374FTB View Datasheet(PDF) - Motorola => Freescale

Part Name

Description

Manufacturer

MC3374FTB

Low Voltage FM Narrowband Receiver

Motorola => Freescale 

Freescale SeMmC3i3c7o4nductor, Inc.

CIRCUIT DESCRIPTION

The MC3374 is an FM narrowband receiver capable of

operation to 75 MHz. The low voltage design yields low

power drain and excellent sensitivity in narrowband voice

and data link applications. In the typical application the mixer

amplifies the incoming RF or IF signal and converts this

frequency to 455 kHz. The signal is then filtered by a 455 kHz

ceramic filter and applied to the first intermediate frequency

(IF) amplifier input, before passing through a second ceramic

filter. The modulated IF signal is then applied to the limiting IF

amplifier and detector circuitry. Modulation is recovered by a

conventional quadrature detector. The typical modulation

bandwidth available is 3.0 to 5.0 kHz.

Features available include buffers for audio/data

amplification and active filtering, on board voltage regulator,

low battery detection circuitry with programmable level, and

receiver disable circuitry. The MC3374 is an FM utility

receiver to be used for voice and/or narrowband data

reception. It is especially suitable where extremely low power

consumption and high design flexibility are required.

APPLICATION

The MC3374 can be used as a high performance FM IF for

the use in low power dual conversion receivers. Because of

the MC3374’s extremely good sensitivity (0.6 µV for 20 dB

(S+N/N, see Figure 3)), it can also be used as a stand alone

single conversion narrowband receiver to 75 MHz for

applications not sensitive to image frequency interference.

An RF preamplifier will likely be needed to overcome

preselector losses.

The oscillator is a Colpitts type which must be run under

crystal control. For fundamental mode crystals choose

resonators, parallel resonant, for a 32 pF load. For higher

frequencies, use a 3rd overtone series mode type. The coil

L2 and RD resistor are needed to ensure proper operation.

The best adjacent channel and sensitivity response occur

when two 455 kHz ceramic filters are used, as shown in

Figure 1. Either can be replaced by a 0.1 µF coupling

capacitor to reduce cost, but some degradation in sensitivity

and/or stability is suspected.

The detector is a quadrature type, with the connection

from the limiter output to the detector input provided

internally. A 455 kHz LC tank circuit must be provided

externally. One of the tank pins (Pin 8) must be decoupled

using a 0.1 µF capacitor. The 56 kΩ damping resistor (see

Figure 1), determines the peak separation of the detector

(and thus its bandwidth). Smaller values will increase the

separation and bandwidth but decrease recovered audio and

sensitivity.

The data buffer is a noninverting amplifier with a nominal

voltage gain of 2.7 V/V. This buffer needs its dc bias

(approximately 250 mV) provided externally or else

debiasing will occur. A 2nd order Sallen–Key low pass filter,

as shown in Figure 1, connecting the recovered audio output

to the data buffer input provides the necessary dc bias and

some post detection filtering. The buffer can also be used as

an active filter.

The audio buffer is a noninverting amplifier with a nominal

voltage gain of 4.0 V/V. This buffer is self–biasing so its input

should be ac coupled. The two buffers, when applied as

active filters, can be used together to allow simultaneous

audio and very low speed data reception. Another possible

configuration is to receive audio only and include a

noise–triggered squelch.

The comparator is a noninverting type with an open

collector output. Typically, the pull–up resistor used between

Pin 14 and VCC is 100 kΩ. With RL = 100 kΩ the comparator

is capable of operation up to 25 kHz. The circuit is

self–biasing, so its input should be ac coupled.

The regulator is a 1.07 V reference capable of sourcing

3.0 mA. This pin (Pin 17) needs to be decoupled using a

1.0 – 10 µF capacitor to maintain stability of the MC3374.

All three VCCs on the MC3374 (VCC, VCC2, VCC3) run on

the same supply voltage. VCC is typically decoupled using

capacitors only. VCC2 and VCC3 should be bypassed using

the RC bypasses shown in Figure 1. Eliminating the resistors

on the VCC2 and VCC3 bypasses may be possible in some

applications, but a reduction in sensitivity and quieting will

likely occur.

The low battery detection circuit gives an NPN open

collector output at Pin 20 which drops low when the MC3374

supply voltage drops below 1.2 V. Typically it would be pulled

up via a 100 kΩ resistor to supply.

The 1.2 V Select pin, when connected to the MC3374 supply,

programs the low battery detector to trip at VCC < 1.1 V. Leaving

this pin open raises the trip voltage on the low battery detector.

Pin 15 is a receiver enable which is connected to VCC for

normal operation. Connecting this pin to ground shuts off

receiver and reduces current drain to ICC < 0.5 µA.

APPENDIX

Design of 2nd Order Sallen–Key Low Pass Filters

R1

Input

C1

+

R2 C2

–

Bias Avo = K

Low Pass Output

0 to fo Hz

The audio and data buffers can easily be configured as active

low pass filters using the circuit configuration shown above.

The circuit has a center frequency (fo) and quality factor (Q)

given by the following:

+ Ǹ fo

1

2p R1R2C1C2

Ǹ Ǹ Ǹ + Q

1

R2C2

) ) R1C1

R1C2

R2C1

(1–K)

R1C1

R2C2

If possible, let R1 = R2 or C1 = C2 to simplify the above

+ ńǸ + equations. Be sure to avoid a negative Q value to prevent

instability. Setting Q 1 2 0.707 yields a maximally flat

filter response.

6

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