MC3335 Просмотр технического описания (PDF) - Motorola => Freescale

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MC3335

LOW POWER DUAL CONVERSION FM RECEIVER

Motorola => Freescale 

CIRCUIT DESCRIPTION

MC3335

The MC3335 is a complete FM narrowband receiver from

antenna input to audio preamp output. The low voltage dual

conversion design yields low power drain, excellent

sensitivity and good image rejection in narrowband voice and

data link applications.

In the typical application diagram, the first mixer amplifies

the signal and converts the RF input to 10.7 MHz. This IF

signal is filtered externally and fed into the second mixer,

which further amplifies the signal and converts it to a 455 kHz

IF signal. After external bandpass filtering, the low IF is fed

into the limiting amplifier and detection circuitry. The audio is

recovered using a conventional quadrature detector.

Twice–IF filtering is provided internally.

The input signal level is monitored by meter drive circuitry

which detects the amount of limiting in the limiting amplifier.

The voltage at the meter drive pin determines the state of the

carrier detect output which is active low.

APPLICATIONS INFORMATION

The first local oscillator can be run using a free running LC

tank, as a VCO using PLL synthesis, or driven from an

external crystal oscillator. At higher VCC values (6.0 to

7.0 V), it has been run to 170 MHz. The second local

oscillator is a common base Colpitts type which is typically

run at 10.245 MHz under crystal control.

The mixers are doubly balanced to reduce spurious

responses. The first and second mixers have conversion

gains of 18 dB and 22 dB (typical), respectively. Mixer gain is

stable with respect to supply voltage. For both conversions,

the mixer impedances and pin layout are designed to allow

the user to employ low cost, readily available ceramic filters.

Overall sensitivity is shown in Figure 5. The input level for

20 dB (S + N)/N is 1.3 µV using the two–pole post–detection

filter as demonstrated.

Following the first mixer, a 10.7 MHz ceramic bandpass

filter is recommended. The 10.7 MHz filtered signal is then

fed into one second mixer input pin, the other input pin being

connected to VCC. Pin 5 (VCC) is treated as a common point

for emitter–driven signals.

The 455 kHz IF is typically filtered using a ceramic

bandpass filter, then fed into the limiter input pin. The limiter

has 10 µV sensitivity for – 3.0 dB limiting, flat to 1.0 MHz.

The output of the limiter is internally connected to the

quadrature detector, including a quadrature capacitor. A

parallel LC tank is needed externally from Pin 11 to VCC.

A 39 kΩ shunt resistance is included which determines the

peak separation of the quadrature detector; a smaller value

will increase the spacing and linearity but decrease

recovered audio and sensitivity.

A data shaping circuit is available and can be coupled to

the recovered audio output of Pin 12. The circuit is a

comparator which is designed to detect zero crossings of

FSK modulation. Data rates of up to 35000 baud are

detectable using the typical application. Hysteresis is

available by connecting a high–valued resistor from Pin 13 to

Pin 14. Values below 120 kΩ are not recommended as the

input signal cannot overcome the hysteresis.

The meter drive circuitry detects input signal level by

monitoring the limiting of the limiting amplifier stages.

Figure 2 shows the unloaded current at Pin 9 versus input

power. The meter drive current can be used directly (RSSI)

or can be used to trip the carrier detect circuit at a specified

input power. To do this, pick an RF trip level in dBm. Read the

corresponding current from Figure 2 and pick a resistor such

that:

R9 = 0.64 Vdc / I9

Hysteresis is available by connecting a high–valued

resistor RH between Pin 9 and 10. The formula is:

Hysteresis = VCC/(RH x 10 – 7) dB

4

MOTOROLA ANALOG IC DEVICE DATA

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