MC3356

3

MOTOROLA WIRELESS SEMICONDUCTOR

SOLUTIONS – RF AND IF DEVICE DATA

Figure 3. Output Components of Signal,

Noise, and Distortion

Figure 4. Meter Current versus Signal Input

10

0

–10

–20

–30

–40

–50

–60

0.01

0.1

1.0

10

INPUT (mVrms)

S + N + D

fO = 100 MHz

fm = 1.0 kHz

∆f = ± 75 kHz

N + D

N

700

600

500

400

300

200

100

0

0.010

0.1

1.0

10

100

1000

PIN 20 INPUT (mVrms)

GENERAL DESCRIPTION

This device is intended for single and double conversion

VHF receiver systems, primarily for FSK data transmission

up to 500 K baud (250 kHz). It contains an oscillator, mixer,

limiting IF, quadrature detector, signal strength meter drive,

and data shaping amplifier.

The oscillator is a common base Colpitts type which can

be crystal controlled, as shown in Figure 1, or L–C controlled

as shown in the other figures. At higher VCC, it has been

operated as high as 200 MHz. A mixer/oscillator voltage gain

of 2 up to approximately 150 MHz, is readily achievable.

The mixer functions well from an input signal of

10

µVrms, below which the squelch is unpredictable, up to

about

10 mVrms,

before

any

evidence

of

overload.

Operation up to 1.0 Vrms input is permitted, but non–linearity

of the meter output is incurred, and some oscillator pulling is

suspected. The AM rejection above 10 mVrms is degraded.

The limiting IF is a high frequency type, capable of being

operated up to 50 MHz. It is expected to be used at 10.7 MHz

in most cases, due to the availability of standard ceramic

resonators. The quadrature detector is internally coupled to

the IF, and a 5.0 pF quadrature capacitor is internally

provided.

The –3dB limiting sensitivity of the IF itself is

approximately 50

µV (at Pin 7), and the IF can accept signals

up to 1.0 Vrms without distortion or change of detector

quiescent dc level.

The IF is unusual in that each of the last 5 stages of the

6 state limiter contains a signal strength sensitive, current

sinking device. These are parallel connected and buffered to

produce a signal strength meter drive which is fairly linear for

IF input signals of 10

µV to 100 mVrms (see Figure 4).

A simple squelch arrangement is provided whereby the

meter current flowing through the meter load resistance flips

a comparator at about 0.8 Vdc above ground. The signal

strength at which this occurs can be adjusted by changing

the meter load resistor. The comparator (+) input and output

are available to permit control of hysteresis. Good positive

action can be obtained for IF input signals of above 30

µVrms. The 130 kΩ resistor shown in the test circuit provides

a small amount of hysteresis. Its connection between the

3.3 k resistor to ground and the 3.0 k pot, permits adjustment

of squelch level without changing the amount of hysteresis.

The squelch is internally connected to both the

quadrature detector and the data shaper. The quadrature

detector output, when squelched, goes to a dc level

approximately equal to the zero signal level unsquelched.

The squelch causes the data shaper to produce a high (VCC)

output.

The data shaper is a complete ‘‘floating’’ comparator,

with back to back diodes across its inputs. The output of the

quadrature detector can be fed directly to either input of this

amplifier to produce an output that is either at VCC or VEE,

depending upon the received frequency. The impedance of

the biasing can be varied to produce an amplifier which

“follows” frequency detuning to some degree, to prevent data

pulse width changes.

When the data shaper is driven directly from the

demodulator output, Pin 13, there may be distortion at Pin 13

due to the diodes, but this is not important in the data

application. A useful note in relating high/low input frequency

to logic state: low IF frequency corresponds to low

demodulator output. If the oscillator is above the incoming

RF frequency, then high RF frequency will produce a logic

low (input to (+) input of Data Shaper as shown in Figures 1

and 2).

APPLICATION NOTES

The MC3356 is a high frequency/high gain receiver that

requires following certain layout techniques in designing a

stable circuit configuration. The objective is to minimize or

eliminate, if possible, any unwanted feedback.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com