MC3356_1 Datasheet, PDF (3/8 Pages) Motorola, Inc – Wideband FSK Receiver

Freescale Se_Mm_C3i₃c₅o₆nductor, Inc.
Figure 3. Output Components of Signal,
Noise, and Distortion
10
S+N+D
0
fO = 100 MHz
–10
fm = 1.0 kHz
∆f = ± 75 kHz
–20
–30
N+D
–40
–50
N
–60
0.01
0.1
1.0
10
INPUT (mVrms)
Figure 4. Meter Current versus Signal Input
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.
MOTOROLA
SOLUTIONS
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