SL6619KG MITEL [Mitel Networks Corporation], SL6619KG Datasheet - Page 9

• Manufacturer Part Number: SL6619KG
• Description: Direct Conversion FSK Data Receiver
• Manufacturer: MITEL [Mitel Networks Corporation]

OPERATION OF SL6619
Low Noise Amplifier
porate a Low Noise RF Amplifier at the front end of the
receiver. This is easily biased using the on-chip voltages and
current source provided. All voltages and current sources
used for bias of the RF amplifier, receiver and mixers should
be RF decoupled using 1nF capacitors. The receiver also
requires a stable Local Oscillator at the required channel
frequency.
Local Oscillator
phase quadrature. This can be achieved with the use of two
RC networks operating at their 23dB/45 transfer character-
istic. The RC characteristics for I and Q channels are com-
bined to give a full 90 phase differential between the LO ports
of the device. Each LO port also requires an equal level of
drive from the oscillator. This is achieved by forming the two
RC networks into a power divider.
Gyrator Filters
may be adjusted by changing the value of the resistor con-
nected between pin 13 and GND. This allows adjustment of
the filters’ cutoff frequency and allows for compensation for
possible process variations.
Audio AGC (Fig. 3)
led by the audio (baseband) signal. It has three parameters
that may be controlled by the user. These are the attack (turn
on ) time, decay (duration) time and threshold level. The
attack time is simply determined by the value of the external
capacitor connected to TCADJ. The external capacitor is in
series with an internal 100k
of this circuit dictates the attack time of the AGC.
connected in parallel with the capacitor CTC. The decay time
is simply
input to the AGC circuit, the variable current source is turned
on. This causes a voltage drop across R13. The voltage
potential between V
current to flow in pin 30. This charges up C18 through the
100k
increases, a current source is turned on and this sinks current
from pin 32. The current sink on pin 32 can be used to drive
To achieve optimum performance it is necessary to incor-
The Local Oscillator signal is applied to the device in
The on-chip filters include an adjustable gyrator filter. This
The Audio AGC consists of a current sink which is control-
The decay time is determined by the external resistor
When a large audio (baseband) signal is incident on the
internal resistor. As the voltage across the capacitor
i.e. t
REF
t
DECAY
ATTACK
and the voltage on pin 31 causes a
= R173C18
= 100k 3C18
resistor and the time constant
the external AGC circuit by causing a PIN diode to conduct,
reducing the signal to the RF amplifier.
RF AGC
protects the mixer’s RF inputs, Pins 3 and 5, from large out of
band RF signals. The loop consists of an RF received signal
strength indicator which detect the signal at the inputs of the
mixers. This RSSI signal is then used to control the LNA
current source (pin 1).
Regulator
a suitable PNP transistor to achieve regulation. As the transis-
tor forms part of the regulator feedback loop the transistor
should exhibit the following characteristics:
sourcing capability of the regulator is limited to 30 A.
Automatic Frequency Control (Fig. 4)
circuit which gives a current output at AFC OP whose magni-
tude and sign is a function of the difference between the local
oscillator (f
is then filtered by an off-chip integrating capacitor. The
integrator’s output voltage is used to control a voltage control
crystal oscillator. This closes the AFC feedback loop giving
the automatic frequency control function. For an FSK modu-
lated incoming RF carrier, the AFC OP current’s polarity is
positive, i.e.current is sourced for f
negative, i.e. current is sunk, for f
magnitude of the AFC OP current is a function of frequency
offset and the transmitted data’s bit stream. If the carrier
frequency, (f
then the magnitude of the current is zero.
BIT RATE FILTER CONTROL
the 1st order bit rate for a given bit rate filter capacitor at pin
27. This allows the cutoff frequency to be changed between
f
function is achieved by changing the value of the current in the
4f detector’s output stage. A logic zero (0V to 0·1V) on pin 26
gives a cutoff frequency of f
gives a cut off frequency of 2f
gives a cutoff frequency of 0·43f
C
, 2f
The RF AGC is an automatic gain control loop that
The on-chip regulator should be used in conjunction with
If no external transistor is used, the maximum current
The Automatic Frequency Control consists of a detection
The logic level on pin 26 controls the cutoff frequency of
C
and 0·43f
LO
) and carrier frequencies (f
C
), equals the local oscillator frequency, (f
H
C
FE
through the logic level on pin 26. This
.100 for V
C
a logic one (V
C
and an open circuit at pin 26
CE
C
.
. = 0·1V
LO
LO
C
.f
,f
). This output current
CC
C
C
.f
,f
220·3V to V
LO
LO
24kHz. The
14kHz and
CC
LO
2)
9
)