SL6649-1 Zarlink Semiconductor Inc, SL6649-1 Datasheet - Page 5

• Manufacturer Part Number: SL6649-1
• Description: 200MHz DIRECT CONVERSION FSK DATA RECEIVER
• Manufacturer: Zarlink Semiconductor Inc

SL6649-1
PRINCIPLE OF OPERATION
converted to baseband. The two paths are produced in phase
quadrature (see Fig 2) and detected in a phase detector which
provides a digital output. The quadrature network must be in
the local oscillator path.
4.5kHz, the input to the system has a demodulation index of 18.
This gives a spectrum as in Fig 3. f
state’ frequencies (i.e. modulated with continuous ‘1’ and ‘0’
respectively). The spectrum in Fig 3 is for reversals (a
0-1-0-1-0-1 etc. pattern) at the system bit rate; f
carrier frequency).
continuous ‘0’ or ‘1’ will produce an audio frequency, at the
output of the mixers corresponding to the difference between f
and f
output signal will be at the same frequency regardless of the
data state; nevertheless, the relative phases of the two paths
will reverse between ‘0’ and ‘1’ states. By applying the amplified
outputs of the mixers to a phase discriminator, the digital data is
reproduced.
TUNING THE CHANNEL FILTERS
SL6649-1 receiver is determined by the channel filters. To
obtain optimum adjacent channel rejection, the channel filters’
cut off frequency should be set to 8kHz. The process tolerances
are such that the cut off frequency cannot be accurately
defined, hence the channel filters must be tuned. However the
receiver characteristics on the previous page can be achieved
with a fixed 270kΩ resistor between pin 3 and GND.
circuits. This changes the values of the gyrator’s equivalent
inductance. The cut off frequency is tuned to 8kHz. To
accurately define the cut off of the channel filters, a gyrator
based Colpitts oscillator circuit has been included on the
SL6649-1. The Colpitts oscillator and channel filters use the
same type of architecture, hence there is a direct correlation
between oscillator frequency and cut off frequency. By knowing
the Colpitts oscillator frequency the channel filter cut off
frequency can be estimated from Figure 4.
necessary to disable the Colpitts oscillator. The Colpitts
oscillator is disabled by connecting the Colpitts oscillator
output/disable pin (pin # 16) to V
Colpitts oscillator may impair the performance of the receiver.
4
The incoming signal is split into two parts and frequency
At a data rate of 512 baud and a deviation frequency of
When the LO is at the nominal carrier frequency, then a
The adjacent channel rejection performance of the
Tuning is performed by adjusting the current in the gyrator
Once the channel filters have been tuned it may be
C
or f
1
and f
C
. If the LO is precisely at fc, then the resultant
CC
1
2. This is needed since the
and f
0
represent the ‘steady
C
is the nominal
0
k
Figure 5: Channel Filter Response
Figure 3: Spectrum Diagram
Figure 4
k
k