Sailor RM2042 Technical Manual page 71

5 CIRCUIT DESCRIPTION AND SCHEMATIC DIAGRAMS RM2042
An exact copy of this de-emphasis filter, with the same component values, is used in the receiver signal
path from the connected VHF radiotelephone. This filter is build-up around the operational amplifier
U7.3.
The signals from the build-in channel-70 receiver and the connected VHF radiotelephone are led to
the input of the second switch inside U6, where the signal selection is performed.
The selected signal is filtered by a 6th. order Gaussian high pass filter with a cutoff frequency of about
1 kHz followed by a 4th. order Chebychev low pass filter with a cutoff frequency of about 3 kHz. These
filters are all together optimized with respect to stopband attenuation and group delay distortion.
The 6th. order Gaussian filter is realized as an infinite-gain multiple-feedback filter and is build-up
around the three operational amplifiers U3.2, U3.3 and U3.4.
The 4th. order Chebychev filter is realized as a voltage controlled voltage source filter (VCVS filter)
and is build-up around the two operational amplifiers U8.3 and U8.4.
The output signal from the 4th. order Chebychev filter is led to the voltage divider consisting of R112
and R113, where the signal level is attenuated to about 10mV . This signal is used as input to the
RMS
FSK decoder.
FSK DECODER/ENCODER
The FSK (Frequency-Shift Keying) decoder and encoder is integreted in U2, which is a 1200 baud
SINGLE CHIP MODEM of type MSM6927 from the manufacturer OKI.
FSK decoder
The FSK decoder consist of receive filter, FSK demodulator and AF signal detect circuit.
The receive filter is a 12th. order band pass filter with a lower cutoff frequency at 600 Hz and an upper
cutoff frequency at 2700 Hz, which gives a bandwidth of 2.1 kHz.
The AF signal detect circuit consist of an AC to DC converter, that produce a DC-voltage proportional
to the input signal level. This DC-voltage is compared with a reference voltage, which can be modified
by changing the voltage divider consisting of the two resistors R64 and R65. The output of this
comparator is led to pin 12 (CD1) at U2, which will be logical low, if an AF signal is detected.
The bit stream output from the FSK demodulator is available at pin 10 (RD), but the output will be keept
logical high, if no AF signal is detected.
FSK encoder
The FSK encoder consist of FSK modulator and transmit filter.
The bit stream input to the FSK modulator is led to pin 9 (XD) and to enable the FSK modulator, the
REQUEST TO SEND signal (RS1) at pin 4 must be logical low. The FSK modulator is generating a
tone signal, where the frequency is altered between 1300 Hz (mark = '1') and 2100 Hz (space = '0')
in accordance with the input bit stream. The frequency of the mark and space signals is controlled by
the crystal X2, which is working at 3.5795 MHz.
The generated FSK signal is led through the transmit filter to the analog output (Aout) at pin 34. The
output signal is then led to the external transmit filter, which is a 2nd. order Chebychev low pass filter
with 0.1 dB ripple and a cutoff frequency of 3.9 kHz. This filter is build around the operational amplifier
U7.4 and the output level is 0 dBm (600 ) 2 db.
9341 PAGE 5-15