Page 52 of 101
DWG ID: 181-0101-00A
Date: 2003-09-26
If J8 is jumped the system will come up in the Maintenance Program mode, when turned
on, where the frequency of operation is set and various transmit and receive levels are
adjusted and set. If J8 is not jumped, then the system will come up in its Operation
mode.
The
µ
P, U1, communicates with a four-level frequency shift keying (4-LFSK) modem
IC, U3. The modem IC, U3, provides the four-level coded signal to FSK modulate the
radio transceiver transmitter and decode the demodulated FSK signal from the radio
transceiver receiver.
Because of dc offsets and to minimize bit error rate the TX and RX paths are dc coupled.
The four-level coded signal emanates from the modem IC at terminal 20, U3-20, at an
amplitude of 1 V pp and a dc offset of Vcc/2 ( 2.50 V dc). The signal is sent through a
RC low-pass filter, R3 and C13, to the input of buffer amplifier AR2D, an op-amp
voltage follower. The signal at the output of the buffer amplifier then goes to the top of
the DEVIATION setting software controlled potentiometer, U2C.
VBIAS from the modem IC is made available at terminal 21, U3-21, that is equal to
Vcc/2 ( 2.50 V dc) that the modem IC uses internally. This dc voltage is buffered by
AR1A and then goes to the bottom of DEVIATION setting software controlled
potentiometer U2C and to R8, one of two inputs to the inverting summing junction of
AR1C. The FREQuency OFFSET software controlled potentiometer, U2F, adds a dc
offset to the AR1C op-amp stage. The output of AR1C can be set for the proper signal
level to drive the transmitter to the proper deviation. The dc offset level of 2.50 V affects
the frequency of the transmitted and received signal because this voltage goes directly to
the master oscillator on the radio transceiver via terminal 7 of the radio interface
connector, P1-7.
Transmitter RF output power level is set with software controlled potentiometer U2G.
This dc level is buffered with AR1D.
For receiving, the RSSI signal from the radio transceiver is first looked at to see if there is
a signal of sufficient amplitude. The RSSI signal at P1-13 is sent to a times two non-
inverting op-amp amplifier, AR2B, and then to AR2A that acts as a voltage comparator.
The RSSI ADJUST software controlled potentiometer U2E is adjusted twice.
Adjustment is done one time for a turn on level and a second time for a turn off level.
This adds hysteresis so that a good signal of specified level being received will not drop
out if, for some reason, the signal level drops a number of dB. Also the RSSI signal is
looked at before transmitting to determine if there is a signal on the air or not.
The DISC_OUT (discriminator output) signal at P1-14 is sent to RX GAIN software
controlled potentiometer U2D for adjustment of the amplitude. This demodulated four-
level signal goes to op-amp AR2C where a dc offset voltage from the RX ADJ 2.5 V
software controlled potentiometer, U2B, is added. The discriminator output signal rides
on a dc offset voltage from the radio transceiver. Adjustment of the RX GAIN