TB9100 Reciter Service Manual Network Circuitry 73
© Tait Electronics Limited January 2006
5.5.3 1 PPS Input
The 1 PPS input circuitry is similar to that for the general-purpose digital
inputs, except that the filter (R641 and C653) time constant is considerably
reduced to approximately 500ns. Having a much lower nominal time delay
preserves the required timing accuracy of the 1 PPS edges, despite
component tolerance variations, etc.
The incoming 1 PPS signal is normally supplied via a 50Ω coaxial cable
from a distribution amplifier outputting a 5Vpp level. To ensure that there
are no reflections, which might cause jitter on the leading edge of the timing
pulse, this cable is AC terminated with network R600 and C609. AC
termination is used to maximize the peak signal level; the full 5Vpp level is
available as against the 2.5Vpp that would be available with DC termination.
5.5.4 Antenna Relay Control
A single high-current output driver is provided for general-purpose use,
although its normal usage is for controlling an antenna switchover relay. It
has an open collector output and can sink currents of up to 250mA from
supply voltages up to 30V. Instead of controlling a relay, it is also suitable for
driving sounders, LEDs, and small incandescent lamps. With incandescent
lamps, their size should be limited such that the initial turn-on current does
not exceed 250mA, to avoid overstressing the output transistor.
The relay driver is controlled by parallel output port PB20 of the MPC.
The current drive from this port is boosted by pre-driver transistor Q601,
which, in turn, drives the output transistor, Q600. Q600 is protected from
voltage spikes from inductive loads by Zener diode D602. Excess current
protection is provided by fuse F600. The output can also be used for driving
logic inputs that are TTL compatible, as resistor R636 pulls the output up
to +5V when Q600 is turned off.
Should the output function not be required, the driver transistor can be
turned off permanently and the line used for an extra input, DIG4_IN. An
input circuit similar to that used for the other digital inputs is paralleled with
the output circuit.
Note The antenna relay feature is not supported in all software versions.
5.5.5 RSSI Output
The reciter generates a differential analog RSSI signal strength indication
output. This differential output is converted to single-ended, amplified and
buffered by U601 on the ASIF. Ignoring transistors Q603a and Q603b,
resistors R651, R657, R658 and R660-R663 set the gain of op-amp U601
to amplify the difference between RSSI+ and RSSI- by approximately 1.64.
They also compensate for the 2.4V standing bias levels on the RSSI+ and
RSSI- lines. The output of U601 is protected by resistors R601 and R631,
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