Sailor RE2100, Technical Manual

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PAGE 5-5
MICROPROCESSOR CONTROLLING
The output from the comparator U01/1 is led to the microprocessor. If the microprocessor detects a high
voltage, it will turn the squelch switch ON and the AF-signal will then reach the loudspeaker, the earpiece
and the 0 dBm amplifier.
When a conversation stops, C95 will slowly be discharged through R119. After about seven seconds the
voltage across C95 will fall below the reference voltage and the output of comparator U01/1 will go low.
This will be detected by the microprocessor, which will turn the squelch OFF.
In scan mode the squelch circuit is used to control scanning by detecting whether the receiver is scanning
a channel with voice activity.
If the microprocessor detects a high voltage at the output of the comparator U01/1, it will immediately reset
it by discharging C95 through Q20.
If the receiver actually receives a voice signal it will quickly build-
up a new voltage across C95 and the output of the comparator will go high again. This cycle of measuring
the comparator voltage and resetting it will be repeated six times to prevent noise spikes from stopping
the scanning.
If the microprocessor after these six dischargings still detects a high voltage, it will stop the scanning for
a period of about 3 secs. In this period the squelch switch will be turned ON and the AF signal will then
reach the loudspeaker.
SIGNAL STRENGTH DETECTOR
The signal strength detector generates a DC voltage to light the LED bar at the display unit (module 7).
The circuit consists of a logarithmic amplifier and a buffer amplifier.
LOGARITHMIC AMPLIFIER
The AGC voltage is fed to the input of the operational amplifier U03/4. With the potentiometer R76 the
output voltage is adjusted so that the first LED in the bar is just about to light with no antenna connected
to the receiver. Because of R69, R72, and D12 the amplifier has an approximately logarithmic
caracteristic, which means that the RF signal level must be much higher to light the last LED’s than to light
the first LED’s.
BUFFER AMPLIFIER
The output from the logarithmic amplifier is fed to a voltage divider, which consists of the resistor R213
and the transistor Q18. With this voltage divider it is possible to ground the signal strength voltage and
thereby turn off the light in the LED bar. From the voltage divider, the signal is fed to the unity gain buffer
U01/2. This buffer is included to avoid the loading by the resistor R213, which is necessary for the wanted
function of the voltage divider.
9324
5 CIRCUIT DESCRIPTION AND SCHEMATIC DIAGRAMS RE2100