Tx Switch, + Switch, Analog Regulator N900

AE/LZB 119 1902 R1A

Tx Switch (V192)

The Tx Switch consists of transistor V192. Transistor V192 is normally off (receive). When the Delayed Push-To- Talk (DPTT) line goes high (transmitter keyed), transistor V192 conducts and the base of B+ Switch PNP transistor V191 goes low.

B+ Switch (V191)

B+ Switch transistor V191 supplies 7.0 volts to the directional coupler in the antenna circuit. When the base of this PNP transistor goes low, 7.0 volts is on the collector and can be metered at test point TP191. This voltage goes to directional coupler W100, Pin 1 (PORT 1). The B+ Switch circuit and the TX Switch circuit are tagged BUFFER on the Block Diagram (Sheet 1 of the Schematic Diagram).

Analog Regulator (N900)

Battery voltage connects directly to analog regulator N900, Pin 8 (Input). Capacitor C900 provides filtering at this point. To enable N900, Pin 3 (EN) must be grounded. This is accomplished by grounding one end of resistor R912 when the power switch is turned on. The voltage divider relationship between pull-up resistor R909 and resistor R910 is enough to produce a ground which will enable N900. This signal is typically < 0.6 Volts when the radio is on and

>2.0 volts when the radio is off and can be metered at test point TP904. The ground at R912 can be metered at test point TP907 and is 0 Vdc when the power switch is on and 7.5 Vdc when the power switch is off. When enabled, the output on N900, Pin 1 is a regulated +5 Volts. This voltage can be metered at test point TP901. Filtering is provided by capacitor C901. The output of N900 becomes the +5V_A line. The +5V_A connects to the base of transistor V905, causing V905 to conduct. Transistor V905 conducting causes the collector (PWR_ENABLE_B) to go low and enable the RF regulators. The +5V_A also goes to the emitter of PNP transistor V906. This caused V906 to conduct placing an active high on the Shut Down (SHDN) input of +5V_D switching regulator N902. The +5V_A also goes to the TX/RX audio circuits in DEBBIE (D601).

+5V-D Switching Regulator (N902)

B+ is connected to N902, Pins 1, 15 and 16 (U+). When an active high is on N902, Pin 2 (SHDN), N902 turns on and produces +5V_D on the output to power all digital circuits.

Processor Reset

+5V_D is connected through resistor R901 to open collector output N900, Pin 5 (ERROR). Timing is provided by pull-up resistor R901 and capacitor C905. The ERROR voltage can be metered at test point TP903. This voltage is normally + 5 Volts or 0 Vdc on reset. This voltage also connects back to N902, Pin 7 (SS) Soft-Start. When capacitor C905 charges up, the output of buffer gate D900 goes high. This high on D701, Pin 1 (RESPOW_B) starts the processor.

Power Switch Turned Off

Due to software programming, when the power switch is switched off, the power to all circuits may not be immediately turned off. For example, the processor may need to transmit log off messages before that happens. When the radio is turned on and the processor is started, a PWR_LATCH signal (2.6 Volts) is applied to the base of transistor V903 turning it on. The collector of V903 goes low and the enable line to N900 is latched in a low state, holding N900 on. N900 can be held on indefinitely if programmed to do so. A PWR_SW_SENSE line monitors the condition of the power switch so the processor knows when to start the power down program. When the power switch is on, diode V904, Pin 3 is low. The +5V_A through resistor R911 forward biases V904 and holds the PWR_SW_SENSE line low. When the power switch is turned off, the diode is no longer forward biased. Pull-up resistor R911 now pulls the PWR _SW_SENSE line high to indicate to the processor that the power switch has been turned off.

Low Battery Power Down

The battery voltage is monitored with A/D input to ATTIE. To protect the battery against deep discharge conditions a LOW_BATT_PWR_OFF line is provided to shut the radio down until a fresh battery is attached. When the battery voltage falls below 5.6 Volts, diode V900 is forward biased conducting through V902. When the LOW_BATT_PWR_OFF line is switched high, V901 and V902 latch on forcing enable pin N900-3 high, which results in shutting the radio off.