CIRCUIT DESCRIPTION

POWER DISTRIBUTION (ALL MODELS)

Flash EPROM and also EEPROM U102. This information is programmed when data is downloaded to the transceiver by the programming software described in Section 4.

A second microcontroller (U2) on the display board controls the front panel display and tri-color indicator and also detects the option switch, Select switch, and microphone on/off hook state. Control information is exchanged between U101 and this microcontroller via a serial bus. The use of a second microcontroller minimizes the number of interconnections that are required between the audio/logic and display boards. The operating program for this micro- controller is permanently stored on the chip and cannot be changed.

3.1.5 RECEIVER

The receiver is a double conversion type with the following intermediate frequencies:

UHF Models - 45 MHz and 450 kHz

800 MHz Models - 52.950 MHz and 450 kHz

900 MHz Models - 45 MHz and 450 kHz

Two bandpass filters in the front end attenuate the image, half IF, injection, and other frequencies outside the selected receive band. Receiver selectivity is enhanced by a four-pole crystal filter and two 450 kHz ceramic filters.

3.1.6 TRANSMITTER

The transmitter amplifies the synthesizer signal to produce a power output of up to either 25 or 40 watts (UHF) or 15 or 30 watts (800/900 MHz) at the antenna jack. Frequency modulation of the transmit signal is performed by modulating the synthesizer TCXO and VCO frequencies. A control circuit senses forward power to maintain constant power output. It also senses final amplifier current and cuts back power if it becomes excessive.

The use of a digital potentiometer allows the power output to be set from the front panel when the test mode is selected and also allows two different power levels to be programmed for each system. The microcontroller also monitors power amplifier ambient temperature and voltage and cuts back power or disables the transmitter if either are excessive.

3.2 POWER DISTRIBUTION AND SWITCHING

3.2.1 POWER SWITCHING CONTROL

A diagram of the power distribution and switching circuits is shown in Figure 3-1.The main power switching is performed on the RF board by Q510 and other transistors. This switch is controlled by the front panel power switch, the ignition sense input, and the microcontroller as shown in Figure 3-2.For the front panel on-off switch to be detected, Q109 must be turned on by a high signal applied through R170 or from the ignition switch. R170 is installed if the ignition switch is not used to control power. Q107 and Q108 provide power switch on and ignition on signals to the microcontroller so that it can sense those conditions.

Q110 allows the microcontroller to hold power on for a time after it has been turned off by the power or ignition switch. For example, when the microcon- troller senses that power was switched off by the front panel switch, it holds power on for a short time so that switch settings can be saved to memory. It also holds power on when a power-off delay is used. Once power turns off, power is also removed from the microcon- troller. Therefore, power can be turned on only by the power and ignition switches.

NOTE: The front panel power switch is a push on, push off type (pressing it toggles between open and closed). It is not a momentary switch.

3.2.2 SUPPLY SWITCHING

When power is turned on by the front panel power switch, the base of Q514 on the RF board is grounded through the power switch. Q514 then turns on which also turns on the Darlington amplifier formed by Q511 and Q512. These transistors are turned on by a 13.6-volt signal applied through R535. Series-pass transistor Q510 is then turned on and 13 volts appears on its collector. The diodes in CR505 and CR506 become forward biased only if the 8-volt supply applied to the collector of Q514 becomes shorted. This provides current limiting which prevents damage to the transistors.

Q513 controls the Q511 base current in order to maintain approximately a 0.8-volt drop across the emitter and collector of Q510. This provides noise

 

February 2001

3-2

Part No. 001-9800-001