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
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
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.
3.2.1 POWER SWITCHING CONTROL
A diagram of the power distribution and switching circuits is shown in Figure
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
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
Q513 controls the Q511 base current in order to maintain approximately a
| February 2001 |
Part No. |