Theory Of Operation; Introduction; Radio Power Distribution; Table 3-1. Voltage Regulators - Motorola HT1250-LS+ Service Manual

Chapter 3

Theory of Operation

3.1

Introduction

This chapter provides a detailed theory of operation for the radio components. Schematic diagrams
for the circuits described in the following paragraphs are located in Figures 5-1 through 5-34.
3.2

Radio Power Distribution

A block diagram of the DC power distribution throughout the radio board is shown in Figure 3-1. Four
voltage regulators are used as follows:
Name
3.3 V Reg
Vdda
Vddd
5 V Reg
The 7.5 V battery supplies radio power (UNSWB) directly to the electronic on/off control circuit, audio
power amplifier, 3.3 V Li-Ion regulator U410, transmitter PA driver and PA Final stage, the Power
Control IC, and the low battery detect circuit.
When the radio on/off/volume control is turned on, the switched battery voltage SWB+ from Q400 is
applied to the various regulators, antenna switch, the 20-pin accessories connector J403, the 40-pin
keypad/option board connector J400, and the transmit LED. The Vddd source from digital 3.3V
regulator U400 supplies operating power to the microprocessor, EEPROM, RAM and Flash ROM IC's.
The Vdda analog 3.3 V regulator (U3201 in 200 MHz models, U203 in 700 MHz models) provides
operating voltage for the ASFICcmp, VCOBIC, synthesizer IC. The 5 V regulator (U3711 in 200, U202
in 700) also powers the synthesizer IC as well as the receiver front end and IF circuitry, the RX and
TX (700 MHz only) VCO buffers, and the opamp audio gain stage. Some stages are supplied only
during receive by 5R, switched by Q3721 (200) or Q201 (700 MHz). Regulator voltage routing is
configurable by jumper placement. All models are configured with R401 and R405 placed, and R402-
4 not placed.
At power up, Q400 is initially turned on by the path through Q403, CR440, and Q405. When C480 has
charged, this path is disabled, and Q400 is maintained on via a logic high ASFICcmp U404 pin 37, via
CR440 and Q405.
The radio turns off when either of the two following conditions occur:
• Radio on/off/volume control is turned off.
• Low battery condition is detected.
If a low battery level or loss of battery voltage is detected by the microprocessor pin 67 through either
of the above conditions, the radio personality data is stored to EEPROM prior to turning off. At this
point U404-37 goes low, turning off Q400.

Table 3-1. Voltage Regulators

200 MHz 700 MHz
U410 U410
U3201 U203
U400 U400
U3711 U202
Application
Li-lon backup battery charging
Analog 3.3 V source (audio, VCO, synth)
Digital 3.3 V source (µP and memory)
Analog 5 V source (RF, IF, synth)
3-1