Motorola Pr400 Detailed 6881096c25 B Manual

6881096C25-BJune, 2005
403-440 MHz UHF Theory Of Operation: UHF Frequency Generation Circuitry 8-5
SPI_CLK, and SYNTH_CS (chip select) lines (U409 pins 100, 1 and 47 respectively). A logic high (3 
V) from U201 pin 4 indicates to the microprocessor that the synthesizer is locked.
Figure 8-3.  UHF Frequency Generation Unit Block Diagram
Transmit modulation from the ASFICcmp (U451 pin 40) is applied to U201 pin 10 (MOD_IN). An 
electronic attenuator in the ASFICcmp adjusts overall transmitter deviation by...

June, 20056881096C25-B
8-6403-440 MHz UHF Theory Of Operation: UHF Frequency Generation Circuitry
One of the auxiliary outputs of the synthesizer IC (AUX3, U201 pin 2) provides the TRB signal which 
determines the operating mode of the VCO, either receive or transmit.
Figure 8-4.  UHF Synthesizer Block Diagram
8.4.2 Voltage Controlled Oscillator (VCO)
The VCOBIC (U251), shown in Figure 8-5, in conjunction with the Fractional-N synthesizer (U201) 
generates RF in both the receive and the transmit modes of...

6881096C25-BJune, 2005
403-440 MHz UHF Theory Of Operation: Keypad 8-7
C267, and varactor D261. High-port audio modulation from the synthesizer IC is applied as 
VCO_MOD to varactor D262 which modulates the transmit VCO.
Figure 8-5.  UHF VCO Block Diagram
8.5 Keypad
The keypad block diagram is shown in Figure 8-6. Pressing a key creates two distinct voltages 
KEYPAD_ROW and KEYPAD_COL. These voltages are sent directly to the radio’s microprocessor 
on the main board. The microprocessor then interprets...

June, 20056881096C25-B
Notes:
8-8403-440 MHz UHF Theory Of Operation: Keypad 

Chapter 9 438-470 MHz UHF Theory Of Operation
9.1 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 Chapter 12 of this manual.
9.2 UHF Receiver
The UHF receiver covers the range of 438-470 MHz and provides switchable IF bandwidth for use 
with 20/25/30 kHz or 12.5 kHz channel spacing systems. The receiver is divided into two major 
blocks, as shown in Figure 9-1.
• Front End...

June, 20056881096C25-B
9-2438-470 MHz UHF Theory Of Operation: UHF Receiver
The output of the RF amplifier is applied to the interstage filter, a fixed-tuned 4-pole Butterworth 
shunt-coupled resonator design having a 3 dB bandwidth of 68 MHz centered at 462 MHz, and 
insertion loss of 3 dB. This filter yields an image rejection of 48 dB at 380 MHz, assisted by a 
transmission-zero at 300 MHz implemented by C524 for the reasons mentioned above.
The output of the interstage filter is connected to the...

6881096C25-BJune, 2005
438-470 MHz UHF Theory Of Operation: UHF Transmitter 9-3
9.3 UHF Transmitter
The UHF transmitter covers the range of 438-470 MHz. Depending on model, the output power of the 
transmitter is either switchable on a per-channel basis between high power (4 watts) and low power 
(1 watt), or is factory preset to 2 watts. The transmitter is divided into four major blocks as shown in 
Figure 9-2.
• Power Amplifier
• Harmonic Filter 
• Antenna Matching Network
• Power Control.
Figure 9-2....

June, 20056881096C25-B
9-4438-470 MHz UHF Theory Of Operation: UHF Frequency Generation Circuitry
9.3.4 Antenna Matching Network
The harmonic filter presents a 50Ω impedance to antenna jack J140. A matching network, made up of 
C140-C141 and L140, is used to match the antenna impedance to the harmonic filter. This optimizes 
the performance of the transmitter and receiver into the impedance presented by the antenna, 
significantly improving the antennas efficiency.
9.3.5 Power Control
The power control...

6881096C25-BJune, 2005
438-470 MHz UHF Theory Of Operation: UHF Frequency Generation Circuitry 9-5
SPI_CLK, and SYNTH_CS (chip select) lines (U409 pins 100, 1 and 47 respectively). A logic high 
(3 V) from U201 pin 4 indicates to the microprocessor that the synthesizer is locked.
Figure 9-3.  UHF Frequency Generation Unit Block Diagram
Transmit modulation from the ASFICcmp (U451 pin 40) is applied to U201 pin 10 (MOD_IN). An 
electronic attenuator in the ASFICcmp adjusts overall transmitter deviation by...

June, 20056881096C25-B
9-6438-470 MHz UHF Theory Of Operation: UHF Frequency Generation Circuitry
One of the auxiliary outputs of the synthesizer IC (AUX3, U201 pin 2) provides the TRB signal which 
determines the operating mode of the VCO, either receive or transmit.
Figure 9-4.  UHF Synthesizer Block Diagram
9.4.2 Voltage Controlled Oscillator (VCO)
The VCOBIC (U251), shown in Figure 9-5, in conjunction with the Fractional-N synthesizer (U201) 
generates RF in both the receive and the transmit modes of...