Motorola Pr400 Detailed 6881096c25 B Manual

6881096C25-BJune, 2005
136-162 MHz VHF Theory Of Operation: VHF Transmitter 4-3
4.3 VHF Transmitter
The VHF transmitter covers the range of 136-162 MHz. Depending on model, the output power of the 
transmitter is either switchable on a per-channel basis between high power (5 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 4-2.
• Power Amplifier
• Harmonic Filter
• Antenna Matching Network
• Power Control
Figure 4-2....

June, 20056881096C25-B
4-4136-162 MHz VHF Theory Of Operation: VHF Frequency Generation Circuitry
4.3.4 Antenna Matching Network
The harmonic filter presents a 50 Ω 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.
4.3.5 Power Control
The power control...

6881096C25-BJune, 2005
136-162 MHz VHF Theory Of Operation: VHF Frequency Generation Circuitry 4-5
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 varying the audio level 
applied to the synthesizer IC. Internally the audio is digitized by the Fractional-N synthesizer and 
applied to the loop divider to provide the low-port modulation. The audio is also routed through an 
internal...

June, 20056881096C25-B
4-6136-162 MHz VHF Theory Of Operation: VHF 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 4-4.  VHF Synthesizer Block Diagram
4.4.2 Voltage Controlled Oscillator (VCO)
The VCOBIC (U251), shown in Figure 4-5, in conjunction with the Fractional-N synthesizer (U201) 
generates RF in both the receive and the transmit modes of...

6881096C25-BJune, 2005
136-162 MHz VHF Theory Of Operation: Keypad 4-7
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 4-5.  VHF VCO Block Diagram
4.5 Keypad
The keypad block diagram is shown in Figure 4-6. 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
4-8136-162 MHz VHF Theory Of Operation: Keypad
Notes: 

Chapter 5 146-174 MHz VHF Theory Of Operation
5.1 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 7 of this manual.
5.2 VHF Receiver
The VHF receiver covers the range of 146-174 MHz and provides switchable IF bandwidth for use 
with 12.5 kHz or 20/25 kHz channel spacing systems. The receiver is divided into two major blocks as 
shown in Figure 5-1.
•Front End
•...

June, 20056881096C25-B
5-2146-174 MHz VHF Theory Of Operation: VHF Receiver
constant at 6.2 mA regardless of device and temperature variations, for optimum dynamic range and 
noise figure.
The output of the RF amplifier is applied to the interstage filter, a fixed-tuned 3-pole series-coupled 
resonator design having a 3 dB bandwidth of 58 MHz and insertion loss of 1.8 dB. This filter has an 
image rejection of 42 dB at 235 MHz, with increasing attenuation at higher frequencies.
The output of the...

6881096C25-BJune, 2005
146-174 MHz VHF Theory Of Operation: VHF Transmitter 5-3
5.3 VHF Transmitter
The VHF transmitter covers the range of 146-174 MHz. Depending on model, the output power of the 
transmitter is either switchable on a per-channel basis between high power (5 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 5-2.
• Power Amplifier
• Harmonic Filter
• Antenna Matching Network
• Power Control
Figure 5-2....

June, 20056881096C25-B
5-4146-174 MHz VHF Theory Of Operation: VHF Frequency Generation Circuitry
5.3.4 Antenna Matching Network
The harmonic filter presents a 50 Ω 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.
5.3.5 Power Control
The power control...