Motorola pro7100 Service Manual download pdf (Page 41)

Theory of Operation 2-27

2.7.1 Front-End Band-Pass Filters and Pre-Amplifier

The received signal from the radio’s antenna connector is first routed through the harmonic filter and

antenna switch, which are part of the RF power amplifier circuits, before being applied to the receiver

pre-selector filter (C3001, C3002, D3001 and associated components). The 2-pole pre-selector filter

tuned by the dual varactor diode D3001 pre-selects the incoming signal (RXIN) from the antenna

switch to reduce spurious effects to following stages. The tuning voltage (FECTRL_1) ranging from 2

volts to 8 volts is controlled by pin 20 of PCIC (U3501) in the Transmitter section. A dual hot carrier

diode (D3003) limits any inband signal to 0 dBm to prevent damage to the pre-amplifier.

The RF pre-amplifier is a surface mount device (SMD) Q3001 with collector-base feedback to

stabilize gain, impedance, and intermodulation. Transistor Q3002 compares the voltage drop across

resistor R3002 with a fixed base voltage from divider R3011, R3000 and R3012, and adjusts the

base current of Q3001 as necessary to maintain its collector current constant at approximately 15-20

mA. Operating voltage is from the regulated 9.3V supply (9V3). During transmit, 9.1 volts (K9V1)

turns off both transistors Q3002 and Q3001. This protects the RF pre-amplifier from excessive

dissipation during transmit mode. A following 3dB pad (R3006 – R3008 and R3016 – R3018)

stabilizes the output impedance and intermodulation performance.

A second 2-pole varactor tuned bandpass filter provides additional filtering of the amplified signal.

The dual varactor diode D3004 is controlled by the same signal FECTRL_1, which controls the pre-

selector filter.

2.7.2 First Mixer and 1st Intermediate Frequency (IF)

The signal coming from the front-end is converted to the 1st IF frequency of 44.85 MHz using a cross

over quad diode mixer (D3031). Its ports are matched for incoming RF signal conversion to the 44.85

MHz IF using high side injection. The high-side injection signal (RXINJ) from the frequency

synthesizer circuit has a level of approximately +13 dBm and is injected via matching transformer

T3002.

The IF output signal (IF) from transformer T3001 pin 2 is fed to the first 2- pole crystal filter FL3101.

The filter output in turn is matched to IF amplifier Q3101 which is actively biased by a collector base

feedback (R3101, R3106) to a current drain of approximately 5 mA drawn from the 5 volt supply. Its

output impedance is matched to the second 2-pole crystal filter FL3102. The signal is further

amplified by a preamplifier (Q3102) before going into pin 1 of IFIC (U3101).

A dual hot carrier diode (D3101) limits the filter output voltage swing to reduce overdrive effects at

RF input levels above -27 dBm.

2.7.3 2nd Intermediate Frequency (IF) and Receiver Back-End

The 44.85 MHz 1st IF signal from the second IF amplifier feeds the IF IC (U3101) at pin1. Within the

IF IC, the 44.85 MHz high IF signal mixes with the 44.395 MHz second local oscillator (2nd LO) to

produce the 2nd IF signal at 455 kHz. The 2nd LO frequency is determined by crystal Y3101. The

2nd IF signal is amplified and filtered by an external pair of 455 kHz ceramic filters FL3112, FL3114

for 20/25 kHz channel spacing or FL3111, FL3113/F3115 for 12.5 kHz channel spacing. These pairs

are selectable via BWSELECT. The filtered output from the ceramic filters is applied to the limiter

input pin of the IF IC (pin 14).

The IF IC contains a quadrature detector using a ceramic phase-shift element (Y3102) to provide

audio detection. Internal amplification provides an audio output level of 120 mV rms (at 60%

deviation) from U3101 pin 8 (DISCAUDIO) which is fed to the ASFIC_CMP (U0221) pin 2 (part of the

Controller circuits).

A received signal strength indicator (RSSI) signal is available at U3101, pin 5, having a dynamic

range of 70 dB. The RSSI signal is interpreted by the µP (U0101, pin 63) and in addition is available

at accessory connector J0501-15.

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Motorola pro7100 Service Manual Page 41

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