Motorola pro7100 Manuel de service télécharger pdf (Page 33)

Theory of Operation 2-19

2.4.1 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 (C4001, C4002, D4001 and associated components). The 2-pole pre-selector filter

tuned by the varactor diodes D4001 and D4002 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 (U4501) in the Transmitter section. A

dual hot carrier diode (D4003) limits any inband signal to 0 dBm to prevent damage to the pre-

amplifier.

The RF pre-amplifier is an SMD device (Q4003) with collector base feedback to stabilize gain,

impedance, and intermodulation. The collector current of approximately 11-16 mA is drawn from the

9.3 volt supply via L4003 and R4002. A 3dB pad (R4006,R4007,R4011 and R4008 - R4010)

stabilizes the output impedance and intermodulation performance.

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

The varactor diodes D4004 and D4005 are controlled by the same signal FECTRL_1, which controls

the pre-selector filter. A following 1 dB pad (R4013 - R4015) stabilizes the output impedance and

intermodulation performance.

2.4.2 First Mixer and 1st Intermediate Frequency (IF)

The signal coming from the front-end is converted to the first IF (44.85 MHz) using a cross over quad

diode mixer (D4051). Its ports are matched for incoming RF signal conversion to the 44.85 MHz IF

using low side injection via matching transformers T4051 and T4052. The injection signal (RXINJ)

coming from the RX VCO buffer (Q4332) is filtered by the lowpass filter consisting of (L4053, L4054,

C4053 - C4055) followed by a matching transformer T4052 and has a level of approximately 15dBm.

The mixer IF output signal (IF) from transformer T4501pin 2 is fed to the first two pole crystal filter

FL3101. The filter output in turn is matched to the following IF amplifier.

The IF amplifier Q3101 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 two 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.

2.4.3 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 low 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 and 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 U3103 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 is available at

accessory connector J0501-15.

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Motorola pro7100 Manuel de service Page 33

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