Motorola LB3 (42.0 - 50.0MHz) 서비스 정보 - 페이지 9

{카테고리_이름} Motorola LB3 (42.0 - 50.0MHz)에 대한 서비스 정보을 온라인으로 검색하거나 PDF를 다운로드하세요. Motorola LB3 (42.0 - 50.0MHz) 26 페이지. Gm series professional radio

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Low Band Receiver

There are two 2-pole 10.7 MHz crystal filters in the high-IF section and two switched pairs of

455 kHz ceramic filters in the low-IF section to provide the required adjacent channel selectivity.

The second IF at 455 kHz is mixed, amplified and demodulated in the IF IC. The processing of the

demodulated audio signal is performed by an audio processing IC located in the controller section.

2.2

Front-End Band-Pass Filters & 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 circuitry, before being applied to the

receiver 5-pole antenna filter (L1001-L1005 and associated components). This filter configuration

provides more rapid attenuation above the passband to provide better rejection of the half-IF

spurious response. A dual hot carrier diode (D1001) limits any inband signal to 0 dBm to prevent

damage to the RF pre-amplifier

The RF pre-amplifier is an SMD device (Q1001) with collector-base feedback to stabilize gain,

impedance, and intermodulation. Transistor Q1002 compares the voltage drop across resistor

R1005 with a fixed base voltage from divider R1006 and R1007, and adjusts the base current of

Q1001 as necessary to maintain its collector current constant at 25 mA. Operating voltage is from

the regulated 9.3V supply (9V3). During transmit, 9.1 volts (9T1) turns on both transistors in U1001,

turning off Q1003 and therefore Q1001-2. This protects the RF pre-amplifier from excessive

dissipation during transmit mode.

A second 5-pole fixed-tuned bandpass filter provides additional filtering of the amplified signal. This

filter configuration also provides steeper attenuation above its passband for best half-IF attenuation.

2.3

First Mixer

The signal coming from the front-end is converted to the high-IF frequency of 10.7 MHz using a

cross over quad diode mixer (U1051). The high-side injection signal (RXINJ) from the frequency

synthesizer circuitry is filtered by a 7-pole low-pass filter (L1012-14 and associated circuitry) which

removes second harmonic content from the injection signal and improves half-IF rejection. The 50-

ohm output of the first mixer is applied to the input of the high-IF circuit block.

2.4

High Intermediate Frequency (IF) and Blanker Switches

The first mixer IF output signal (IF) is applied to a diplexer network consisting of L1101, L1111 and

associated components. This network has three functions: it terminates the mixer output at

frequencies other than 10.7 MHz into 51-ohm resistor R1101; it matches the 50-ohm mixer output to

the first IF amplifier (Q1101) input; and it provides bandpass filtering at 10.7 MHz to prevent the 5.35

MHz half-IF component of the mixer output from creating a second harmonic at 10.7 MHz in Q1101,

which degrades half-IF rejection.

The IF amplifier Q1101 uses ac and dc feedback to stabilize gain and quiescent current

(approximately 28 mA). Operating voltage is from the regulated 9.3V supply (9V3). Its output is

applied to a 10.7 MHz ceramic filter FL1101 which has a 3 dB bandwidth of 270 kHz and provides a

time delay of 2.6 usec. This delay allows enough time for the "Extender" to respond to impulse noise

present at the input of Q1101 and operate the blanker switches Q1102 and Q1103, muting the IF

signal for the duration of the noise pulse. L1104 and L1105 also provide additional selectivity and

time delay. Operation of the "Extender" circuit is explained in Section 8.5 below.

When the blanker switches turn "on" to mute the IF signal, they momentarily change the impedance

of resonant circuits L1104 and L1105 from high to very low. This abrupt impedance change, if

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