Icom IC-T90A manual Transmitter Circuits

4-1-8 AGC CIRCUIT (AF AND RF UNITS)

The AGC (Automatic Gain Control) circuit reduce signal fad- ing and keeps the audio output level constant.

A portion of AF signals from the AM detector circuit (Q201) are applied to the amplifier (Q202). The amplified DC voltage from the Q202, pin 4 is applied to the RF amplifiers (RF unit; Q302, Q303, Q351, Q401, Q402, Q451, Q452) to reduce the amplifier gain when strong signals are received.

4-2 TRANSMITTER CIRCUITS

4-2-1 MICROPHON AMPLIFIER CIRCUIIT (LOGIC UNIT)

The microphone amplifier circuit amplifies the audio signals from the microphone, within +6 dB/octave pre-emphasis characteristics (300 Hz–3 kHz), to a level needed for the modulation circuit. The microphone amplifier circuit is used for both the VHF and UHF bands.

The AF signals from the microphone (MC1) or external [MIC] jack (AF unit; J252) passes through the microphone mute cir- cuit (Q310, pins 4, 2) which is controlled by the CPU (IC1, pin

48)via the “MUTE” signal. The AF signals are applied to the microphone (limiter) amplifier (Q311, Q312) which has +6 dB/octave pre-emphasis characteristics, and are then passed through the low-pass filter (Q310, pins 4, 1). The fil- tered signals are applied to the modulation circuit (VCO unit) as the “VMOD” signal via the each modulation band switch (Q342, R365: for 50 MHz band, Q305, R326: for 145 MHz band, Q304, R325: for the 440 MHz band).

4-2-2 MODULATION CIRCUIT (RF AND VCO UNITS)

The modulation circuit modulates the VCO oscillating signal (RF signal) using the microphone AF signals.

The signals from the modulation band switches (LOGIC unit) are then applied to the 144 MHz VCO or 430 MHz VCO cir- cuits separately.

• 144 MHz VCO

The applied signals from the RF unit change the reactance of D3 to modulate the oscillated signal at the 144 MHz VCO cir- cuit (VCO unit; Q4, Q5).

• 430 MHz VCO

The applied signals from the RF unit change the reactance of D1 to modulate the oscillated signal at the 430 MHz VCO cir- cuit (VCO unit; Q1, Q2).

The modulated signal is amplified at the buffer amplifier (VCO unit; Q7) and is then bypassed divider circuit (VCO unit; IC1) via the divier switch (VCO unit; D8, D10). The sig- nal is amplified at the two LO amplifiers (VCO unit; Q16, Q212), and then applied to the drive/power amplifier circuits.

4-2-3 DRIVE/POWER AMPLIFIER CIRCUITS (RF UNIT)

The drive amplifier circuit amplifies the transmit signal to a level needed for the power amplifier circuit. The power ampli- fier circuit amplifies this to obtain a specified transmit output power.

The signal from the two LO amplifiers (VCO unit; Q16, Q212) is passed through the Transmit/Receive switch (Q51), and is amplified at the buffer amplifier (IC51, pin 1). The signal is applied to the pre-driver (Q54) via the attenuator (D54). The amplified signal is applied to the power amplifier via the 50 MHz line, 144 MHz and 430 MHz line separetly.

• 50 MHz LINE

The amplified signal passes through the low-pass filter (L54, C60, C89, R81, R82) via the D57, and is then applied to the power amplifier (Q51).

• 144 MHz AND 430 MHz LINE

The amplified signal passes through the high-pass filter (L56, C65, C68, C69), and is then applied to the drive amplifier (Q52). The signal is applied to the power amplifier (Q51).

The amplified signal is applied to the each TX filter circuit.

4-2-4 TX FILTER CIRCUIT (RF UNIT)

The amplified signal from the drive/power amplified circuit is passed through the each TX filter circuit to suppress high- harmonics spurious components

(1) 50 MHz TX FILTER CIRCUIT

The amplified 50 MHz signal from the power amplifier pass- es through the low-pass filter (L22, L23, C34, C44, C480) and 1/4λ type antenna switch (D6). The filtered signal is applied to the low-pass filter (L11–L13, L17–L19, C21, C481, C482) again, and is then passed through the triplexer circuit (L5, L6, L24, C43, C483, C486).

(2) 144 MHz TX FILTER CIRCUIT

The amplified 144 MHz signal from the power amplifier pass- es through the bandpass filter (L18, L19, C24, C26, C27) and 1/4λ type antenna switch (D4, D19). The filtered signal is applied to the high-pass filter (L9, C9–C11), and is then passed through the triplexer circuit (L5, L6, L24, C43, C483–C486).

(3) 430 MHz TX FILTER CIRCUIT

The amplified 430 MHz signal from the power amplifier pass- es through the high-pass filter (L16, L456, C32, C33, C98) and 1/4λ type antenna switch (D2, D18). The filtered signal is applied to the low-pass filter (FI1), and is then passed through the triplexer circuit (L1, C1–C3).

The filtered signal is applied to the antenna connector (CHASSIS unit; J1).

Collector voltage for the drive amplifier (Q52) and control voltage for the power amplifier (Q51) are controlled by the APC circuit to protect the power module from a mismatched condition as well as to stabilize the output power.

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