Midland 2001/77002 Two-Way Radio Schematic Diagram

Overall System Architecture

The schematic illustrates a two-way radio system with distinct transmit (TX) and receive (RX) paths. Key components include an antenna, RF amplifiers, mixers, phase-locked loop (PLL) ICs for channel selection, voltage-controlled oscillators (VCOs), filters, detectors, and audio power amplifiers. The system operates by receiving signals, processing them through intermediate frequency (IF) stages, detecting the audio, amplifying it, and outputting to a speaker. For transmission, an audio input (microphone) is filtered, processed, and amplified, then modulated and transmitted via an RF power amplifier to the antenna.

Receive Path

The receive path begins with an antenna connected to an RF amplifier (Q101). The signal is then mixed in a converter (CONV.1, Q102) with a local oscillator frequency. This is followed by another converter (CONV.2, Q105) and an IF amplifier (Q103). A detector (DET, D105) extracts the audio signal, which is then processed by an audio amplifier (AUDIO POWER AMP, IC 201) and sent to the speaker (SP). An Automatic Gain Control (AGC) circuit manages signal levels throughout the receive chain.

Transmit Path

The transmit path starts with a microphone (MIC) input, which is filtered and then processed by a squelch (SQL, Q201) and an ALC (Automatic Level Control) circuit (Q202). The signal is then fed into an RF amplifier (RF AMP, Q301), followed by an RF drive stage (Q302) and an RF power amplifier (RF POWER, Q303). An LC filter is used to shape the transmitted signal before it is sent to the antenna via a rectifier (RECT, D302).

Frequency Control and Channel Selection

A Phase-Locked Loop (PLL) IC (IC 202) is central to frequency control and channel selection. It receives input from a channel switch (Channel SW.) and controls a Voltage-Controlled Oscillator (VCO, 1/2 IC 203) which generates the necessary local oscillator frequencies. The PLL IC also drives a Liquid Crystal Display (LED SW.) to show the selected channel. The VCO outputs frequencies such as 10.24 MHz and 16.725 MHz, which are mixed and processed to achieve the desired operating frequencies.

Key Components and Frequencies

• RF Amplifier (Q101): Operates at 26.965 MHz.
• Converter 1 (Q102): Mixes signals at 26.965 MHz and 10.695 MHz.
• Converter 2 (Q105): Mixes signals at 10.695 MHz and 0.455 MHz.
• IF Amplifier (Q103): Operates at 0.455 MHz.
• PLL IC (IC 202): Manages channel selection and frequency synthesis.
• VCO (1/2 IC 203): Generates local oscillator frequencies (e.g., 10.24 MHz, 16.725 MHz).
• Mixer (1/2 IC 203): Combines frequencies for transmission (26.965 MHz).
• Audio Power Amplifier (IC 201): Amplifies the detected audio signal.
• RF Power Amplifier (Q303): Amplifies the signal for transmission.

Component Details (Illustrative Snippets)

The schematic includes detailed component values and connections. For instance:

• Resistors (R) are specified with values like 22KΩ, 100KΩ, 1.8KΩ, etc.
• Capacitors (C) are specified with values like 0.01µF, 39pF, 470pF, 0.022µF, etc.
• Inductors (L) are shown as coils, with some identified as L101 through L107.
• Diodes (D) are indicated with types such as IS 34, IS 555, etc.
• Transistors (Q) are labeled with part numbers like Q101, Q301, Q302, etc.
• Integrated Circuits (IC) are labeled as IC 201, IC 202, IC 203, etc.
• Test points (TP) are marked for measurement and debugging.

Specific voltage levels are indicated at various points in the circuit, such as 12.04V, 2.04V, 1.52V, 12.7V, 0.66V, etc., providing operational context.

Additional Circuit Elements

The diagram also shows power connections (+B, Ground), switches (Channel SW., T/R, SW.3), a meter for RF power indication, and various filters (e.g., CF.1, CF.2) and tuning elements (e.g., L101, L102) crucial for radio frequency performance.