Amateur Radio Transceiver

Overview

An amateur (ham) radio transceiver is a two-way radio operator's primary instrument for long-distance communication on high-frequency (HF) bands. This unit covers the standard amateur bands from 160 m (1.8 MHz) through 6 m (50 MHz), using single-sideband (SSB) voice and continuous-wave (CW) modulation. The design employs a superheterodyne receiver architecture for selectivity and a solid-state linear power amplifier capable of 100 W output into a 50 Ω antenna.

The core signal path splits into two branches. Received signals enter through the RF Front-End, are amplified and mixed down by the Receiver Chain to intermediate frequency (IF), then demodulated and sent to the speaker. Transmitted voice or keyed CW modulates a carrier generated by the Transmitter Chain, which drives the Power Amplifier to produce the final RF output. The Antenna Tuning Unit matches the PA output to the antenna impedance, and the Control Panel allows the operator to select frequency (via a Encoder), mode (SSB/CW), and transmit/receive switching.

How it works

Reception begins at the antenna input, where an Antenna Changeover Relay switches between transmit and receive paths. The RF Front-End applies band-specific filtering to reject out-of-band signals and unwanted images. An RF Preamplifier preamplifier boosts weak incoming signals while minimizing added noise.

The First Mixer combines the RF signal with a local oscillator (VCO) to produce an intermediate frequency (typically 9 MHz). This IF is then amplified by multiple IF Amplifier Stage stages and filtered by a narrow IF Crystal Filter (often a crystal lattice) for selectivity. The Product Detector (product detector) multiplies the IF signal by a beat oscillator to recover the original audio (voice or CW tone), which is then sent to the Speaker.

For transmission, voice from a microphone or CW from a keyer enters the Microphone Preamplifier, which conditions it for the SSB Modulator. This balanced modulator suppresses the carrier and generates a lower-sideband (LSB) or upper-sideband (USB) signal at audio frequencies. The modulated signal is then upconverted to RF by mixing with the Local Oscillator VCO, amplified by the Driver Stage, and finally boosted by the Power Amplifier.

The PA is a parallel MOSFET design, with two MOSFET Transistors operating in push-pull configuration to generate 100 W of RF power into a 50 Ω load. An Low-Pass Output Filter suppresses harmonics and spurious emissions. The Antenna Tuning Unit allows fine impedance matching when the antenna is not a pure 50 Ω resistor, adjusting both inductance and capacitance to cancel any reactance and maximize power transfer.

Control is handled by an Control Panel featuring a Encoder for frequency selection, a mode switch to choose SSB/CW, and a PTT (push-to-talk) button to toggle transmit/receive. The Display Module shows the operating frequency, mode, and RF power output in real time.

The Power Supply derives regulated DC rails (+13.8 V for RF stages, +12 V and lower rails for logic and audio) from a 12 V external supply, with on-board linear regulation and filtering to maintain clean power and minimize noise floor degradation.

Design rationale

Superheterodyne architecture has been the standard for transceivers since the 1920s because it decouples the frequency-selective and high-gain RF front-end from the fixed-frequency demodulator, avoiding oscillator pulling and enabling simultaneous reception and transmission without crosstalk. The use of a crystal filter at IF ensures excellent shape factor and stopband attenuation at a lower component cost than equivalent RF filtering.

Linear power amplifier design (push-pull MOSFET) was chosen over switching amplifiers because amateur regulations require distortion products to be at least 50 dB below the fundamental (for SSB), and linear stages inherently meet this without the complexity of envelope tracking or other digital correction loops.

The Antenna Tuning Unit is essential for portable or emergency operation because most antennas at HF are compromises or adjusted for one band only; the ability to tune impedance mismatch on the fly extends operating range and efficiency.