Overall Architecture

The design combines four key subsystems:

1. RF signal conditioning (filters and switching)
2. RF measurement (directional coupler and detectors)
3. digital control (ESP32 + MCP23017)
4. operator interface (encoders, display, buzzer)

Together these blocks form the front-end control and monitoring infrastructure of a modern QRP HF transceiver platform

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Block Description

1. Antenna Interface and Directional Coupler

The antenna port feeds a directional coupler built around a binocular ferrite core (BN43-202).
This block samples the RF energy traveling in both directions on the transmission line.

Main functions:

• Sense forward power delivered to the antenna.

• Sense reflected power caused by impedance mismatch.

• Provide low-level RF samples for the measurement circuitry.

Two secondary windings on the binocular core couple a small fraction of the RF current from the transmission line. The resulting signals are rectified by detector diodes and converted to DC voltages proportional to the RF power.

These voltages allow the system to calculate:

• Forward power

• Reflected power

• Standing Wave Ratio (SWR)

This type of coupler is widely used in QRP SWR meters because it is simple and broadband across the HF bands.

2. Low-Pass Filter Bank

The RF signal passes through a bank of relay-selected low-pass filters (LPF) designed to suppress harmonic emissions during transmission.

The filters cover the HF spectrum using three bands:

Each filter is a 7 poles Chebishev LC network built with toroidal inductors and RF capacitors.

Functions:

• Suppress transmitter harmonics

• Maintain regulatory spectral purity

• Provide good impedance matching in the passband

Latch Relays route the RF signal through the correct filter depending on the operating frequency selected by the controller.

3. Relay Control and I/O Expansion

The relay bank is driven by an I²C GPIO expander, the MCP23017.

This device expands the number of digital outputs available to the controller and performs several tasks:

• Drives filter-selection relays

• Controls RX/TX switching relays

• Simplifies wiring by using the I²C bus

Using an expander allows the microcontroller to manage many control lines while using only two pins (SDA and SCL).

4. RF Power Amplifier Interface

The filtered RF signal connects to the RF power amplifier stage.

This block represents the connection point between the low-level RF system and the final amplifier.

Key roles:

• Deliver clean RF drive to the PA

• Route the transmit signal through the filter bank

• Return the received RF path toward the receiver

Proper impedance control (50 Ω) throughout this section is essential to minimize insertion loss and reflections.

See also this project: 1-30 MHz 0.5-1W class C amplifier

5. Power and SWR Detection Circuit

Signals from the directional coupler are processed by an RF detector and analog conditioning network.

Main components include:

• RF detector 1SS86 Schottky diodes
• RC smoothing filters

This block converts the RF samples into DC voltages suitable for measurement by the microcontroller's ADC.

Measured values allow the firmware to compute:

• Forward power

• Reflected power

• SWR

These measurements can then be displayed to the user.

6. Microcontroller Control System

The central controller is an ESP32 microcontroller board.

It manages both the RF control logic and the user interface.

Responsibilities include:

• Selecting the appropriate low-pass filter

• Reading SWR and power measurements

• Managing RX/TX switching

• Driving the display

• Handling user input from rotary encoders and buttons

The ESP32 provides significant processing capability and built-in peripherals, making it well suited for digital control of RF equipment.

7. User...