The Sovina 40m Class-E CW Transmitter is a compact and highly efficient HF transmitter designed specifically for amateur radio operators who enjoy homebrew QRP equipment. This circuit is based on the original work by PY2OHH (Roberto S. Soria) and has become popular due to its simplicity, excellent efficiency, and clean RF output. This CW Transmitter is particularly noted for its reliability and performance.
Overview of the Design
The transmitter operates on the 40-meter band (around 7.030 MHz) and uses Class-E switching techniques to achieve efficiencies above 80%. Instead of linear amplification, the MOSFETs work as RF switches, greatly reducing dissipation and allowing several watts of RF output from a small circuit.
The system is divided into four main sections:
- Crystal oscillator
- Logic gate buffer and keying
- Class-E power stage
- Harmonic filter and antenna matching network
Crystal Oscillator and Logic Buffer
A 74HC02 quad NOR gate IC forms a stable crystal oscillator at 7.030 MHz using a 1 MΩ feedback resistor and 100 pF / 22 pF loading capacitors. Three of the NOR gates are paralleled to provide sufficient drive current for the MOSFET gates.
This approach offers:
- Excellent frequency stability
- Square-wave drive with fast rise times
- Simple CW keying by grounding the gate drive line
Class-ECW Transmitter –Power Stage
Three BS170 MOSFETs are connected in parallel to share current and reduce on-resistance. The drain is fed through a 10 µH RF choke, while a 2.2 nF series capacitor and shunt capacitor establish the Class-E switching condition. This ensures that the MOSFET turns on when drain voltage is near zero, minimizing switching loss.
Output Network and Low-Pass Filter
The output network consists of:
- 770 nH and 360 nH series inductors
- 390 pF and 1000 pF shunt capacitors
- 390 pF antenna coupling capacitor
This network performs two critical functions:
- Shapes the voltage and current waveforms for proper Class-E operation
- Suppresses harmonics and matches the transmitter to a 50-ohm antenna
The result is a clean CW signal suitable for direct connection to an antenna or low-power tuner.
Power Supply and Regulation
A 7805 regulator provides stable 5 V for the 74HC02 logic, while the RF stage is powered directly from a 12–14 V supply. Proper decoupling with 100 nF and 470 µF capacitors prevents RF feedback and ensures stable operation.
Class-E CW Transmitter –Performance on the Air
According to the original author PY2OHH, the transmitter achieves:
- About 4.6 W at 11.5 V with ~86% efficiency
- About 7.4 W at 14.3 V with ~85% efficiency
These figures make it an excellent choice for portable QRP, educational RF experiments, and Class-E amplifier study.
Acknowledgment
This circuit is based on the original Sovina 40m Class-E CW Transmitter designed by Roberto S. Soria – PY2OHH.
Original project page:
https://py2ohh.wixsite.com/py2ohh/sovina-transmitter
All credit for the core design and measurements goes to the original author.
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