A simple 40-metre SDR receiver you can build yourself

A homebrew Software Defined Radio receiver by Neville Marr – ZL2BNE

Build your own simple 40-metre Software Defined Radio receiver using readily available components and a computer.

The receiver uses a small amount of analogue hardware to convert the 40-metre RF signal into I/Q baseband audio. Your computer’s stereo soundcard and SDR software then do the rest — providing tuning, filtering, demodulation and a waterfall display.

The design was created by Neville Marr – ZL2BNE as a practical proof-of-concept SDR that keeps the hardware deliberately simple.

Build your own

[ Build Notes ]    [ Schematic ]    [ Gerbers ]    [ Interactive BOM ]

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What is this project?

Software Defined Radio can sound complicated, but the idea behind this receiver is surprisingly simple.

Instead of building a conventional receiver with all of the filtering and demodulation performed in hardware, this project produces the I and Q signals from the incoming RF and passes them to a computer.

The computer then uses SDR software to process those signals.

For this project, HDSDR was used, although other software capable of accepting I/Q input can also be used.

The result is a simple hardware platform that lets you explore 40-metre signals using software.

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Why build a 40-metre SDR?

This project is aimed at amateur radio operators who want to get hands-on with SDR, RF electronics and homebrew construction without having to build a complicated modern SDR from scratch.

It’s particularly suited to:

• 🔧 Homebrew and electronics enthusiasts
• 📻 Amateur radio operators
• 💻 People wanting to experiment with SDR
• 🧪 Makers and electronics students
• 📡 Operators interested in the 40-metre band
• 🎓 Anyone wanting to understand how a simple SDR works

It is deliberately a simple proof-of-concept design, rather than an attempt to compete with a commercial SDR receiver. That simplicity is part of its appeal.

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What can it receive?

The receiver covers approximately:

7.00 MHz – 7.19 MHz

That’s the 40-metre amateur band region covered by the design.

The hardware produces the signals across this range and the SDR software provides the user interface for:

• Tuning
• Waterfall display
• Mode selection
• Filtering
• Signal processing

The original design uses a 192 kHz computer soundcard, which also acts as the receiver’s ADC.

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How does it work?

The receiver is based on a pair of direct-conversion receivers.

At the heart of the design is a stable local oscillator based around a 28.375 MHz crystal.

A dual flip-flop divides this frequency by four:

28.375 MHz ÷ 4 = 7.09375 MHz

That places the resulting signal right in the middle of the 40-metre band.

The incoming RF is split and fed to two mixer stages.

The two mixers produce signals that are 90 degrees apart in phase:

I — In-phase

and

Q — Quadrature

These are the two signals required by SDR software.

The I and Q outputs are connected to the left and right channels of a stereo computer soundcard.

From there, the computer takes over.

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The signal path

Antenna 
↓
RF splitter
↓
Two mixer stages
↓
I + Q baseband
↓
Stereo soundcard
↓
SDR software
↓
Waterfall / tuning / demodulation

This is what makes the project interesting: the hardware is relatively simple, while the computer provides the sophisticated signal processing.

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What’s inside the receiver?

The hardware consists of several straightforward building blocks.

Crystal oscillator

A 28.375 MHz Colpitts oscillator with a buffer provides the stable local oscillator.

Frequency divider

A 74AC74 dual flip-flop divides the oscillator frequency by four to produce approximately 7.09375 MHz, with the two outputs providing the required 90-degree phase relationship.

RF splitter

The incoming antenna signal is split so it can be presented to both mixer stages.

Mixers

The design uses NE602-family mixers to combine the incoming RF with the I/Q oscillator signals.

The mixers also provide amplification to bring the resulting signals up to a level suitable for the computer soundcard.

Computer soundcard

The stereo soundcard provides the analogue-to-digital conversion.

The:

I → Left channel

Q → Right channel

The SDR software then processes the two channels.

The original design notes that NE602, NE612, SA602 and SA612 devices are functionally interchangeable for this application, although the older devices are no longer manufactured and suitable alternatives may need to be sourced.

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A deliberately simple SDR

The design philosophy is important.

This isn’t intended to be an optimised, high-performance SDR receiver.

It was developed as a proof of concept using the simplest practical hardware and readily available software.

As Neville describes it, the aim was to make a radio that demonstrates the SDR concept while remaining reproducible for someone wanting to build the hardware themselves.

That makes it an excellent learning project.

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Build your own

The receiver is built on a PCB and is designed to be assembled by the builder.

The project provides the information needed to reproduce the receiver, including the schematic, PCB files, component information and construction notes.

The basic build

1. Manufacture the PCB

Have the supplied PCB files professionally manufactured.

2. Source the components

Use the Interactive BOM and schematic to identify the required components.

3. Build the power supply

The receiver uses a regulated 5 V supply with reverse-polarity protection.

The input can be approximately 6–15 V DC.

A 9 V battery is one example of a suitable supply.

4. Build the toroidal transformer

One of the more interesting parts of the project is winding the RF transformer.

The transformer uses:

• 31-turn bifilar winding
• 4-turn single-wire winding

The project includes a dedicated illustrated guide explaining how to construct and test it.

5. Assemble the PCB

Populate the board following the build notes.

6. Test the power supply

Apply 6–15 V and verify that the regulated rail measures 5 V before continuing with the rest of the construction.

7. Complete the receiver

Install the oscillator, divider, mixers, RF input, audio output and remaining components.

8. Connect it to a computer

Connect:

I → left soundcard channel

Q → right soundcard channel

9. Start the SDR software

Use HDSDR or another suitable SDR application capable of processing I/Q input.

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The toroidal transformer

The transformer is probably the most hands-on part of the build.

The supplied construction guide walks through the process of making the bifilar winding, winding the toroid, removing the enamel, tinning the wires and checking the finished transformer.

The guide specifies a 31-turn bifilar pair plus a 4-turn winding.

It also includes continuity checks to make sure the windings have been connected correctly before the transformer is installed.

[ Read the Toroidal Transformer Guide → ]

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Connect it to your computer

The receiver is designed around the idea that the computer becomes part of the radio.

The receiver provides the analogue I/Q signals.

Your computer provides:

• ADC
• Digital filtering
• Demodulation
• Frequency display
• Waterfall
• Mode selection
• Audio processing

The original project used HDSDR, but the I/Q output makes the receiver compatible with other SDR software that supports I/Q input.

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HDSDR

HDSDR provides the software interface for the receiver.

Once the I and Q channels are connected to the computer, the software can display the 40-metre spectrum as a waterfall and provide tuning, filtering and demodulation.

[ HDSDR / SDR Software Information → ]

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Antenna considerations

Depending on your location and the RF environment around you, a front-end band-pass filter may be useful, particularly to reduce interference from strong broadcast stations.

The original design was successfully connected directly to an antenna in its development environment, but local RF conditions will vary.

This is another useful aspect of the project: you can experiment with the receiver, antenna and additional filtering to see how the performance changes.

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Build resources

Everything needed to explore and reproduce the project is collected here.

📖 Original article

The receiver was originally described in the May/June 2024 issue of Break-In.

The article provides the background, theory of operation and complete description of the design.

[ Read “A Simple 40 Metre SDR Receiver You Can Build” → ]

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📐 Schematic

The complete circuit schematic.

[ View the 40M SDR Schematic → ]

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🧩 Interactive BOM

Use the Interactive Bill of Materials to identify components and assist with sourcing.

[ Open the Interactive BOM → ]

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🔧 Build Notes

Step-by-step construction notes covering PCB assembly, power testing and component installation.

[ Read the Build Notes → ]

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🧲 Toroidal Transformer Guide

Detailed instructions for winding and testing the RF transformer.

[ Read the Transformer Guide → ]

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🏭 PCB Gerbers

PCB manufacturing files for the 40M SDR Version 3a.

[ Download the Gerbers → ]

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Project photos

The finished receiver

[INSERT FINISHED V3 PHOTO HERE]

A completed 40-metre HomeBrew SDR receiver.

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The receiver in operation

[INSERT HDSDR SCREENSHOT HERE]

The 40-metre SDR receiving signals using HDSDR.

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The PCB

[INSERT PCB RENDER / COMPONENT PHOTO HERE]

The 40M SDR PCB and components.

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From a handful of components to a software-defined radio

The appeal of this project is that the hardware doesn’t need to do everything.

A relatively small amount of analogue circuitry creates the I and Q signals.

The computer does the complicated work.

That makes the project an excellent way to see the basic principles of SDR in action without needing to build an expensive or complicated receiver.

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About the designer

Neville Marr – ZL2BNE

The 40-metre HomeBrew SDR was designed by Neville Marr – ZL2BNE.

The project was originally developed as a practical experiment in building a simple SDR using readily available hardware and freely available computer software.

The resulting design was subsequently published in the May/June 2024 issue of Break-In.

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Ready to build?

If you’re interested in SDR, RF electronics or homebrew amateur radio, this is a great project to start with.

You don’t need an expensive SDR receiver.

Build the hardware, connect it to a computer, start the software — and you have a 40-metre SDR.

Start here

[ 📖 Read the Original Break-In Article ]

[ 🔧 Build Notes ]    [ 📐 Schematic ]    [ 🧩 Interactive BOM ]    [ 🏭 Gerbers ]

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40-metre HomeBrew SDR

Simple hardware · Computer-based SDR · 40 metres · Homebrew

Designed by Neville Marr – ZL2BNE

Build it. Connect it. Open the waterfall. See what’s on 40 metres.