Building a Simple 2N2222 DSB Transmitter for Amateur Radio

Minimalist RF circuits often teach more than complex modern designs. This small DSB transmitter is a good example. The entire DSB transmitter uses only one active semiconductor device together with a pair of diodes and a few tuned circuits.

At first glance the circuit almost looks too simple to work seriously. Yet it does. When connected to a properly matched antenna, the transmitter can establish contacts with ordinary SSB stations surprisingly well. Most operators on the other end will hardly notice that the signal comes from such a basic setup because the carrier is largely suppressed.

The original concept was published by the well-known experimenter JF1OZL and later reproduced by several European homebrew enthusiasts. Its appeal comes from one thing: simplicity.

DSB Transmitter  – Circuit Description

The transmitter combines three important RF sections into a compact design:

• crystal-controlled RF oscillator
• balanced diode modulator
• tuned bandpass output filter

The oscillator uses a single 2N2222 transistor operating in a conventional crystal-controlled circuit. The crystal stabilizes the operating frequency while coils L1 and L2 provide the required feedback and output coupling.

The RF signal from the oscillator feeds a balanced modulator built around diodes D1 and D2. Audio from a carbon microphone enters the modulator through transformer Tr1. Inside this stage, the audio and RF signals mix together while the carrier wave cancels out.

The result is a DSB signal with suppressed carrier. That’s the reason ordinary SSB receivers can copy the transmission quite normally.

Output Power and Practical Range

The RF output power is extremely low, typically around 5 mW at the antenna connector. The oscillator itself can deliver roughly 40 mW before the filtering stages, but only a fraction reaches the antenna.

That may sound insignificant. Still, with an efficient dipole or properly matched resonant antenna, the transmitter works better than expected.

On the other hand, random indoor wire antennas usually give disappointing results. A poorly matched “wire out the window” setup may only provide very short-range communication. With a tuned outdoor antenna, however, local and even modest DX contacts become possible under good propagation conditions.

Oscillator Stage

The oscillator section is intentionally simple.

The 2N2222 transistor operates with crystal stabilization, giving good frequency stability without complicated circuitry. C1, L1, and L2 are selected according to the chosen crystal frequency.

The original design used a turns ratio of approximately 1:3 between the coupling coil and the tuned winding. For operation on 80 meters around 3.5 MHz, the following values work well:

Suggested Coil Data

L1

• 18 turns
• 0.5 mm enamel copper wire
• 8 mm air core

L2

• 6 turns wound over L1
• close coupling

The oscillator output at L2 is typically around 2 V RF across a 50-ohm load.

Balanced Modulator Section

The modulator is the most interesting part of the DSB transmitter. A trifilar-wound RF transformer couples the oscillator signal into a pair of germanium diodes. At the same time, the audio signal from the microphone transformer drives the diode pair in opposite phase.

Because both diodes conduct symmetrically, the RF carrier largely cancels while the sidebands remain. Good diode matching is important here. Poorly matched diodes increase residual carrier leakage.

Recommended Diodes

Suitable replacements for D1 and D2 include:

• OA90
• OA91
• AA119
• 1N34A
• BAT43
• BAT85

Germanium diodes generally provide the best low-level switching performance.

Audio Input and Transformer

One particularly clever feature of the design is the direct use of a carbon microphone.

An ordinary telephone carbon insert from older telecom equipment provides enough audio level to drive the balanced modulator without additional amplification. The microphone operates directly from the transmitter supply voltage.

Some microphone inserts prefer reduced current, so a resistor between 1 kΩ and 5 kΩ can be added in series. Transformer Tr1 should ideally provide a ratio close to 900 Ω : 100 Ω, similar to old television or transistor radio audio transformers.

Suitable substitutes include:

• old transistor radio driver transformers
• small telecom audio transformers
• 600 Ω : 600 Ω isolation transformers
• miniature modulation transformers

The secondary audio voltage should reach approximately 800 mV peak.

RF Transformer and Filter Network

The RF transformer around L3, L4, and L5 is wound trifilarly on a ferrite toroid or binocular core salvaged from older television tuners or IF sections.

The original turns ratio is approximately 1:6 between the coupling winding and the tuned section.

Suggested Values

L3

• 4 turns

L4

• 24 turns trifilar wound

L5

• 24 turns trifilar wound

After modulation, the signal passes through a capacitively coupled bandpass filter consisting of L6, L7, L8, and associated capacitors. This section suppresses harmonics and improves spectral cleanliness before the signal reaches the antenna.