Learning the Easy BITX circuit in detail for troubleshooting

| March 10, 2026 | Amateur Radio (Ham Radio) | No Comments

BITX means bidirectional transceiver with classic bidirectional topology developed by VU2ESE, where most circuit blocks are used for both transmitting and receiving by simply reversing the signal flow. The “Easy” designation comes from the PCB layout by VU3SUA, which was divided into 15 distinct stages, making it easier for homebrewers to build and troubleshoot section by section. The assembly of two band version for 20 and 40m is not divided into separate sections like that. Each type of components are assembled in a group like all transformers together, all resistors together, all transistors together and all capacitors together.

1. The Core Concept: Bidirectional Amplifiers

The heart of the Easy BITX is the bidirectional amplifier. Instead of separate RX and TX chains, the circuit uses pairs of transistors strapped back-to-back.

  • How it works: When you are in Receive (R), power is applied to one transistor, which amplifies the signal from the antenna toward the speaker.
  • When you press PTT (T): Power is cut from the RX transistor and applied to its twin, which is oriented in the opposite direction. This sends the signal from the microphone toward the antenna.
  • Benefits: This significantly reduces the component count and ensures that the filters (Crystal and Band-Pass) are used for both modes, ensuring perfect frequency alignment between TX and RX.

2. Signal Path Breakdown

A. Receiver Mode (RX)

  1. Antenna & LPF: The signal enters through a Low Pass Filter (which removes harmonics).
  2. Band Pass Filter (BPF): A double-tuned circuit that selects only the desired band (e.g., 40m or 20m).
  3. RF Amplifier: A bidirectional stage that provides the first level of gain.
  4. First Mixer: A diode-ring mixer combines the RF with the VFO signal (Variable Frequency Oscillator) from CLK0 of Si5351 to produce the Intermediate Frequency (IF).
  5. Crystal Filter: A “ladder” filter made of matched crystals that strips away everything except the narrow SSB signal (approx. 2.4 kHz wide).
  6. IF Amplifier: Boosts the filtered signal.
  7. Product Detector: Mixes the IF with the BFO (Beat Frequency Oscillator) coming CLK1 of Si5351 to convert the signal down to audible frequencies.
  8. Audio Amplifier: LM386 IC that drives the speaker or headphones.

B. Transmitter Mode (TX)

  1. Microphone Amp: Boosts your voice.
  2. Balanced Modulator: Combines your voice with the BFO to create a Double Sideband (DSB) signal.
  3. Crystal Filter: Crucially, the same filter used in RX now “chops off” one sideband and the carrier, leaving a clean Single Sideband (SSB) signal at the IF frequency.
  4. Post-Filter Amp: Boosts the IF signal.
  5. Second Mixer: Mixes the SSB IF signal with the VFO to translate it back up to the operating frequency (RF).
  6. Power Chain: The signal passes through the BPF, an RF pre-driver, a driver, and finally a Power Amplifier (PA)—using an IRF510 to reach 4W on 20m and 7W on 40m.