11/06/2025
📡 Using an AM Receiver to Receive SSB with a BFO
📘 Introduction
AM receivers are designed to demodulate standard amplitude modulated signals. However, SSB signals (USB/LSB) do not have a carrier, which makes them unintelligible on a standard AM receiver. To demodulate SSB, a carrier must be reinserted using a BFO (Beat Frequency Oscillator).
This write-up explains how a conventional AM receiver can be adapted for SSB reception using:
1. An external BFO, or
2. A simple BFO circuit using a single transistor to produce a beat frequency that can be zero-beated with the incoming SSB signal.
---
🎛️ Principle of SSB Reception
In standard AM reception:
Demodulated signal = Envelope of (Carrier + Upper Side Band + Lower Side Band)
In SSB:
Carrier is suppressed; only USB or LSB is transmitted.
Therefore, the AM detector has nothing to lock onto unless you reinsert a carrier—this is what the BFO does.
The BFO signal mixes with the incoming SSB signal inside the detector or IF stage to recreate a carrier and allow detection.
---
🔧 Method 1: Using an External BFO
🔌 Circuit Overview
You can use a standalone oscillator (usually a Colpitts or Hartley oscillator) tuned to the intermediate frequency (IF) of the receiver, typically 455 kHz in AM radios.
💡 Implementation Steps:
1. Build or use a signal generator tuned near the IF (e.g., 455 kHz).
2. Place the BFO coil close to the IF transformer in the AM receiver. Magnetic coupling is often enough.
3. Fine-tune the BFO frequency while listening to an SSB signal until the voice becomes intelligible.
4. Adjust frequency for "zero beat" – this occurs when the BFO frequency nearly matches the suppressed carrier frequency of the SSB signal, resulting in clear audio.
✅ Pros:
No internal modification required.
Easy to prototype.
❌ Cons:
Frequency drift in cheap oscillators.
Tuning can be unstable without a stable VFO.
---
🔩 Method 2: Adding a Transistor-Based BFO Inside the Radio
🧪 Basic BFO Circuit (Colpitts Oscillator Example)
Components:
NPN transistor (e.g., BC547, 2N2222)
Capacitors and inductor to set frequency (~455 kHz)
Variable capacitor or varactor for tuning
Power source: 9V or regulated 5V
🔁 Placement:
The oscillator output is loosely coupled (via small capacitor or magnetic coupling) into the IF transformer (455 kHz) of the AM radio.
Alternatively, you can inject directly near the detector diode or mixer.
🎚️ Operation:
The transistor BFO generates a steady sine wave at 455 kHz.
When a 455 kHz SSB signal (USB or LSB) reaches the detector stage, the BFO signal mixes with it.
This creates a new beat frequency in the audio range, which the audio amplifier can handle.
Zero beating allows the listener to adjust the pitch of the received voice to a natural tone.
---
🧠 Example Use Case
Let’s assume:
You’re listening to a 7.100 MHz LSB signal.
Your receiver uses a 455 kHz IF.
Your BFO must produce a signal at 455 kHz, inserted into the IF stage.
When the BFO is adjusted just right, you’ll hear clear human speech. If not tuned properly, it’ll sound garbled or high-pitched.
---
🛠️ Tips for Best Performance
Shield the oscillator to avoid drift from hand capacitance.
Use a stable inductor and capacitors (NP0/C0G types).
If possible, regulate the power supply to reduce frequency drift.
Use a fine-tuning capacitor for exact beat adjustment.
---
🔄 Final Thoughts
Modifying or augmenting a simple AM receiver for SSB reception is a great educational experiment and can extend the usefulness of otherwise basic equipment. Using an external or internal BFO circuit, especially with a transistor-based design, is a practical and cost-effective way to enjoy SSB signals, such as ham radio QSOs, with minimal resources.
---
🧰 Sample Transistor BFO Circuit (for 455 kHz)
+V (9V)
|
R1 (10k)
|
Base --- C1 --- L1 --- C2 --- GND
| |
GND Emitter
|
Collector
|
R2 (1k)
|
GND
C1 and C2: tuning capacitors (adjust for resonance)
L1: 455 kHz inductor (IF coil)
Output: weak magnetic coupling to IF transformer in the AM receiver
