Software Defined Radio (SDR) is a radio communication system where components traditionally implemented in hardware (mixers, filters, amplifiers, modulators/demodulators, detectors) are instead implemented in software on a computer or embedded system.
- Flexibility: Change radio parameters through software updates
- Cost-Effective: One device can operate across multiple frequency bands and modes
- Upgradability: Add new features and protocols without hardware changes
- Experimentation: Easy to test and develop new signal processing techniques
- Education: Visualize and understand radio signals in real-time
The rate at which a radio wave oscillates, measured in Hertz (Hz). Common ranges:
- kHz (kilohertz): 1,000 Hz - Used for AM broadcast, maritime
- MHz (megahertz): 1,000,000 Hz - Used for FM broadcast, amateur radio, aviation
- GHz (gigahertz): 1,000,000,000 Hz - Used for satellite, WiFi, radar
The range of frequencies occupied by a signal. Different modes require different bandwidths:
- CW (Morse Code): ~100 Hz
- SSB (Single Sideband): ~2.7 kHz
- AM Broadcasting: ~10 kHz
- FM Broadcasting: ~200 kHz
- Digital TV: Several MHz
The number of times per second the SDR samples the incoming signal. Must be at least twice the highest frequency of interest (Nyquist theorem).
Example: To receive a 20 MHz wide signal, you need a sample rate of at least 40 MS/s (mega-samples per second).
The middle frequency of the range you're observing. The SDR samples frequencies around this center point based on the sample rate.
If center frequency = 100 MHz and sample rate = 2 MHz, you can see signals from 99-101 MHz.
Shows signal amplitude over time. Useful for:
- Observing modulation patterns
- Detecting interference
- Measuring signal timing
Shows signal power at different frequencies. Useful for:
- Finding active signals
- Identifying signal types
- Measuring bandwidth
- Detecting interference sources
A time-history of the spectrum, showing how frequency content changes over time. Color represents signal strength:
- Blue/Purple: Weak or no signal (noise floor)
- Green/Yellow: Moderate signal strength
- Red/White: Strong signal
Measured in dBm (decibel-milliwatts) or as S-units:
- S9: Strong signal (-73 dBm for HF)
- S9+20dB: Very strong signal
- S1-S3: Weak signal
- Below S1: Too weak to decode reliably
The ratio of signal power to noise power. Higher is better:
- >20 dB: Excellent quality
- 10-20 dB: Good quality
- 0-10 dB: Marginal quality
- <0 dB: Usually unreadable
The ratio between the strongest and weakest signals the SDR can handle simultaneously. Measured in dB. Higher values mean better performance in crowded spectrum.
Controls the amplification of incoming signals before digitization:
- High gain: Better for weak signals, but risk of overload with strong signals
- Low gain: Better for strong signals or crowded bands
- AGC (Automatic Gain Control): Automatically adjusts gain
Mutes audio output when signal strength falls below a threshold. Prevents listening to noise between transmissions.
Determines how wide a frequency range passes through. Narrower filters:
- Reject more interference
- Improve weak signal reception
- May cut off parts of the desired signal if too narrow
- Choose your frequency: Use frequency allocation charts to find interesting signals
- Set appropriate mode: Match the modulation to the signal type
- Adjust bandwidth: Start wide, then narrow to isolate your signal
- Tune RF gain: Avoid overload while maintaining good sensitivity
- Fine-tune frequency: Center the signal in your passband
Most SDR users operate in receive-only mode, which is generally legal worldwide for most frequencies.
Transmitting requires:
- Appropriate license for the frequency and mode
- Type-approved equipment
- Power limits compliance
- Spurious emission limits compliance
- Never record or share private communications
- Respect encryption and privacy
- Follow local laws regarding monitoring
- Use responsibly and ethically
- Explore Frequency Allocations to find signals of interest
- Learn about Modulation Types to decode different signals
- Review Common Use Cases for practical applications
- Study Signal Analysis techniques