Audio Monitoring and FM Slope Demodulation
The tinySA Ultra features an internal AM demodulator and a 3.5mm audio jack for signal monitoring. While it does not feature a dedicated FM demodulator or hardware volume control, you can monitor both AM and FM signals using specific techniques.
Hardware Configuration
- Audio Output: The 3.5mm jack supports three-pole (stereo) connectors, but the output signal is strictly mono.
- Audio Level: Fixed at a comfortable listening volume (no software or hardware volume control).
- Headphones: High-impedance earbuds are recommended. If audio is weak or distorted, try pulling the plug out by one click.
AM Demodulation Procedure
- Activate a narrow sweep span so the target signal is clearly visible with multiple data points.
- Align the sweep marker with the exact peak frequency of the signal.
- Navigate to
LEVEL -> LISTEN. This freezes the frequency sweep at the marker's current position.
- Adjust the Resolution Bandwidth (RBW). Setting the RBW too wide or too narrow relative to the AM modulation bandwidth will degrade audio quality.
- Fine-tune the marker position slightly to maximize audio clarity.
FM Listening via Slope Demodulation
Because the tinySA Ultra only natively demodulates AM, tuning directly to the center frequency of an FM signal yields little to no audio. To hear FM signals, you must use a technique called slope demodulation, which converts FM frequency variations into AM amplitude variations using the filter edge.
Filter Shape
/\
/ \ <-- Place FM signal here (on the slope)
________/____\________
- Narrow the sweep span to approximately 100 kHz so the target signal appears broad on the display.
- Engage the
LISTEN function.
- Off-center the marker, placing it roughly halfway between the noise floor and the signal peak along the sloping edge of the RBW filter.
- As the FM signal shifts in frequency, the sloped filter translates those shifts into amplitude changes that the AM demodulator can process.
Limitations of FM Slope Demodulation
- Audio Quality: Recovered audio is generally low-quality and prone to distortion.
- Optimization: Audio recovery depends heavily on finding the right balance between the FM modulation index and the RBW setting. Experimentation with both is required.
- Bandwidth: The recovered audio bandwidth is strictly limited. It cannot capture a 19 kHz stereo pilot tone or any multiplexed subcarriers.
