RF safety guide for SDR experiments¶
This page defines the minimum safety discipline for all RF-facing labs in the course. It is written for educational SDR experiments with Zynq/AD9363-class boards, RTL-SDR receivers, attenuators, coax cables and controlled low-power signals.
Warning
Do not connect an SDR transmitter directly to a sensitive receiver input unless the expected power level, attenuation and maximum input rating have been checked. When in doubt, add attenuation and measure with a safe instrument first.
Safety goals¶
The goal is to protect:
- SDR receiver inputs from overload or permanent damage;
- RF front-end components on the Zynq/AD9363 board;
- test equipment and USB receivers;
- other spectrum users;
- learners from ambiguous or unsafe lab setups.
Minimum RF connection checklist¶
Before any RF experiment, record these items in the lab notebook or measurement report:
| Item | Required note |
|---|---|
| Signal source | Board, generator, replay file, antenna, or synthetic data. |
| Center frequency | Expected RF or complex baseband frequency. |
| Sample rate / bandwidth | SDR sample rate and approximate occupied bandwidth. |
| TX gain / output level | Configured gain or estimated output power. |
| RX gain | Manual gain setting when possible. |
| RF path | Coax, attenuator chain, splitter, antenna, or over-the-air path. |
| Attenuation | Nominal attenuation in dB and power rating. |
| Receiver input limit | Known safe input level or conservative assumption. |
| Observation tool | HDSDR, GNU Radio, MATLAB, Python, spectrum analyzer, or logs. |
Recommended connection patterns¶
Pattern A: Synthetic data only¶
Use this for early labs and CI validation.
Python / MATLAB model -> IQ file -> reader / analyzer -> plots / metrics
Risk level: low. No RF hardware is connected.
Pattern B: External observation receiver¶
Use this for the first practical RF observation.
Zynq/AD9363 low-power signal -> controlled RF path or weak OTA signal -> RTL-SDR -> HDSDR / IQ recording
Risk level: medium. Keep TX power low, use attenuation when using coax, and avoid unknown gain settings.
Pattern C: Conducted RF loopback¶
Use this only when attenuation is explicitly calculated.
TX port -> fixed attenuator chain -> optional DC block -> RX port / RTL-SDR -> analyzer
Risk level: high if attenuation is missing or incorrect.
Conservative attenuation rule¶
For first experiments, prefer excessive attenuation over receiver overload. A practical educational starting point is:
TX output -> 30 dB to 60 dB attenuation -> receiver input
The exact value must be adapted to the board output level, receiver limit and measurement objective. Never assume that a receiver input is protected just because the signal is generated by another SDR board.
Gain discipline¶
Use manual gain settings whenever possible. Automatic gain control can hide overload and make measurements non-reproducible.
| Stage | Recommended practice |
|---|---|
| TX gain | Start low and increase only after observing the signal. |
| RX gain | Start low or moderate; avoid maximum gain during first connection. |
| HDSDR / SDR software | Disable AGC for measurement-oriented captures when practical. |
| Analysis scripts | Record gain, bandwidth, sample rate and file format in metadata. |
Overload symptoms¶
Stop the experiment and reduce input level if you observe:
- a flat-topped waveform;
- a spectrum filled with unexpected wideband products;
- many harmonics or mirrored images that do not match the model;
- unstable noise floor when gain is unchanged;
- receiver software showing clipping or ADC overload warnings.
Spectrum and regulatory discipline¶
Educational SDR experiments should normally use shielded or conducted setups, dummy loads, attenuators, or very low-power short-range links. Do not intentionally transmit on frequencies or power levels that require authorization.
For public demonstrations, prefer:
- synthetic IQ files;
- recorded captures;
- conducted loopback with sufficient attenuation;
- receive-only observations of known allowed signals.
Lab report RF safety section¶
Every RF-facing lab report should include:
RF safety summary:
- Source:
- Center frequency:
- Sample rate / bandwidth:
- TX gain or estimated output power:
- RF path:
- Attenuation:
- RX gain:
- Receiver protection assumption:
- Observed overload symptoms: yes/no
- Corrective actions:
Definition of a safe course lab¶
A lab is safe enough for course publication when it has:
- an explicit RF path diagram;
- attenuation assumptions;
- receiver protection notes;
- gain settings;
- a fallback synthetic-data mode;
- clear warnings around conducted TX/RX connections;
- metadata fields for frequency, bandwidth, sample rate, gain and file format.