Lab 6.2 — Gain staging and overload¶
Lab 6.2 — Gain Staging and Overload¶
Goal¶
Learn how to choose safe RF levels, observe overload symptoms and document gain settings for repeatable SDR experiments.
The lab answers the practical question:
How do we know whether the RF receiver is seeing a clean signal or an overloaded/distorted one?
RF level chain¶
flowchart LR
TXGAIN[TX gain / output level] --> ATT[External attenuation]
ATT --> CABLE[Cable / RF path]
CABLE --> RXGAIN[RX gain]
RXGAIN --> ADC[ADC input]
ADC --> FFT[Spectrum]Safe starting point¶
Use conservative settings for the first cabled experiment:
| Item | Recommended start | Comment |
|---|---|---|
| TX gain | minimum or strongly reduced | avoid receiver damage/overload |
| External attenuation | 30–60 dB | mandatory for direct cable tests |
| RX gain | low/manual | disable AGC for repeatability |
| Signal type | single tone | easiest overload indicator |
| Tone offset | 50–200 kHz | away from DC and band edge |
| Observation span | within RX bandwidth | avoid edge effects |
Do not skip attenuation
A direct TX-to-RX cable without attenuation can overload or damage a sensitive receiver. Start with more attenuation than you think you need.
Normal vs overloaded spectrum¶
| Observation | Normal mode | Overload mode |
|---|---|---|
| Main tone | narrow stable peak | distorted or flat-topped region |
| Harmonics | absent or low | strong harmonics/spurs |
| Noise floor | stable | rises with signal level |
| Gain response | predictable | compressed or unchanged |
| Time waveform | sinusoidal-like | clipped / squared |
Gain sweep procedure¶
- Configure a fixed frequency plan from Lab 6.1.
- Set external attenuation.
- Disable AGC if possible.
- Start with low TX gain and low RX gain.
- Record the main peak level and noise floor.
- Increase RX gain in small steps.
- Stop when overload symptoms appear.
- Return to the last clean setting.
- Repeat for TX gain if needed.
Measurement table¶
| Step | TX gain | RX gain | External attenuation | Peak level | Noise floor | Spur level | Verdict |
|---|---|---|---|---|---|---|---|
| 1 | clean / overload | ||||||
| 2 | clean / overload | ||||||
| 3 | clean / overload | ||||||
| 4 | clean / overload |
Simple overload metrics¶
Headroom estimate¶
headroom = clipping_level - peak_level
Spur-free dynamic range estimate¶
SFDR = main_peak_level - largest_spur_level
Signal-to-noise estimate¶
SNR = main_peak_level - noise_floor_level
These estimates are spectrum-display approximations. For rigorous work, define bandwidth, window, averaging and calibration method.
Common mistakes¶
| Mistake | Result | Fix |
|---|---|---|
| AGC enabled during measurement | gain changes during experiment | use manual gain |
| no external attenuation | overload or damage risk | add attenuator |
| tone too close to DC | DC spur hides signal | shift tone away from DC |
| tone near band edge | filter roll-off changes level | move tone toward center |
| only screenshot, no settings | experiment cannot be reproduced | record metadata |
Report checklist¶
- [ ] Draw the RF level chain.
- [ ] State TX/RX frequencies.
- [ ] State TX/RX gains.
- [ ] State external attenuation.
- [ ] State sample rate and bandwidth.
- [ ] Record clean spectrum observation.
- [ ] Record overload spectrum observation or safe margin.
- [ ] Choose recommended safe setting.
- [ ] Attach IQ metadata.
Engineering conclusion template¶
The clean operating region was observed for TX gain ____ and RX gain ____ with ____ dB external attenuation.
Overload symptoms appeared at ______. The recommended setting is ______ because it provides stable peak level,
low spur content and enough headroom for repeatable measurements.