Tutorial Spectrum analyser

Tutorial: Spectrum analyser (wide sweeps)

What it is. The spectrum analyser sweeps a wide frequency range quickly, like a bench instrument, and draws the result as a trace with peak hold, averaging, markers and a band overlay. Use it to survey what is on the air between 1 and 6000 MHz, find interference, check an antenna, or choose a frequency for one of the decoders. It is different from the Spectrum explorer, which watches one band live and measures a signal in detail.

What you need

• A radio. A HackRF uses its native sweep and covers 1 to 6000 MHz in about a second (in 20 MHz steps). Other radios sweep by stepped retuning, which is slower. The mode asks for 20 Msps and works down to 1 Msps. See Supported radios and Radio: HackRF.
• An antenna that suits the range you look at. A wideband antenna shows everything weakly; a band antenna shows its band well.
• No decoder and no frequency list are needed. The default centre is 100 MHz.

Before you start

• A sweep joins many short looks. A signal that only appears occasionally can be missed in one sweep; use the peak trace to catch it.
• A very strong signal (a nearby transmitter) overloads the radio and creates false signals elsewhere in the sweep, "images" and spurs. Reduce the LNA and VGA gains, or turn Amp off, if the display fills with signals that move when you change the gain.
• The radio's own DC spike or LO leak shows at the centre of each 20 MHz step; Remove spurs takes it out.

Step by step

1. In the mode list open General & tools and choose Spectrum analyser (second line Sweep, 1 - 6000 MHz).
2. In the tuner, pick a Range preset or type your own limits (MHz). Presets: Full 1 - 6000 MHz, FM broadcast (87.5 to 108), Airband (108 to 137), 2 m (144 to 146), 70 cm (430 to 440), ISM 433 (433 to 435), ISM 868 (863 to 870), Mobile 700 - 2700, L-band (1000 to 2000), 2.4 GHz (2400 to 2500), 5.8 GHz (5650 to 5925), Satellite IF (950 to 2150) and Ku LNB IF 739-2150.
3. Pick the resolution RBW: 2.8 kHz, 5, 10, 25, 100, 250, 500 kHz or 1 MHz. Finer RBW is slower over a wide range (the HackRF sweep goes no finer than about 2.8 kHz).
4. Set Avg (1 to 64 sweeps averaged) and set the gains LNA, VGA and Amp if your radio has them.
5. Press Start. The Analyser tab draws the trace.
6. Click in the chart for marker 1, shift-click for marker 2, right-click to clear. The readout shows each marker's frequency and level and the difference between them.

What you should see

• A chart with frequency across and level in dBFS up. Live is the latest sweep, Peak the highest value seen at each frequency since you pressed Reset peak, Average the mean over the Avg sweeps, Persistence a density display that shows how often each level occurs. A waterfall of the sweeps scrolls below.
• Bands overlays the EU band plan as labelled strips, so you see what service each peak belongs to.
• The state line reads: HackRF native sweep (20 MHz steps) or stepped retuning, the number of tuning steps per sweep, the number of bins and bin width, the number of sweeps, how many had gaps, and the input rate.

Reading the plot

• Noise floor: the flat bottom of the trace between signals. It depends on RBW (a finer RBW lowers the floor by 10 times the log of the ratio) and on gain. Compare signals to the floor, not to zero.
• Peak hold: the Peak trace never falls. Press Reset peak to start again. A peak that appears once and stays shows an intermittent signal (a remote control, a pager).
• Average: smooths the noise floor so weak signals stand out; it hides short bursts.
• Persistence: bright where a level is common, dim where it is rare; it shows a bursty signal sitting on a steady one.
• Level is in dBFS (relative to the radio's full scale), not dBm, unless your radio has a calibration. Differences in dB are meaningful; absolute values are not.

Options and settings

• Remove spurs: removes the DC spike or LO leak of each step.
• Export CSV writes every bin at the chosen RBW, streamed to the file; the expected size is shown first.
• Auto-save CSV every n seconds into a Folder... with names like specan_YYYYMMDD_HHMMSS.csv; keep n files (0 keeps all); Auto-save on starts it.

Troubleshooting

• Strange signals everywhere. Overload; reduce gain or turn off Amp.
• Sweep is slow. Radios other than the HackRF retune step by step; narrow the range or coarsen RBW.
• Gaps in the trace. The status shows sweeps with gaps; a slow USB link drops samples.
• A signal appears at two places. The image of an overloaded front end; reduce gain.
• Absolute levels disagree with another instrument. Levels are relative; calibrate against a known source.

Tips

• Sweep first, then switch to the Spectrum explorer or a decoder at the frequency you found.
• Leave Peak on while you press a key fob or switch a device to find its frequency.

Legal and privacy notes

Looking at the spectrum is passive and legal almost everywhere. What you decode from a signal you find is subject to local law.

Under the hood

The HackRF native sweep retunes in 20 MHz steps with its own sweep mode and the FFT of each step is placed on a common frequency axis; other radios use stepped retuning. Bins are averaged, the DC spike removed, and the trace is stored as live, peak and average.

Related

Mode page: Spectrum analyser | Spectrum explorer | Signal workbench | Radio: HackRF | Supported radios