The RTL-SDR workspace: spectrum, VFOs, gain and squelch

An RTL-SDR does not tune to one station the way the ATS Mini does. It captures a whole slice of spectrum at once, up to a few megahertz wide, and the app picks stations out of that slice in software. Everything in the workspace follows from that: the spectrum shows the slice, and each VFO listens to one signal inside it.

The layout

• VFO bar at the top: the VFO tabs, the frequency, then Mode, Bandwidth (BW), Step, a Preset, the squelch and the signal meter. Everything you need while listening lives here.
• Spectrum and waterfall in the middle: the live spectrum above, the history below, each VFO drawn as a coloured passband with its letter.
• Receiver settings on the left: Source, Radio, Audio, Display and Recording. This is setup you change once in a while.
• Side panel on the right: Bookmarks, Stations and the Scanner.
• Output bar at the bottom: Signals in view, RDS, Recordings and, when available, Decoders.

Tuning

Click a signal on the spectrum to move the active VFO there. The mouse wheel over the spectrum tunes by the step; with Ctrl (⌘ on a Mac) it changes the bandwidth, with Shift it shifts the passband. The wheel over the frequency axis zooms, and dragging it pans.

To type a frequency, focus the readout and press Enter. The entry understands units and a width: 145.5M, 7074k, 162.4/12.5k. The arrow keys step the digit you are on. Press ? for the full list of keyboard shortcuts.

The modes are NFM (narrow FM: two-way radio, PMR, ham repeaters), WFM (broadcast FM), AM and SAM (airband, shortwave broadcast), DSB, USB and LSB (single sideband), CW (Morse) and RAW. Picking a mode sets a sensible bandwidth; the built-in presets (Broadcast FM, Airband, 2 m / 70 cm voice, PMR446, SSB voice, CW narrow) set the bandwidth, filter and audio for a whole kind of listening in one go, never the frequency.

Several VFOs at once

Press + next to the VFO tabs to add a VFO, up to six. A new one lands on the strongest signal no VFO covers yet, or else nearby, and copies its mode, bandwidth and squelch. All of them play together; use Mute and Solo in the VFO bar to choose what you hear. A VFO only works while its frequency is inside the capture. When you tune one outside, it goes quiet and the bar says so: re-centre the capture on it, or bring it back in.

Sample rate and decimation

The sample rate (in Source) sets how wide a slice you capture: 2.4 MS/s shows about 2.4 MHz. Wider costs more USB bandwidth and CPU. The app offers 0.25 to 3.2 MS/s and warns above 2.56 MS/s, where many sticks start dropping samples. Device info and USB test measures what your stick and USB port actually manage. Decimation narrows the view inside the app for finer detail without touching the stick.

Gain and overload

Gain starts on Auto, which suits most listening. Manual gain runs from 0 to 49.6 dB; raise it for weak signals and lower it when strong ones overload the stick. Under Gain stages you can set the tuner’s three amplifiers separately.

When the stick’s converter clips, the status strip shows an Overload badge. Overload sounds like stations appearing where they should not be, or a raised noise floor everywhere. Lower the gain first. If a strong FM broadcast or a nearby transmitter keeps causing it, a filter in front of the stick does more than any setting.

The Correction field fixes the stick’s crystal error in ppm. Sticks with a TCXO (the RTL-SDR Blog V3 and V4) are usually within 1 ppm and need nothing. For others, Calibrate in Radio tools measures the error against a signal whose frequency you trust; let the stick warm up for five minutes first.

Squelch

Squelch keeps a VFO silent until a signal comes up. Set the threshold in the VFO bar, in dBFS. Auto puts it 6 dB above the noise it measures right now. Under Radio you can open the squelch on SNR instead of level, set a tail so word endings are not cut, and on NFM require a CTCSS tone so that on a shared channel you hear only one group.

On a phone or tablet

On a phone the Radio view is the page: the frequency, the spectrum and the controls you use most. Settings, Bookmarks, Stations and the Scanner open as sheets from the bottom bar, and Tune opens a keypad with your recent frequencies and quick bands. Tap the spectrum to tune, pinch to zoom, and long-press for the actions menu. Under Audio, Keep playing in the background lets you listen with the screen off.

Looking for something to listen to? The Stations tab has the EiBi schedule, live amateur spots (POTA, DX Cluster and PSK Reporter) and aviation frequencies near you. Scanner and bookmarks covers the rest.