Tutorial General receiver

Tutorial: General receiver (NFM, WFM, AM, USB, LSB, CW)

What it is. The general receiver is the all-round radio of OnAir: you click on a signal in the spectrum or waterfall, choose how to demodulate it, and listen. It covers narrow FM (NFM), wide FM (WFM, mono), AM, upper and lower sideband (USB, LSB) and CW, with adjustable filter, squelch, AGC, an S-meter, frequency steps and audio recording. Use it for amateur voice, marine and air band checks, weather satellite audio, scanning around, or anything that has no mode of its own.

What you need

• Almost any radio. The mode asks for 2 Msps and works down to 250 ksps; the mode table covers 1 to 6000 MHz. An RTL-SDR is a fine start. HF needs a radio that reaches HF (or direct sampling, or an upconverter). See Supported radios.
• An antenna for the band you listen to.
• Frequencies from the frequency database (data/frequencies/amateur.tsv and others), for example:
  • 145.500 MHz, FM calling channel on 2 m (Region 1), NFM
  • 146.520 MHz, FM calling frequency 2 m (US), NFM
  • 144.300 MHz, SSB calling frequency 2 m, USB
  • 432.200 MHz, SSB/CW calling 70 cm, USB
  • 50.200 MHz, SSB calling 6 m, USB
  • 28.400 MHz, SSB centre of activity 10 m, USB
  • 29.600 MHz, FM calling 10 m, NFM
  • 137.100 MHz, NOAA 19 APT (analogue picture; the general receiver shows the audio) The default start frequency is 145.5 MHz.

Before you start

• For sideband, the correct sideband above 10 MHz is USB; below 10 MHz radio amateurs use LSB (the 40, 80 and 160 m bands). Many 2 m and 70 cm SSB contacts are USB.
• Gain: raise it until the noise floor lifts clearly, and no further. If the S-meter bar is stuck near 0 dBFS on every signal, the radio is overloaded.
• Avoid putting the signal at the exact centre of the spectrum, where the radio's DC spike is.

Step by step

1. In the mode list open General & tools and choose General receiver (second line NFM / AM / SSB / CW).
2. Set the source and press Start.
3. In the Listen tab choose the demodulator with the radio buttons NFM, WFM, AM, USB, LSB, CW.
4. Click on a signal in the spectrum or the waterfall. The light band (the channel marker) moves there and the frequency under Listening on updates. Drag the edges of the light band to change the filter, or press a width preset (the filter buttons).
5. If the sound is noisy between transmissions, tick Squelch.
6. To tune the radio so that the channel is in the middle again, press Centre.

What you should see

• The listened frequency in MHz with four decimals (the dial plus the marker offset).
• An S-meter bar from -130 to 0 dBFS with the squelch threshold marked.
• A readout: Listening on (frequency and offset from the dial), Input (sample rate and demodulation rate), Filter (edges around the channel in kHz), Noise in the channel, Squelch threshold, FM offset and Audio level in dBFS.

Reading the screen

• Frequency step: the drop-down offers 10 Hz, 100 Hz, 1 kHz, 2.5, 5, 6.25, 8.33, 9, 10, 12.5, 25, 50 and 100 kHz; the arrows (hold for continuous change) and clicks on the spectrum snap to it. Set 9 or 10 kHz for medium wave.
• Filter presets, in kHz per demodulator: NFM 6.25, 12.5, 25; WFM 150, 180, 200; AM 4, 6, 9, 12; USB and LSB 1.8, 2.4, 2.7, 3.0; CW 0.1, 0.25, 0.5, 1. Defaults are WFM 180, AM 9, SSB 2.7 kHz.
• Squelch has two modes: Auto opens when the signal is 6 dB over the noise beside the channel (for USB and LSB, when the voice passband is 6 dB over its own noise, so no carrier is needed); Level opens at a fixed threshold in dBFS. Set puts the threshold 3 dB over the current level (use it while nobody transmits). A hang time keeps the squelch open between words.
• AGC on or off; with it off a fixed gain slider (0 to 80 dB) applies.
• De-emphasis: off, 50 us, 75 us (broadcast FM) or 750 us (pre-emphasised narrow FM).
• CW pitch: a slider from 200 to 1500 Hz sets the tone a carrier on the channel gives (the BFO).
• Volume.

Options and settings

• Record: records what you hear (after the squelch, before the volume) to a 16-bit 48 kHz WAV in the audio-recordings folder. Tick FLAC for lossless files about half the size, with the frequency and date as tags, or MP3 (64 kbit/s mono, needs the LAME library). One file per transmission starts a file each time the squelch opens, with a pre-roll so the first syllable is kept, and stays open through the hang time. The file name carries the time, frequency and mode.
• Channel IQ: records the IQ of the channel (check the panel for what it writes).
• Noise remover (neural): a speech noise remover with a strength slider; it adds about 20 ms of delay. Use it on speech only, not on music or data modes (see the noise-remover documentation).
• Channels: listen to more than one channel of the same capture at once; each extra channel is another demodulator and costs CPU. Drag the amber lines on the spectrum onto the signals; you hear the first channel, the others show their level and open/closed state; Listen swaps one to the front and Rec records each transmission on it.
• Presets (AM broadcast mode): the separate AM broadcast entry in the mode list is this receiver in AM with medium wave and shortwave presets (MW 531, 999, 1368, 1566 kHz and the 49, 41, 31, 25, 22, 19, 16, 13 and 11 m bands).

Troubleshooting

• Hiss only. Squelch is off, or the demodulator does not match the signal (WFM on a narrow signal, for example). Check the filter width and demodulator.
• Voice sounds like Donald Duck. Wrong sideband, or the marker is off the carrier. Move the marker in small steps (10 or 100 Hz).
• Squelch chops words. Raise the hang time or lower the threshold; use Auto and watch the S-meter.
• Distortion on strong signals. Overload at the radio. Reduce the gain.
• CW silent. Narrow the filter (100 to 500 Hz) and set the pitch; tune so that the carrier gives your pitch.
• High CPU with extra channels. Lower the sample rate.

Tips

• Use 2.5 or 5 kHz steps for NFM channels, 25 kHz and 8.33 kHz for airband AM (see Airband).
• With a synthetic source you can try every demodulator: a station on the dial frequency modulated by a 1 kHz tone, over noise.
• Record with One file per transmission to log a repeater or a net unattended.

Legal and privacy notes

Listening is legal in most countries, but some forbid listening to certain services (police, mobile telephony, others). Know your local law. Do not use recordings of private communications in ways the law forbids. OnAir's Transmit feature requires a licence on the bands you use (see Transmit).

Under the hood

Each channel is frequency-shifted to baseband, filtered, decimated and demodulated: FM by phase differentiation with de-emphasis, AM by envelope with AGC, SSB by filtering the wanted sideband and AGC, CW with a BFO at the chosen pitch. Squelch compares the channel level with the noise measured beside the channel. Audio is 48 kHz.

Related

Mode page: General receiver | FM | Spectrum explorer | Signal workbench | Supported radios | Troubleshooting