Understanding Slices and VFOs

In AetherSDR, a slice is an independent receiver within a panadapter. Each slice has its own VFO frequency, mode, filter, and audio settings. The FLEX-8600 supports up to eight simultaneous slices (labeled A through H), letting you monitor multiple frequencies at once within the same or different panadapters.

Before you start

• AetherSDR must be connected to a FLEX-8600 radio. Slices only exist when a radio connection is active.
• The RX Controls applet must be visible. If it is not, click the RX tray button on the right sidebar.

How slices work

Each slice is a fully independent receive channel. It has:

• A VFO frequency — the center tuning frequency for that slice, shown in the Frequency label in the RX Controls applet.
• A mode — USB, LSB, CW, AM, SAM, FM, NFM, DFM, DIGU, DIGL, or RTTY — set with the Mode combo.
• A filter passband — adjustable via filter width presets or by dragging the Filter passband widget.
• Its own AF gain, AGC, squelch, RIT, and XIT settings.
• Assigned RX and TX antennas.

A slice is always bound to a panadapter. The panadapter shows the FFT spectrum for the slice's band segment, and the slice's VFO marker appears as a line on that spectrum.

Slices and the panadapter

The panadapter's Spectrum / waterfall display shows the slice's current VFO position. Clicking or dragging on the spectrum tunes the active slice. The panadapter title bar shows which slice is bound to it (for example, Slice A).

In multi-slice mode, each panadapter can contain one or more slice markers. Clicking the spectrum of a different panadapter activates that panadapter and its associated slice.

Switching between slices

The RX Controls applet shows a row of tabs labeled A through H (up to the radio's maximum slice count). Click a tab to bind the RX Controls applet to that slice. The Slice badge indicator in the applet updates to show the active slice letter, colored by slice identity. The badge supports rich text rendering for the slice letter.

The tab row is hidden when only one slice is in use. When the radio is disconnected, clearSliceButtons() tears down all tab buttons and restores the static slice badge.

The TX slice

Only one slice transmits at a time. The currently transmitting slice is the TX slice. To make a slice the TX slice, click its TX (badge) button in the RX Controls applet. This routes transmit through that slice's frequency, mode, and TX antenna.

RIT and XIT

RIT (Receive Incremental Tuning) offsets the receive frequency without moving the VFO. Enable it with the RIT button; adjust with the RIT offset spinbox (10 Hz steps); reset with RIT 0.

XIT (Transmit Incremental Tuning) offsets the transmit frequency without changing the receive frequency. Enable it with the XIT button; adjust with the XIT offset spinbox (10 Hz steps); reset with XIT 0.

Both are independent per slice.

Locking a slice

To prevent accidental retuning, click the 🔓 button in the RX Controls applet. The icon changes to 🔒 and the slice ignores frequency changes until unlocked.

AF gain and pan

Adjust the AF gain slider (0–100) to set the slice audio output volume. Use the L / R pan slider (0–100) to position the slice audio in the stereo field: 0 is full left, 50 is centre, 100 is full right. Double-click the pan slider to reset to centre. The pan slider now shows a text indicator: "C" for centre, "L{n}" for left offset, or "R{n}" for right offset.

Squelch

Enable the squelch by clicking the SQL button, then adjust the Squelch level slider (0–100) to set the threshold. The squelch only takes effect when SQL is toggled on.

Squelch is automatically disabled in RTTY and digital modes (DIGU, DIGL) where squelch would notch out FSK characters and break decoding.

The Manual squelch threshold is persisted client-side across sessions. When auto-squelch mode is active, the radio may overwrite the slice's squelch level with algorithm-suggested values, so AetherSDR remembers your last manual preference and restores it.

AGC

Select the AGC mode from the AGC mode combo box: Off, Slow, Med, or Fast. The AGC threshold slider adjusts the AGC threshold level. When AGC mode is Off, the slider sets the off-level instead. The mode combo is hidden in FM family modes (FM, NFM, DFM).

AGC-T noise calibration

Right-click the AGC threshold slider to open a context menu, then select Calibrate AGC-T against noise floor… to launch the AGC-T noise calibration panel for the current slice. The calibration panel uses the noise floor measurement to compute an optimal threshold. The tooltip on the slider advertises this feature.

FM repeater duplex

When operating in FM, NFM, or DFM mode, the FM duplex controls appear:

• Tone mode (FM) — Select "CTCSS TX" to enable CTCSS tone transmission.
• CTCSS tone value — Select the CTCSS tone frequency from 41 standard EIA/TIA-603 tones (67.0 Hz to 254.1 Hz). Only enabled when Tone mode is set to CTCSS TX.
• Offset (FM) — Set the repeater offset frequency (0.0–100.0 MHz in 0.1 MHz steps).
• − (offset down) — Click to set TX frequency below RX.
• Simplex — Click to set TX frequency equal to RX (default).
• + (offset up) — Click to set TX frequency above RX.
• REV — Click to invert the TX offset sign for a reversed repeater pair.

Antenna selection

RX antenna

Click the ANT1 (RX antenna) button to open a menu listing available receive antennas. Selecting an antenna calls setRxAntenna() on the slice. The menu is populated from the slice's rxAntennaList() when available, otherwise from the panadapter's antenna list. Each menu item carries the antenna token as its data value and shows a display label with tooltip and status tip.

TX antenna

Click the ANT1 (TX antenna) button to open a menu listing TX-capable antennas. RX-only antenna ports (prefix "RX") are filtered out. Selecting an antenna calls setTxAntenna() on the slice. Each menu item carries the antenna token as its data value and shows a display label with tooltip and status tip.

Filter width presets

Click a Filter width presets button to apply a preset filter width. Right-click a preset button to save the current filter width as a preset. Presets are per-mode and hidden for FM/NFM/DFM modes.

The Filter width label indicator displays the current filter bandwidth (e.g., "2.7K", "3.3K", "500", "6.0K"). The filter width readout is shared with the VFO panel for consistent display, using mode-aware logic so SSB/digital modes display the correct labelled width.

Use the Filter passband widget to drag the low and high edges and adjust the filter passband manually.

Step filter width

Use the Widen and Narrow commands to step through the per-mode filter preset list. Each press moves to the next wider or narrower preset in the list. The command walks the per-mode preset list so it always produces mode-correct passband edges.

Mute

Click the 🔊 / 🔇 button to mute or unmute the slice audio output. Single-click mutes/unmutes the current slice. Double-click mutes/unmutes all owned slices at once. The mute button is not checkable — the icon updates only when the radio acknowledges the mute state change via the slice model, ensuring the displayed state always matches the radio.

Per the Radio-Authoritative Settings Policy (#2489), mute state is NOT saved or restored on reconnect — the radio is the source of truth for audio mute.

QSK indicator

The QSK indicator lights amber when CW break-in (QSK) is active. This is read-only and controlled via the CW applet Breakin button.

Frequency entry

The Frequency edit field now uses FreqLineEdit (a FrequencyEntryParser-derived widget) with hint text "MHz". Entering a frequency above 54.0 MHz without explicit MHz notation (e.g., "144600000" for 144.6 MHz) is treated as a VHF/UHF band entry and auto-scaled accordingly. Explicit MHz notation over 54.0 MHz (e.g., "144.600") grants access to frequencies up to 50000.0 MHz without requiring an XVTR antenna.

WFM software demodulator

The WFM toggle button enables a software FM demodulator for the current slice. When enabled, the button lights green. The demodulator processes wideband FM signals received via DAX IQ through the Hi-Fi Cable. Toggle the button on to activate the WFM overlay, or off to deactivate it.

The WFM overlay is automatically torn down when you change the slice mode via the Mode combo — selecting any real radio mode (USB, LSB, CW, etc.) deactivates WFM on that slice. The WFM button state is synchronised with the radio: when another part of the application activates or deactivates WFM on the same slice, the button updates accordingly.

Slider text indicators

Both the AF gain and pan sliders now display live text readouts: - AF gain: Shows "X%" (e.g., "70%") - Pan: Shows "C" for centre, "L{n}" for left offset, or "R{n}" for right offset (e.g., "L20", "R15")

SWR sweep overlay

V0.9.4 adds a SWR sweep overlay that draws SWR versus frequency data directly on the panadapter spectrum. When a sweep is active, each data point maps its frequency (in MHz) to a horizontal position on the spectrum and plots the corresponding SWR value as a line overlay. The overlay is drawn on both the GPU-accelerated and software-rendered painting paths.

The overlay has three states:

State	Description	Notes
No data	Overlay is not drawn. Call clearSwrSweepPoints() to return to this state.
Sweep in progress	Overlay is drawn and a cursor marks the current sweep frequency. Set running = true and supply currentFreqMhz when calling setSwrSweepPoints().
Sweep complete	Overlay is drawn without a cursor marker. Set running = false when calling setSwrSweepPoints().

An optional source label (for example, the name of the antenna tuner or analyser providing the data) can be passed via the sourceLabel parameter and is displayed on the overlay.

To update the overlay, call setSwrSweepPoints() with a vector of SwrSweepPoint values. Each point carries:

• freqMhz — frequency of the measurement, in MHz (default 0.0).
• swr — SWR value at that frequency (default 1.0).

Points with non-finite freqMhz or swr values are silently skipped. Points whose mapped x-coordinate falls outside the visible spectrum area are not drawn.

To remove the overlay, call clearSwrSweepPoints().

Tips

• The Frequency label displays the VFO frequency with dotted grouping (for example, 14.225.000). Click it to enter edit mode and type a frequency in MHz, then press Enter to tune and re-center the panadapter. The frequency editor supports up to 450 MHz when the slice is on an XVTR antenna, and up to 50000.0 MHz when an explicit MHz entry above 54.0 MHz is entered.
• The STEP spinbox controls how far the VFO moves per scroll-wheel click or per press of the < / > buttons. Step sizes are per-mode — for example, SSB steps are 1, 10, 50, 100, 500, 1000, 2000, or 3000 Hz; CW steps are 1, 5, 10, 50, 100, 200, or 400 Hz.
• The default step size is 100 Hz (index 2 in the per-mode list).
• Pressing Escape in the frequency edit field cancels the entry, restores the previous frequency, and dismisses the editor.
• The AF gain slider default is 70. The L / R pan slider default is 50 (centre).
• The Squelch level default is 20. The manual squelch level is remembered across sessions.
• The AGC threshold default is 65.
• Right-click the AGC threshold slider to calibrate AGC-T against the noise floor.

Related

• RX Controls overview
• [Switch between multiple slices using the A..H tab row](../../features/rx/switch-between-multiple-slices-using