Spectrum Analyzer

Spectrum Analyzer

A swept spectrum analyzer that works like a bench instrument. It runs in landscape and covers anything from a 10 kHz span up to the whole 1 MHz – 6 GHz range. It has three traces, four markers, a waterfall, and controls modelled on the softkey menus of a benchtop analyzer.

Location: Receive → Spectrum (external app, green icon)

Note

Level readings are relative, not calibrated. The app converts dBFS into an input-referred level by subtracting the LNA, VGA and amp gain. To read absolute dBm, measure a known source and set AMPT → Offst (reference offset) so the reading matches.

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Holding the device

The app turns the screen to landscape. By default, hold the PortaPack turned 90° anticlockwise from its normal position. The D-pad turns with the screen, so "left" is always left on the screen as you see it.

If you'd rather hold it the other way, use DISP → Flip to rotate the picture 180°. The keys and touch input follow.

When you leave the app, the screen goes back to portrait.

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Screen layout

┌──────────────────────────────────────────────────────────────┐
│ Ref -40.0dB 10dB/ LNA24 VGA20 AMP-               CONT 12.3ms │  status line
│ M1 433.92MHz -52.31dB  D2 +25kHz -18.40dB                    │  marker readout
├──────────────────────────────────────────────────────────────┤
│                                                              │
│                spectrum graticule (10 × 10 div)              │
│                                                              │
├──────────────────────────────────────────────────────────────┤
│                         waterfall                            │  (optional)
├──────────────────────────────────────────────────────────────┤
│ CF 433.92MHz  Span 20MHz  RBW 156.2kHz  VBW 156.2kHz   1tune │  annotation
│ Center                                  433.920 000 MHz      │  active function
│ < │FREQ │SPAN │AMPT │ BW  │TRACE│ MKR │DISP │                │  softkeys
└──────────────────────────────────────────────────────────────┘

Area	Contents
Status line	Reference level, dB/div scale, LNA/VGA gain, amp on/off, reference offset (if not zero). On the right: sweep state (CONT continuous, SGL single sweep in progress, HOLD stopped) and the time one sweep takes.
Marker readout	The selected marker, plus the next one that is on. Shows Mn freq level for a normal marker, or Dn ±Δf ±ΔdB for a delta marker.
Graticule	10 vertical divisions (dB/div), 10 horizontal divisions (span/10), one display point per pixel column (320 points).
Waterfall	History of past sweeps, newest at the top. Colours run from black (bottom of screen) through blue, cyan, yellow and red to white (at the reference level).
Annotation	Center frequency, span, RBW, VBW, and how many tunes one sweep needs. SETTLE LOW appears in orange if the settle time is below the safe minimum for your hardware.
Active function	The parameter the encoder and Up/Down keys change right now: its name on the left, its value on the right.
Softkeys	< (back) followed by the current menu.

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Controls

The app uses the usual analyzer model: you pick a softkey, and it becomes the active function. You then change its value with the encoder or the arrow keys.

Input	Action
Left / Right	Move the softkey cursor (yellow highlight).
Select	Press the highlighted softkey.
Encoder	Change the active function in fine steps.
Up / Down	Change the active function in coarse steps (usually one division).
Back	Go back to the top menu; from the top menu, exit the app.
< softkey	Same as Back.

Softkey behaviour

• Menu keys (FREQ, SPAN, …) open their submenu.
• Value keys (Centr, Span, Ref, RBW, …): the first press makes the key the active function, shown with a blue background. For frequencies, a second press opens the frequency keypad. For list settings (dB/div, RBW, Window, …), a second press steps to the next value. That way you can use the app with touch alone.
• Toggle keys (Amp, Fill, Wfall, Flip, …) turn on and off right away. A green label means on.
• Action keys (Peak, Zoom+, >CF, …) run right away.

Touch

• Tap a softkey to press it.
• Tap or drag on the graticule to move the selected marker to that spot. If the marker was off, this turns it on.
• Tap the active-function line while Center, Start, Stop or Span is active to open the frequency keypad.

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Menus

FREQ

Key	Function	Fine (encoder)	Coarse (Up/Down)
Centr	Center frequency	~span/100 (rounded to 1-2-5)	span/10 (one division)
Start	Start frequency (span changes, stop stays put)	same	same
Stop	Stop frequency (span changes, start stays put)	same	same
Keys	Opens the frequency keypad for Start, Stop or Span if one of those is active; otherwise for Center

The tuning range is 0 – 7.25 GHz. How well the hardware performs at the very ends of that range depends on your device.

SPAN

Key	Function
Span	Sets the span in 1-2-5 steps (10 kHz minimum). Press again to type a value on the keypad.
Full	Sweeps 1 MHz – 6 GHz.
Zoom+	Halves the span around the center.
Zoom-	Doubles the span around the center.
Last	Returns to the previous span.

AMPT (amplitude)

Key	Function	Range / step
Ref	Reference level (top line of the graticule)	−150 … +30 dB; fine 0.1 dB, coarse one division
dB/dv	Vertical scale	1, 2, 5, 10, 20 dB/div
LNA	RF LNA gain	0 … 40 dB, 8 dB steps
VGA	Baseband VGA gain	0 … 62 dB, 2 dB steps
Amp	RF amp (+14 dB) on/off	toggle
Offst	Reference offset added to every reading (for calibration or external attenuators and preamps)	±100 dB; fine 0.1 dB, coarse 10 dB
AutoR	Sets the reference level so the highest point of trace 1 sits just under the top line (next 5 dB step at least 5 dB above the peak).	action

Changing a gain clears the traces, because old data would be at the wrong level. The displayed level already takes LNA, VGA and amp gain into account, so changing a gain moves the noise floor, not the signals.

Tip

If strong signals show spurs or a raised noise floor across the band, the front end is overloaded. Lower the LNA gain or turn the Amp off.

BW (bandwidth and sweep)

Key	Function
RBW	Resolution bandwidth. Auto picks the smallest FFT whose bins are no wider than one display point. You can also force an FFT size of N = 256, 512, 1024 or 2048. The readout shows the effective RBW (including the window's noise bandwidth) and N.
VBW	Video bandwidth: how many FFTs are averaged per tune (1, 2, 4, 8, 16, 32). Higher values give a smoother noise floor but a slower sweep. VBW is shown as RBW ÷ averages.
Windw	FFT window: Rectangular, Hann, Blackman-Harris (default), or Flat top. Use Flat top for accurate amplitude readings and Blackman-Harris for good dynamic range.
Detct	How several FFT bins are combined into one display point when the span is wide: Peak (default, never misses a narrow carrier), Average, Sample, or Neg peak.
Bins	Which FFT bins each tune uses. See Bins layout.
Settl	Settle time after each retune before samples are used. 0 … 5000 µs; fine 50 µs, coarse 500 µs. Shows <min in orange if set below the safe minimum.

Bins layout

• Upper sideband: uses only the bins above the LO and keeps away from the anti-alias filter edge. This keeps the DC/LO spike and the image of strong signals off the trace. It is the right choice for a standard zero-IF HackRF One, and the default there.
• Both sidebands: uses bins on both sides of the LO. Each tune covers about 2.2× more spectrum, so the sweep is much faster. Use it on hardware that already keeps DC and the image out of band (PRALINE, where it is the default). On a HackRF One it shows a spike at every tune's LO and image signals.

Settle time

The tuner needs a short time to settle after each retune. If samples are taken too early, a clean carrier shows up smeared across part of the screen.

Hardware	Default	Safe minimum
HackRF One	400 µs	350 µs
PRALINE	100 µs	0 µs

Settle time only matters when one sweep needs more than one tune (see the tunes count in the annotation line). Narrow spans that fit in a single tune never retune between sweeps.

TRACE

There are three independent traces: Tr 1 yellow, Tr 2 cyan, Tr 3 magenta. By default Tr 1 is Clear/Write and Tr 2 and Tr 3 are Blank.

Key	Function
Tr n	Selects which trace the other keys act on.
mode	Trace mode of the selected trace (the label shows the current mode).
AvgN	Number of sweeps the Average mode averages over: 2 … 128 in powers of two (default 8).
Fill	Fills the area under trace 1.
Clear	Clears the selected trace and restarts its hold/average.

Trace modes:

Mode	Label	Behaviour
Clear/Write	ClrW	Shows the latest sweep.
Max hold	MaxH	Keeps the highest level seen at each point. Good for catching bursts and hoppers.
Min hold	MinH	Keeps the lowest level seen at each point.
Average	Avg	Exponential average in dB over AvgN sweeps. It settles quickly after a clear.
View	View	Freezes the trace as it is. Handy for comparing against a live trace.
Blank	Blnk	Hidden and not updated.

Changing the mode clears the trace. View is the exception: it keeps what is already there.

MKR (markers)

There are four markers, M1 … M4, drawn as small coloured arrows on their trace. When the menu opens, the cursor is on Peak.

Key	Function
Mkr n	Makes the selected marker the active function, so the encoder moves it one point and Up/Down move it one division. Press again to select the next marker. Selecting a marker that is off runs a peak search to place it.
mode	Steps through Off → Norm → Delta. Delta reads frequency and level relative to M1. M1 has no delta mode. A marker sits on the trace that is selected when you turn it on.
Peak	Moves the selected marker to the highest point of its trace.
NxtPk	Moves to the next lower peak. A peak only counts if it rises at least 3 dB above the valley between it and the current marker, so noise is skipped.
>CF	Sets the center frequency to the marker frequency.
>Ref	Sets the reference level to the marker level.
AllOf	Turns all markers off.

DISP (display and sweep)

Key	Function
Wfall	Turns the waterfall under the spectrum on or off. With the waterfall off, the spectrum uses the full height.
DLine	Display line: a dashed horizontal reference line. Press once to turn it on and make it active, then set its level with the encoder (0.1 dB) or Up/Down (one division). Press again while it is active to turn it off.
Cont / Singl	Switches between continuous sweeping and single sweep.
Run	Restarts the sweep. In single mode, it takes one new sweep.
Flip	Rotates the picture 180° so you can hold the device the other way round.

The waterfall adds at most about 30 lines per second. When the sweep is faster than that, the sweeps in between are peak-held into one line, so short bursts still show up.

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Typical workflows

Find and measure a signal

1. Go to SPAN → Full for an overview, or FREQ → Centr and type a frequency.
2. Go to MKR → Peak to jump to the strongest signal, then >CF to center it.
3. Go to SPAN → Zoom+ a few times to look closer. RBW gets narrower on its own.
4. Go to AMPT → AutoR to bring the peak near the top line.

Catch intermittent or hopping signals

1. TRACE: set Tr 1 to MaxH.
2. Leave it running. Watch the waterfall for when the signal appears.
3. Press Clear to start over.

Compare before and after

1. TRACE: with Tr 1 in ClrW, select Tr 2, set it to ClrW, wait one sweep, then switch it to View.
2. Tr 2 now holds a reference copy, and Tr 1 keeps updating live.

Measure an offset between two signals

1. MKR: put M1 on the first signal (Peak).
2. Select Mkr2, set its mode to Delta, then move it to the second signal with NxtPk, the encoder, or a tap.
3. The readout shows D2 ±Δf ±ΔdB.

Calibrate the level readout

1. Connect a source of known level.
2. Use MKR → Peak to read its level.
3. Go to AMPT → Offst and adjust until the marker reads the true level. The offset is saved.

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Tips

• Slow sweep? Wide spans with a narrow RBW need many tunes. To speed up: use RBW Auto, lower VBW averaging, or switch Bins to Both sidebands if your hardware supports it. The status line shows the sweep time and the annotation line shows the tune count.
• Spike at center on HackRF One? That is LO feedthrough. Keep Bins on Upper sideband so it stays off the trace.
• Smeared carriers or "ghost" humps between tunes? The settle time is too short. Raise Settl until they go away.
• Settings are saved. Frequency, span, levels, gains, bandwidths, trace modes, markers and display options are kept between sessions (settings file rx_specan).