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Source Tab

The Source Tab is where you choose and configure the signal input. Before you can analyze signals, you need to select a source type and start the analyzer.

Starting the Analyzer

The Start Button (▶️) is located to the right of the source type dropdown menu. After selecting a source, tap this button to start sampling and display the FFT and waterfall plot.

Most settings can be changed while the analyzer is running, but there are some exceptions, especially for file playback.

Selecting a Source

The dropdown menu in the Source Tab lets you choose between different signal sources:

  • RTL-SDR - Receive live radio signals using an RTL-SDR USB dongle.
  • HackRF - Works similarly to RTL-SDR but supports wider bandwidths.
  • Airspy - A high quality SDR supporting 12bit resolution and up to 10MHz bandwidth.
  • HydraSDR - A high quality SDR supporting 12bit resolution and up to 10MHz bandwidth.
  • File Source - Replay a previously recorded IQ file.

Automatic Source Detection

When you plug in a RTL-SDR or HackRF while the app is in the foreground, the correct source type is automatically set by the app.

Start/Stop Button

The play button in the upper right starts and stops the signal analyzer. When started, the app begins receiving and displaying IQ samples from the source. Once started, the analyzer keeps running even when the app is moved into the background. A notification icon is displayed in the status bar. Tap the notification to bring the RF Analyzer app back into the foreground or use the Stop Analyzer button in the notification to stop and terminate the app.

Each source type changes the available settings. Let’s go over them one by one. We start with some settings that are common to most of the source types:

Common Source Settings

Tune Frequency

This field sets the center frequency (in Hz) to which the SDR device should tune. You can input the frequency manually. The app does also automatically adjust the tune frequency when the Channel Frequency is set to a value outside of the current signal range. Scrolling outside of the current signal also causes an automatic retune of the device.

Info

The app limits the frequency values based on the tuner chip specification inside the connected SDR. For some custom RTL-SDR devices this may be undesired. In this case consider to enable the option Allow Out-of-Bound Frequency in the Settings Tab.

Sample Rate

When Automatic Sample Rate is disabled, the dropdown lets you choose the fixed sample rate manually.

By default, Automatic Sample Rate is enabled. The sample rate automatically follows the zoom level of the spectrum display. This provides an intuitive way to adjust bandwidth without needing to manage sampling directly. However, the adjusting of the sample rate does affect the signal which is received from the source which might be undesirable.

Gain Controls

Depending on the specific SDR device there are multiple gain controls available. These sliders allow you to optimize the signal level by adjusting different gain stages.

Note that excessive gain may cause distortion or clipping in case of very strong signals.

LNA Gain

Adjusts the Low Noise Amplifier gain at the RF stage. Sometimes this is also just refered to as 'Gain'.

VGA Gain

Controls the Variable Gain Amplifier (VGA). Useful for fine-tuning the baseband signal level before digitization.

Mixer Gain

Adjusts the Mixer gain at the conversion stage from radio frequency (RF) to intermediate frequency (IF).

IF Gain

This option is only available for RTL-SDR devices with a E4000 tuner chip. It allows the fine-tuning of the intermediate frequency (IF) gain.

Antenna Port Power

This is sometimes refered to as Bias Voltage, Bias Tee or RF Bias. It enables a power supply on the antenna port of the SDR. This is useful for powering active antennas or external devices requiring a bias voltage.

  • HackRF: 3.3V (max 50mA)
  • RTL-SDR Blog V4: 4.5V (max 50mA)
  • Airspy: 4.5V (max 25mA)
  • HydraSDR: 4.5V (max 25mA)

Warning

Not all devices or antennas are compatible. Use caution to avoid damaging external equipment.

Frequency Converter Offset

If you are using an upconverter or other frequency-shifting hardware, enter the offset (in Hz) here. This value (which can be negative) is added to the tuned frequency to display the actual RF frequency.

This feature helps when using hardware like an up-/downconverter that shifts the spectrum into a different frequency range.

Example: Ham-It-Up

For the Ham-It-Up converter the frequency offset setting should be set to -125000000Hz. That means that the app will display the incoming signals as if they were 125MHz lower than they actually are. Now when tuning the app to 14MHz you see the signals that are actually incoming at 139MHz (this is where the 14MHz band is found after up- converting it with the Ham-It-Up converter).

RTL-SDR Specific Settings

All of the following options can be adjusted while the Analyzer is already running.

Gain Controls

Hardware AGC

Activate the RTL-SDR's build-in automatic gain control. This is not usually recommended as it will add too much noise to the signal.

Manual Gain

Enable this switch to manually set the Gain and IF Gain, otherwise the RTL-SDR driver will automatically set these values. Refer to the Gain Controls section for more information about the specific gain stages.

External RTL-SDR Server

Connect to an RTL-SDR network server instead of a local USB device. This must be done before starting the analyzer. Input the IP and TCP port of the rtl_tcp server and start the analyzer to connect. Make sure the Server is not blocking the connection with a firewall.

Warning

Streaming IQ samples from a remote RTL-SDR onto your phone does consume a very high amount of data volume (~4MB/s)! It is highly recommended to do that only over a WiFi connection and not over a paid data plan.

Frequency Correction

This setting is passed to the RTL-SDR driver and allows to compensate for small frequency offsets due to hardware imperfections. The value has the unit PPM (parts-per-million) and can also be negative.

HackRF Specific Settings

When HACKRF is selected as the signal source, a range of hardware-specific settings becomes available.

The HackRF has two gain stages (LNA and VGA) which are explained in the Gain Controls section. Additionally, it features an amplifier:

Amplifier

This switch enables or disables the 11dB internal RF amplifier in the HackRF. This is an additional gain stage and may be helpful when receiving very weak signals, though it can introduce distortion if overused.

Airspy Specific Settings

When AIRSPY is selected as the signal source, a range of hardware-specific settings becomes available.

Advanced Gain Control

The easiest way of controlling the gain of the Airspy is by specifying the Linearity and Sensitivity gain values.

Internally, the Airspy has three gain stages (LNA, Mixer, VGA - see the Gain Controls section above). These three stages are automatically managed by the Airspy when setting the linearity and sensitivity gain values.

If you wish to specify the LNA, Mixer and VGA values explicitly, enable the Advanced Gain Control switch.

HydraSDR Specific Settings

When HYDRASDR is selected as the signal source, a range of hardware-specific settings becomes available.

Advanced Gain Control

The easiest way of controlling the gain of the HydraSDR is by specifying the Linearity and Sensitivity gain values.

Internally, the HydraSDR has three gain stages (LNA, Mixer, VGA - see the Gain Controls section above). These three stages are automatically managed by the HydraSDR when setting the linearity and sensitivity gain values.

If you wish to specify the LNA, Mixer and VGA values explicitly, enable the Advanced Gain Control switch.

RF Port

The HydraSDR has 3 ports from which RF signals can be captured:

  • AIR_IN (also called ANT or RX0): Default antenna port
  • CABLE1 (also called RX1): First internal port
  • CABLE2 (also called RX2): Second internal port

File Source Settings

If you select File Source, you can load and replay an IQ recording. This is useful for analyzing captured signals at a later time. IQ recordings can be created via the PC software tools rtl_sdr and hackrf_transfer. RF Analyzer can also record IQ files directly (see Recordings).

Once playback has started, the tune frequency, sample rate, and file format cannot be changed. Unlike live SDR sources, an IQ file has fixed characteristics and cannot be "retuned". If these values are incorrect the recording will not be correctly replayed.

Selecting a Source File

The Open File button opens a file chooser to choose an IQ file from the phone's storage. These files contain raw IQ data and no information about the file format, frequency and sample rate. Therefore, these values need to be specified after selecting the file.

Auto-Detection

If you select a recording file from the app’s internal storage, frequency, sample rate, and file format are automatically filled in from information extracted from the file name. The file name should contain the values in this format: hackrf_123000kHz_2Msps.iq. The arrangement can be different and the app recognizes the units MHz, KHz and Hz for frequency values and MSps, KSps and Sps for sample rate values.

The View Recordings button lets you browse saved recordings within the app. By tapping on a recording and pressing the play button, the recording file is automatically loaded into the File Source and started. File format, frequency and sample rate do not need to be entered manually.

Tune Frequency

This value sets the center frequency at which the recording was done.

Note that this frequency value is not important for the display and demodulation of the signals. It is only used to display the frequency grid. If unknown, you may set the frequency to 0Hz which is similar to enabling the Relative Frequency setting.

Sample Rate

This value specifies the sample rate at which the file should be replayed.

File Format

The SDR devices use different file formats:

  • RTLSDR: interleaved, 8-bit, unsigned IQ samples
  • HackRF: interleaved, 8-bit, signed IQ samples
  • AIRSPY: interleaved, 16-bit, signed IQ samples
  • HYDRASDR: interleaved, 16-bit, signed IQ samples

In all cases, the in-phase component (I) is first, followed by the quadrature component (Q).

Repeat

If this setting is enabled, the file will start playing again from the beginning once the end of the file is reached. It will loop forever until the Analyzer is manually stopped by the user.

Wrapping Up

Now that you understand Source Tab and the FFT is running, you are ready to start exploring signals! Next, we will dive deeper into FFT & Spectrum Analysis and discuss how to interpret and optimize the FFT display.