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.
- 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.
RTL-SDR Settings
All of the following options can be adjusted while the Analyzer is already running.
Tune Frequency
This field sets the center frequency (in Hz) to which the RTL-SDR 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 RTL-SDR.
Info
The app limits the frequency values based on the tuner chip specification inside the connected RTL-SDR. If this is undesired, consider the option Allow Out-of-Bound Frequency in the Settings Tab.
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.
Gain
Choosing a higher gain will result in a stronger signal but also increases the noise.
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.
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. On modern hardware, selecting the highest available sample rate manually results in the best experience.
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.
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).
HackRF Settings
When HACKRF is selected as the signal source, a range of hardware-specific settings becomes available. These settings allow you to control the tuning frequency, gain stages, amplifiers, and more.
Tune Frequency
This field sets the center frequency (in Hz) to which the HackRF 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 HackRF.
Gain Controls
These sliders allow you to optimize the signal level by adjusting different gain stages of the HackRF.
Note that excessive gain may cause distortion or clipping in case of very strong signals.
VGA Gain
Controls the Variable Gain Amplifier (VGA). Useful for fine-tuning the baseband signal level before digitization.
LNA Gain
Adjusts the Low Noise Amplifier gain at the Intermediate Frequency (IF) stage.
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.
Antenna Port Power
Enables a 3.3V power supply (max. 50mA) on the antenna port. This is useful for powering active antennas or external devices requiring a bias voltage.
Warning
Not all devices or antennas are compatible. Use caution to avoid damaging external equipment.
Sample Rate
When Automatic Sample Rate is disabled, the dropdown lets you choose the fixed sample rate manually. Valid sample rates depend on the capabilities of the HackRF hardware.
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.
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.
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 HackRF and RTL-SDR devices use different file formats:
- RTLSDR: interleaved, 8-bit, unsigned IQ samples
- HackRF: interleaved, 8-bit, signed IQ samples
In both 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.