Portsdown Band Viewer

Introduction

The "Band Viewer" software is included with the Portsdown 4 and Portsdown 2020 and can be used to provide a basic spectrum display of band occupancy using the receive section of a LimeSDR, Pluto (Portsdown 4 only), Airspy R2 or RTL-SDR. It only works with the specified SDRs and the 7 inch screen (or the DSI-connected 5 inch screen), not the GPIO-connected 3.5 inch screen.

To start the software, connect the SDR by USB select Menu 2, Band Viewer. The capabilities of each SDR are as follows:

SDR Characteristics

SDR	Frequency Coverage	Span Widths	Notes
LimeSDR	30 MHz - 3.5 GHz	500 kHz, 1, 2, 5, 10, 20 MHz	LimeSDR Mini, LimeSDR USB or LimeNET Micro
Pluto	70 MHz - 6 GHz	2, 5, 10, 20, 50 MHz	Portsdown 4 only
Airspy R2	24 MHz - 1.7 GHz	1, 2, 5, 10 MHz	Only Airspy R2 supported
RTL-SDR	model-dependant	1, 2 MHz

Starting the Software

BandViewer can be selected from Menu 2, BandViewer. The BandViewer for specific SDRs can also be selected from Menu 2, Test Equipment. On selection of the generic BandViewer button, the software checks for the presence of the SDRs in the order: Lime, Airspy, Pluto, RTL-SDR. It uses the first SDR that it finds, or displays an error message if it cannot find an SDR.

BandViewer can also be selected from the Terrestrial (not QO-100) Receiver menu. The BandViewer is automatically set to the selected receive frequency.

To Exit the software back to Portsdown, press the "Exit to Portsdown" button twice.

Frequency

The centre frequency can be directly entered from the keyboard (selected from the Settings menu) and a number of programmable presets are available. The centre frequency can be shifted down or up by 10% of the span width by using the left and right buttons on the main menu. The presets can be configured fronm the Mode, Set Config, Set Freq Presets Menu. Stored frequency and presets are specific to each SDR type.

Level Display

Although the display is calibrated over 80 dB of dynamic range, only about 50 dB is usable. The Lime gain can be adjusted from the settings menu to optimise the display. Careful screening and grounding of the LimeSDR can produce a reduction in the noise level. There are a number of spurious signals that are visible in the passband; this is a limitation of using the LimeSDR.

Measurements and Recording

A measurement marker is available to track peaks in the spectrum. There is also a "Peak Hold" function. The display can also be frozen and/or captured to a .jpg file which is saved in the /home/pi/snaps folder. These can be viewed using the Snap Viewer selected from the System Menu, or transferred to another computer using a network connection.

Resolution

The software uses a Fast Fourier Transform with a Bin Size of 1/500th of the display width. So, for example, on the 500 kHz span width the resolution is 1 kHz.

Known Bugs and Limitations

Some display corruption of the button text occasionally occurs during marker operation. This seems to be limited to the areas of text around the display and is not critical.

On the Portsdown 2020, the first start of the Band Viewer after each boot is delayed to prevent a conflict with other screen-writing activity. Subsequent starts are unaffected.

If the 20 MHz span does not work on your LimeSDR Mini, it is a symptom of low supply voltage at the LimeSDR. Measure the voltage at the LimeSDR Mini edge terminals - it needs to be at least 4.9 volts. To cure such problems, make sure that your Raspberry Pi is supplied with exactly 5.2 volts and use a good quality (not necessarily expensive - just low resistance) USB cable.

Exit

To exit back to the Portsdown, select the System Menu. The Viewer can also be restarted from here.

Acknowledgements

Thanks to Phil M0DNY for the LimeSDR fft code, and to Michel HB9DUG for encouraging the development.