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), SDRPlay RSPdx (Portsdown 4 only), Airspy R2 or RTL-SDR. It only works effectively with the specified SDRs and the 7 inch screen (or the DSI-connected 5 inch screen). It has a limited capability with the Waveshare 3.5 inch screen on the Portsdown 2020, but only with the RTL-SDR. See here for details.

Note - references to the SDRPlay RSPdx are in advance of the capability being released. I'm trying to get the documentation up to date first!!

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
Pluto 5th	6 GHz - 30 GHz	2, 5, 10, 20, 50 MHz	Much Reduced Sensitivity
RSPdx	1 kHz - 2 GHz	100, 200, 500 kHz, 1, 2, 5, 10 MHz	Portsdown 4 only
Airspy R2	24 MHz - 1.7 GHz	1, 2, 5, 10 MHz	Only Airspy R2 supported
RTL-SDR	model-dependent	500 kHz, 1, 2 MHz

Pluto Set-up

BandViewer has only been tested with USB-connected Plutos. If you have changed the IP Address of your USB-connected Pluto, you also need to set it in the Portsdown Menu 3, Pluto Config, Set Pluto IP for Portsdown.

Starting the Software

BandViewer can be selected from Portsdown 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: Airspy, SDRplay, LimeSDR, 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. In this case, 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 from the Mode, Set Config, Set Freq Presets Menu. Stored frequency and presets are specific to each SDR type.

The Pluto 5th Harmonic mode extends BandViewer coverage into the range 6 GHz - 30 GHz. This mode has much reduced sensitivity and a lot of spurious responses, but can be useful for checking the spectrum of microwave receiver LOs and microwave transmitter outputs. The mode is automatically selected when a frequency above 6 GHz is set as the centre frequency.

Gain Control and Accuracy of Level Display

Although the display is calibrated over 80 dB of dynamic range, it not all is usable - particularly with the RTL-SDR. The SDR gain can be adjusted from the settings menu to optimise the display. Careful screening and grounding of the SDR can reduce the noise level and spurii, although not all can be eliminated.

The relative levels displayed at a single frequency are accurate within about 1dB. However, the absolute level is uncalibrated, and there is some variation in level calibration at the outer edges of the scan.

From the Mode menu, it is possible to reconfigure the display to show only 20dB of range. This is ideal for fine alignment of systems.

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.

Web Viewing

The displayed spectrum can be viewed from a browser on the same network as the BandViewer. Simply type in the Portsdown's IP address and select "Touchscreen" and you should see the spectrum, which is updated once a second.

For this to work, "Web Control" needs to be enabled in th Portsdown software (Menu 2, System Config, Web Control Enable/Disable). Note that, unlike the Portsdown and Signal Generator software, web control of the Band Viewer is not possible - it is display only. Settings need to be adjusted using the touchscreen. Exceptionally, web control is available on the SDRPplay BandViewer - see below.

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

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.

The software sometimes crashes when the span width is changed. Simply restart the BandViewer and it should restart with the new Span Width.

SDRplay RSPdx Capability - Portsdown 4 Only

The SDRplay RSPdx is a complex SDR with good performance. It needs special drivers, which sometimes take a while (up to 90 seconds) to be loaded when selected. It should be connected to one of the USB2 ports on the Raspberry Pi 4. The USB3 ports do not work for the SDRplay.

There are gain settings for both the IF Gain and the RF Gain. A good starting point for BandViewer is an RF Gain of 0 dB, and an IF gain of -10 dB. The AGC facility does work, but is not recommended with BandViewer.

On the "Waterfall Config" menu (more about the waterfall below), the desired Antenna input for the RSPdx can be selected. Enter 0 for input A, 1, for input B and 2 for imput C. The 4.7v Bias T LNA supply on input B can also be switched on an off from this menu.

Although the SDRplay RSPdx BandViewer might work with other SDRs from the SDRplay family (such as the RSP 1a or RSP 2), it has not been tested with them. The gain settings may not be optimum, and the input and LNA switching is unlikely to work.

Unlike other versions of BandViewer, the "Set Frequency Presets" menu is accessed from the "System" menu.

Web view is available for the SDRplay BandViewer, and also web control. Note that web view only updates the screen once per second so spectrums and waterfalls will appear to be jerky.

Waterfall Capability

The SDRplay BandViewer includes a waterfall capability. This is selected from the Mode Menu. There are 2 waterfall modes: mix mode, where the top quarter of the screen displays a spectrum and the lower three quarters displays a waterfall, and Waterfall mode, where the waterfall fills the screen.

The waterfall is configured from the "Waterfall Config" menu. "Set Waterfall Base Level" sets the signal strength below which the waterfall will paint black. "Set Waterfall Range" adds to the base level to set the signal strength above which the waterfall will paint white. These settings should be adjusted so the the base level is just below the noise floor, and the range includes the strongest signals. Selecting "Mode" and "Classic SA Display" will show a small strip of the waterfall spectrum to the right of the normal bandviewer display to enable these levels to be set properly. Good starting values for these are a base level of -70 and a range of 60. For clearer display of weaker signals, the range should be reduced.

The other waterfall setting is the Waterfall Time Span. This sets the time taken for any event to move from the top of the screen to the bottom of the screen (in seconds). A good starting setting is 10 seconds. The waterfall moving speed is the same in Mix mode, so the time from top to bottom is reduced by 25% as the available screen is shorter. The minimum time is about 3 seconds - this is limited by computing speed.

At slow waterfall speeds, the peak level of all events that occur between the writing of each new line on the waterfall is displayed. This is why the background brightness appears to increase at low waterfall speeds, because more noise peaks are being captured.

It is hoped to add the waterfall capability to the BandViewer for other SDRs when time permits.

Portsdown 2020 with Waveshare 3.5 inch Screen and the RTL-SDR

The BandViewer software was written to only support the 7 inch screen with an 800x480 picture size. The Waveshare 3.5 inch screen has a framebuffer size of 720x576, so there is no room for the control buttons on the right hand side. However, the RTL-SDR BandViewer can be run in fixed-frequency, fixed span width and fixed gain mode by using the following procedure:

• Connect an RTL-SDR and start Portsdown
• Select RX. If you have no MiniTiouner connected, touch the screen to continue - it does not matter.
• On the receiver menu, Make sure that you have "Terrestrial", not "QO-100" selected in the top left button. Touch the button to change it if required.
• Select the centre frequency that you want to tune BandViewer to. If it is not on one of the presets, select the button labelled keyboard and you can enter the frequency.
• Select the bottom right button "BandViewer on RX Freq". If the button is grey, not blue, it means that you do not have an RTL-SDR connected.
• After a pause, BandViewer should start on your selected centre frequency, with a 2 MHz span width.
• You cannot adjust the settings.
• To return to Portsdown (and possibly select a new centre frequency) simply touch the screen.
• Having set the correct centre fequency, the BandViewer can be reselected directly from Portsdown Menu 2.

Advanced users can adjust the settings by editing the file /home/pi/rpidatv/src/rtlsdrview/rtlsdrview_config.txt before starting BandViewer, but this needs to be done carefully to avoid illegal settings.

Note that this Waveshare screen capability is only for the RTL-SDR, not the LimeSDR or Airspy R2.

Exit

To Exit the software back to Portsdown, press the "Exit to Portsdown" button twice. To restart BandViewer (if it freezes), use the Restart button on the System Menu.

Acknowledgements

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