Portsdown 2020 Getting Started Guide

The Portsdown 2020 build is primarily intended to be used with the LimeSDR Mini loaded with the custom LimeDVB Firmware. If you do not intend to use this new firmware it is not, at this stage (Feb 2020), worth switching to the Portsdown 2020 build - the more mature 2019 build could be better.

Background

The Raspberry Pi 3 has to work quite hard to drive a LimeSDR to transmit DVB-S and DVB-S2. In response to this, custom firmware has been developed for the LimeSDR Mini (only) that enables it to take on some of the processing load of generating QPSK and 8PSK modulation.

This "LimeDVB" firmware must be loaded over an existing firmware load, and must be driven with different software on the Portsdown. Because it works differently, the power output is lower than the conventional software, but a cleaner signal is generated. Initial tests indicate that the normal receive capabilities of the LimeSDR are retained with the LimeDVB firmware loaded.

Setting up to use LimeDVB

1. First of all, you need to set up the Portsdown to generate the signals to drive the LimeSDR Mini with the custom Lime DVB firmware. Connect your LimeSDR Mini to your Portsdown Raspberry Pi (preferably through a powered USB hub) and power on. Select LimeDVB as the output mode for the Portsdown: On Menu 1 (the main Menu) on the touchscreen, select "Output to" and then select "Lime DVB" (which should not be greyed out). Confirm that Lime DVB is shown on the "Output to" button on Menu 1.
2. Next check the firmware on the LimeSDR Mini. Select Menu 3 (M3), then "Lime Config". Then select "Lime FW Info". Towards the bottom of the displayed list, you will see Gateware version 1, and Gateware revision 26, 27, 28 ,29 or 30.
3. If the revision number is not 30 (so not Firmware 1.30) you need to update your LimeSDR Mini firmware to 1.30. Select "Update to FW 1.30". You should see a message "Upgrading Lime Firmware to 1.30" and then after 20 seconds or so "Firmware Upgrade Successful. Now at Gateware 1.30 Touch Screen to Continue". Touch the screen.
4. If your firmware was at 1.30, or you have successfully upgraded, you then need to load the custom LimeDVB firmware. Select "Update to DVB FW". You should see a message "Upgrading Lime Firmware to Custom DVB" and then after 20 seconds or so "Firmware Upgrade Complete DVB Touch Screen to Continue". Touch the screen.

You can now set the LimeSDR Mini to only calibrate if it needs to, by pressing the "Calibrate Every TX" button. This will cycle through "Never Calibrate" (not recommended) "Calibrate if Needed" (recommended) and "Calibrate Every TX" (should be used if you connect a different LimeSDR Mini). Leave it set to "Calibrate if Needed" and it will only calibrate on major frequency changes.

Now press exit to get back to the main menu

Setting up for Transmission

From the main menu you can set the Modulation (S2QPSK recommended), Encding (H264 recommended), Format (4:3 recommended), Source (Pi Cam, Test Card or C920 Webcam recommended), Frequency, Symbol Rate and FEC.

You should set the Lime Gain once you can check the output level is correct for your transmitter. At Lime Gain 100, the Output levels are about 4dB lower than indicated on this chart LimeSDR_Mini_Output_Power_Levels. At Lime Gain 80, the reduction is about 20 dB, but the steps are very non-linear.

PTT Swtching

The PTT line on the Raspberry Pi GPIO pin 40 goes high after 7 seconds, giving time for the LimeSDR Mini to calibrate if required before the PA is keyed up. GPIO 8 on the LimeSDR Mini also goes high when the calibration (or initialisation if not calibrating) is complete.