Using LimeSDR with Portsdown

Introduction

Software version 201811100 introduces experimental support for the Lime SDR Mini and the Lime SDR USB.

Powering the Lime

The Lime (both Mini and USB) has a greater power consumption than the RPi can comfortably power from its USB Port, so a powered USB Hub should be used to provide power to the Lime.

Installing the Lime Software

The software to drive the Lime SDR is not included in the normal Portsdown build; it is an additional download. To install the software, select Menu 3 and then select "Lime Config". Then select "Install Lime". The update will report on progress and ask for a screen touch when it finishes. No reboot is required. Note that the software loaded is LimeSuite 18.04 (from April 2018) as more recent builds do not currently work with the Portsdown. To enable further testing to solve this issue, the button "Break Lime" loads the latest software (currently 18.10) - which of course will not work. You can revert to the working software (or load any future Portsdown updates for the Portsdown Lime software) by selecting "Install Lime" again.

Lime Firmware and Gateware

Selecting "Lime FW Info" displays the current Firmware and Gateware versions. Portsdown works best with the versions matching LimeSuite 18.04. This is Firmware Version 5 and Gateware version/revision 1.24 for the Lime Mini and Firmware version 4 and Gateware version/revision 2.16 for the Lime USB. You should load these versions by pressing the "Update Lime FW" button. The version appropriate to the currently loaded LimeSuite version is loaded by this process.

The Lime firmware/gateware version that works with the Portsdown also works with the current version of DATV Express.

If you want to update the Lime Firmware and Gateware to the latest version for use with other computers and applications, you should upgrade the Portsdown to the latest (non functioning) version of LimeSuite (by selecting "Break Lime"), then "Update Lime FW". You can downgrade the Portsdown to a working version by selecting "Install Lime" (but you should then downgrade the firmware as well).

Transmitting

Select Lime Mini or Lime USB from the "Output to" Menu. The detected device should be "not greyed out". Om Menu 1, you will see the Lime Gain button for the appropriate band (distinct Lime Gains between 0 and 100 are stored for each band). This has yet to be calibrated, but I suggest that settings of over 90 risk distortion in the Lime final amplifier.

Selecting the modulation button will allow you to choose between DVB-S and DVB-S2 modes. Once a DVB-S2 mode is selected, the appropriate FECs are available on the FEC button. Note that different FECs are available (and not available) for different modulations.

The RPi has to work quite hard to transmit the higher modes such as 32APSK; I have noticed that it runs out of processing power while transmitting 32APSK at 2MS. This should not be an issue; full HD pictures can be transmitted (from a C920 webcam) at 32 APSK 1MS).

Limitations

After selecting transmit, the Lime self-calibrates with RF transmissions throughout the spectrum before transmitting on the desired frequency with the desired modulation. For this reason, switching on of the RPi PTT line is delayed for 7 seconds. At the end of the transmission it is dropped immediately.

Carrier mode is not yet available for the Lime; neither is a receive capability.

GPIO switching is not available because Myriad RF had not incorporated this capability in the Gateware while I was developing the Lime capability. It looks like this may now have been rectified: https://discourse.myriadrf.org/t/limesdr-mini-gpio-read-not-supported/3510.

Please report any further bugs that you find in the Development build directly to G8GKQ. After formal release, bugs should be reported on the Forum.

RF Switching for Multiple Bands

The BATC 4-band and 8-band PTT drive and RF switching boards can continue to be used with the Lime. They need to be connected to the RPi GPIO because Myriad RF had not incorporated Lime GPIO switching capability into their Gateware at the time that I was developing the Lime capability. It looks like this may now have been rectified: https://discourse.myriadrf.org/t/limesdr-mini-gpio-read-not-supported/3510.

As the maximum output of the Lime is lower than that of the Portsdown Filter-Modulator board, the gain provided by the BATC 8-way RF switch is useful to maintain current levels (5 - 10 mW) at the output. There will be some component adjustments needed to ensure the correct levels are available. Details will be published soon.

On the Lime USB the output is from "TX1 Low" below 2 GHz and from "TX1 High" above 2 GHz. The Lime Mini only has a single output.

Acknowledgements

Thanks to Evariste F5OEO, Andrew Back at Myriad RF, Charles G4GUO and Malcolm G0UHY for their help in implementing this capability.