The Pluto SDR is technically capable of directly processing and generating the frequencies of 1130MHz and 2400MHz respectively that occur when receiving and transmitting via the QO100. However, the reference clock of Pluto (unmodified 40MHz) is not stable enough for narrowband operation, nor is the LNB of the receiving antenna. Usually, the TCXO's have to be replaced by better types or OCXO's or external clock sources have to be used.

The transmitting power of Pluto at 2400MHz is hardly more than +0dBm.  For a sufficiently strong signal on the transponder with an SNR > 20dB, an amplification to approx. +37dBm is required.

For the third build of my QO-100 TRX, I took an approach that reduces the number of system components.
The previous setup is  described here.

On the receiving side, the satellite downlink (10,489.5MHz) is mixed down to 1129.5MHz with a modified LNB (24MHZ ext. clock). This signal is routed directly to the Pluto SDR by the DownLink Unit.

The DownLink Unit also generates the reference frequencies for the LNB and the Pluto-SDR (both 24MHz). The 5V for the Pluto SDR as well as the PTT switched 5V for the driver stage are also provided by this unit.

On the transmit side, the Pluto SDR generates the uplink frequencies from 2400MHz directly. They are amplified by a driver stage according  to DB4UM and fed to the 13cm PA from SG-Labs. The output power is about 37dBm.

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The entire system is compactly housed in two boxes. This makes the system easy to transport and can be used for demonstrations, field days, etc. Since the PA is operated with 24V, there is no longer an external 12V connection.

The connections of both boxes are reduced to a minimum.

Pluto SDR / Downlink

The downlink block contains the Adalm-Pluto and the DownLink Unit.

On the left front of the box is the driver PA. It amplifies the TX signal to approx. +27dBm. The Pluto SDR can thus be operated at less than +0dBm output with a very clean output signal.  The driver is built according to the design of DB4UM .

I have made the following modifications to Adalm Pluto:

1.     The original TCXO has been removed and replaced with an external clock feed. Here, the 24MHz reference signal is supplied from the DownLink Unit.

2.     I connected a PTT circuit to the GPIO header of Pluto. The suggestion for this comes from F5OEO. However, the realization is different, as an opto-coupler seemed indispensable to me with respect to the PTT relay operated with 12V.
Attention – the High level at the GPIO is only 1.3V. Not all couplers are suitable here. I'm using a TLP 127 from Toshiba.

To be able to use this option, I run the Pluto SDR with the DATV firmware from F5OEO.

The main purpose of the DownLink Unit is to provide the supply voltages for the Pluto-SDR (5V), the Driver-PA (5V, PTT switched) and the LNB (12V, bias-T function).

In addition, the DownLink Unit generates the 24MHz clock for the LNB as well as the reference clock for Pluto-SDR. In order to keep the effort of generating clocks from an OCXO as low as possible, the Pluto SDR is also supplied with 24MHz. The Pluto-SDR has been configured accordingly.

The Schematic and BOM of the DownLink Unit are shown here.

The OCXO is a DOCSC022F-024M from CONNOR WINFIELD. This type is readily available (DIGIKEY) and comparatively inexpensive. The specification is completely sufficient for the intended use (20ppb, -40 to +85°C).

                        Schematic and layout designed with Diptrace V4.

In addition to the 13cm Power Amplifier from SG-Labs, the uplink block contains  the 24V/50W power supply and the 24V/12V voltage converter to supply the DownLink Box.

As you can see in the pictures, I mounted the PA on a solid 20mm ALU block for heat dissipation.

The antenna for RX and TX is an 85cm offset dish.

According to the description of DJ0ABR, I modified the LNB (Megasat Diavolo Twin)  to an external clock feed, i.e. one F-socket of the Twin LNB supplies the reception signal of the NB transponder. The 24MHz clock is fed into the second F-socket by the DownLink-Unit.

In front of the LNB a helix antenna with 4.5 turns is mounted.

The reflector is a 1.5mm aluminium plate, cut to a size of 120x120mm.

The Helix is wound from 4qmm copper wire. Compared to earlier setups made of 2.5qmm wire, this construction is much more stable. However, the attenuation of the reception signal is also slightly higher. But this is not a problem with the levels in the satellite downlink.

For dimensioning the helix I used the following tool:

http://jcoppens.com/ant/helix/calc.en.php

The acrylic glass plate behind the reflector has a U-shaped cutout. It is inserted between the feed holder and the feed-horn of the LNB and screwed to the reflector. Due to the contact pressure of the Helix reflector on the LNB, the construction is firmly in place. There is no need for any mechanical intervention on the feed arm or the LNB.

The weather protection of the Helix is made of 1mm PET foil.

                                Helix Attachment

   Antenna mounted on the carport.

The signal on the NB transponder is sufficiently strong despite the increased attenuation.

The screenshot also shows the very clean reception spectrum due to the type of frequency processing.

The UP- and downlink boxes are installed in the carport directly under the antenna. The connection to the Shack is only via LAN.

Last Update:  8.02.2024   DB8BH

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