Does the Pluto suffer calibration RF spikes like the Lime SDR?

[Basil]

I have just bought a Bison three semiconductor PA, that will be on its way to me soon, and the seller warns that my Pluto, (and then an Analog Devices preamp) can cause high level out of band TX spikes as it occasionally self calibrates. These can apparently kill the PA. I have not heard of this before, and have not had an issue with my current LAN connected Pluto - AD preamp - SG Labs 20W power amp into a POTY.



How should I go about stopping this risk?



I want to have as much gear as possible in the cabinet, which has mains power available, under the dish, including the new PA and AD preamp,



At the moment a LAN cable and the LNB coax for the wide band polarisation connection to the 4 port LNB are all that need to come back to the shack. This set up allows me to run SSB using SDR Console on the narrow band transponder via the LAN to the Pluto, and one LNB port, and low power to the wide band transponder for DATV, with a Minitiouner for RX off the LNB cable into the shack, from another LNB port. I am currently using a Portsdown for TX. The Portsdown is LAN connected to the Pluto at the dish.



Adding this much more powerful PA is looking more complicated than I thought as the SG Labs PA uses VOX instead of electrical PTT. Are others troubled by this calibration spike when running higher power PA's and need to get the Pluto TX'ing before turning on the PA, presumably manually or with some sort of PTT timer arrangement?



I have built the PTT relay board to go inside the Pluto, but had no need to fit it yet as the SG Labs 20W PA uses VOX to go to TX mode. I am getting a bit confused as to what's needed to protect the new PA now, with the bigger Bison PA which has been built to have a lot of gain meaning the Analog Devices preamp should very easily drive it.



Any advice most welcome, thank you.



*EDIT*



I have now read a BATC article describing an out of band high level TX when the Lime Mini SDR is self calibrating, I wonder if the vendor is getting confused and thinks the Pluto SDR also do does this, or is the one that has this issue?



2E0ILY
​

[G8GKQ]

Hi

The Pluto does have a calibration spike, but it does not seem to be quite as high a power as the Lime.

The PTT signal from the Portsdown (GPIO pin 40) is delayed to take account of this,but I am not sure about the PTT signal from the Pluto relay, which is generated by software that I do not have visibility of, or control over.

I would run a separate cable to the PA box to enable the PA once the Portsdown GPIO (pin 40 on the RPi) goes high (that is what I do - with an extra enable switch in-line). If that is not possible, you could use a coax relay to switch the RF path from the Pluto to the PA, so that it only connected after the PTT signal went high.

Dave

[g0mjw]

Spikes are mostly but not entirely a Lime issue. However, make sure the Pluto and pre- PA amplifier can't, under any fault condition, possibly deliver a drive level that would damage the PA. Unless the Bison PA requires 20W of drive, then don't drive it from a 20W amplifier. That's just asking for trouble. I think the Bison needs about 20mW of drive for 200W out so use attenuators where necessary to make not exceeding this an absolute certainty, even when you inevitably make a mistake and get the full 5-10mW from the Pluto.

As an intermediate licensee, you are only permitted 50W of RF power output, which is sufficient and at that level the PA ought to be clean. Given that the Pluto on its own might already have enough drive, but a filtered pre-driver like the one made by G0MRF might help ensure linearity. It has about 200mW output so a 3-6 dB attenuator on its output would be a good idea.

It may have enough gain to put the Pluto in the shack. Why? It is best never to put too much gain in one box or it will oscillate. Again, don't put too much gain in one box as it will oscillate. Much better to have your Pluto indoors and a cable run to the box. Gain is cheap at low power levels on 13cm, even if the coax loses 10-20 dB it is easy to make that up. This way the Pluto low level signals are not close to the high power emitted from the feed which will inevitably find its way into your dish box, how ever hard you try. Those who don't have issues with stability are either lucky or have paid a lot of attention to avoiding them.

Mike

[G8GKQ]

Well said Mike.

Perhaps I should have added that my LimeSDR is in the shack with a small amplifier after it. I then have a 20m run of Sat Coax to the PA which is next to the dish.

Dave

[Basil]

Ok, thanks for the excellent and insightful replies. Forgive me another newbie question please, if I bring the Pluto inside, what are the pros and cons of connecting to it by LAN versus USB? Inside I would have a choice.

Do either present less of a "bottleneck" to a data stream either from my Portsdown or from say from OBS Studio direct? I get audio break up quite easily when TX 'ing but don't know enough to judge why and where.

At the moment all cables bar the armoured mains supply to the steel cabinet go overhead, I need to bury a duct to allow changes in the future, with pull through a, as I expect this set up will constantly evolve.

Thanks again.

[g0mjw]

The only advantage of Ethernet is that the Pluto does not have to be plugged in to the PC you are using. If the Pluto is next to the PC then you might as well use USB. If like mine it is in different room, or you have multiple PCs needing to access it, then use Ethernet.

Mike

[Basil]

Thanks again, it would be very good if I could keep the Pluto outside in the steel cabinet. There's not enough room in it for the Bison amp and its three in series HP server power supplies. They will have to go in a separate steel cabinet below the existing one. Would the amp and its supplies being in a separate steel cabinet with the doors to both grounded with braid to the main cavities, and to the deeply buried 4 inch OD steel pole the dish and cabinets are on be enough isolation please?

[hb9tob]

It definity does
Here is what I got when stating the pluto from sdrConsole

23202404-8f6a-4988-a4fa-5e1e0169befc (1).jpg (84.13 KiB) Viewed 1770 times