Portsdown 4 Major Update

[radiogareth]

CPC have them and you get free delivery too...
https://cpc.farnell.com/raspberry-pi/ra ... 20displays

*** EDIT**** well I'm sure they had them last night......now showing as late July....

Gareth

[G8DGR]

Hi
Got one from Rapid Electronics .
Delivery to-morrow.
Pi 4 and pluto I have and a Hammond 1550J (Black) is coming.
Also located the Mech Drawing for the display to do the cutout in the box.
Dave said the Micro SD card is underway.
So I'm all set I think
Rod G8DGR

[G8DGR]

Ok it all arrived and works
Only one problem - The Pi cant detect the Pluto.
Pluto works on a PC with DATVEasy
So clearly I do not have the Pluto set up to suit the Portsdown 4
I need to know the correct Pluto firmware version and config.txt parameters to get the Pi to see the Pluto

Rod G8DGR

[g0mjw]

See the Wiki https://wiki.batc.org.uk/Portsdown_4_Pluto - the firmware version required is listed there.

[radiogareth]

What PSU are you using and can you measure the voltage accurately on the RPI. If its much over 5.25V you will find the Pluto can't be found. Many users add a diode in the +VE supply cable to the Pluto (cut open the USB cable and insert a suitable low volts drop diode in line, usually the RED. The Portsdown should report the software version found in the Pluto, even if its the wrong one. In the short term you could try turning the RPI volts down to 5, but it gets very crotchety at that point.
With that mod to the pluto usb lead and the PD4 on 5.25 volts, I have very few problems.
Good luck...
Gareth

[G8GKQ]

Hi
The BATC shop says out of stock for the Portsdown 4 Micro SD card
When will these be available please?
Rod G8DGR

Following a correction to the website, the spurious "Out-of-stock" message should no longer appear. Portsdown 4 (and 2020) SD cards continue to be available from the BATC shop.

Dave, G8GKQ

[g4saq]

I have a query about setting up the PD4 noise figure meter. I am using a Pluto and an ‘Rx Gen’ noise source from Kevin, G3AAF. I am using the BATC switching PSU for the noise source (driven from pin 26 on the Pi4.)
The Rx Gen is specified as having a ‘flat’ ENR of 10dB (so, frequency independent). The Pluto is working fine in both PD4 and Langstone modes.
I followed the Wiki instructions, last updated 27/4/24.
I set the NF meter ENR to 10db, centre frequency to 1296.2 and bandwidth to 2MHz.
I connected the Rx Gen (powered up) to the Pluto Rx port.
Iintitially I adjusted the gain setting to -45.
The message “Increase Pluto gain.” appeared.
I could not get rid of “Increase Pluto gain” until I set the gain to 100%, whereupon the message disappeared. At this point the screen displayed “Noise on -44.8; Noise off -51.”.
I got the same results on 70 cms and 2m.
Having been through this process, the NF meter seems to be working fine but I have no way of evaluating its accuracy.
Should I be concerned that I am having to set the gain to 100%? Is this likely to adversely affect my measurements?

Many thanks to G8GKQ for a great piece of test gear!
73,
Russ, G4SAQ

[g0mjw]

How about using a device with a known noise figure for comparison? E.g. a 3dB attenuator.

Mike

Edit - might be worth explaining the noise figure of an attenuator is to a first approximation, equal to its loss. That is assuming a matched circuit and that match does not change between on and off. We also need there to be sufficient gain the in Pluto, which there should be but you might need some more if it has gone a little deaf like one of mine has.

Ideally, with a perfect amplifier, you should see a 10dB increase with a 10 dB noise head connected directly. You won't, because the amplifier and the Pluto have a noise figure of their own. This compensated for in the calibration where the noise figure of the Pluto needs to be taken out of the equation. If the noise figure of the pluto is high and the gain of the amplifier is low, errors are likely to be worse. So setting the Pluto to 100%, minimizing its noise figure, is a good idea as long as it is not saturating.

You might want to read this as it explains well how the Y factor method works.

https://scdn.rohde-schwarz.com/ur/pws/d ... Figure.pdf

𝐹(𝑑𝐵) = 𝐸𝑁𝑅(𝑑𝐵) − 10 𝑙𝑜𝑔(𝑌 − 1)

Your 6.2dB difference corresponds to a Y factor of 4.2 from 10dB ENR head, assuming that's at room temperature. Plugging in those gives a noise figure for the Pluto of 5dB. That seems better than I would have expected but maybe it is right. If you now add a 3dB attenuator, the difference should be only 3.2dB. That's a sensitive test as small errors are going to make a big difference.

What tends to happen is the impedance of the noise head changes between noise on and noise off. This can upset some preamps, especially if their input match isn't very good. A 10dB ENR noise head is probably a much larger noise temperature diode, say 30dB with a 20dB attenuator in front of it to minimise this effect.

There are many other things that can influence the errors. A pre-amp powered by a switching supply for example, or a nearby mobile base station, or spurious responses in the Pluto. Fortunately, as long as the measurement isn't too disappointing, all you normally want to do is minimize it.

[g4saq]

Many thanks Mike. I shall read that a few more times tomorrow and try the tests you recommend. Will report back.
73
Russ.

[g4saq]

Mike - reporting back:
I tried a 3dB attenuator, as you suggested. Off figure unchanged at -50.8. On figure now -46.5.
Device NF 2.6dB Device gain -3dB

I then tried a different Pluto, having firstly flashed it to PD4.
Like the first Pluto, this also required the gain to be set to 100% in order to eliminate the "Increase Pluto gain" message.
With a 3dB attenuator the off was 43.4 and the on was 49.5. The display showed Device NF 2.8dB and Device gain -2.9dB.

I conclude that it is likely all Plutos will need the gain set to 100% in the PD4 NF meter.
The experience of other users would be interesting.

Thanks for your advice.
Russ, G4SAQ