First tests

[gi7ugv]

I've just finished temporarily assembling the transmitter to test things are working before fixing and shortening cables and the likes. Not having seen or used one before I'm wondering about the results I'm observing and what I should be expecting.

I realise the below is a bit vague but any comments and suggestions for improvements or to test the setup would be greatly appreciated, this is very new to me so keep it simple please

I've done a few tests in shack and although it is working, the constellations are not as tight as I'm seeing with output from other setups and with a lower MER than I'm expecting with something not looking right at 333kS:

• 18dB at 125kS
• 14/15dB at 333kS
• 19dB at 1000kS

At 333kS, the timing lock bumps around 50-70% whereas on 125 or 1000 it sits at 100%. Additionally, ugly mode at 333kS has lock at 100% and a similar or higher MER than IQ mode despite the constellations being more spread so I assume the receive side is okay.

A few screenshots below at 333kS in case I'm doing anything that's obvious wrong:

1. IQ without LO filter: https://i.imgur.com/EZXRidW.png
2. IQ with LO filter: https://i.imgur.com/pXj9NHV.png
3. Ugly mode: https://i.imgur.com/kXsxWYm.png

Thanks!
John

[G8GKQ]

Hi John

Really pleased to see that you have pictures from your Portsdown. Well done!

The MER for 125 KS is exactly what I would expect, however, the 333 KS MER is lower than normal, as is the 1000 KS MER. I would suggest that you check the wiring from the RPi to J02 of the F-M board, which switches the filters. The board labelling is not as clear as it might be, and this has caught a few constructors out. The best thing is to check the voltages on Pins 1 and 3 of U20, U21 and U22 against the truth table on this page: https://wiki.batc.tv/images/5/5b/PdFM_V2_Sht_1.pdf. Make sure that the ground side (pin 1) is properly connected it relies on the RPi's ground connection, and is not grounded on the F-M board.

The Ugly mode constellation looks typical of what I would expect, and the difference with and without the LO filter is textbook.

Let us know how you get on

Dave

[M0DHP]

Hi John and Dave

For comparison, here are readings for my Portsdown on 437MHz, connected to MiniTiouner via 60dB attenuator and running V0.7b MiniTioune software.

Poor on 333 but better on 300 - Dave, in a previous thread, I think you said this might be due to filter shape?

Code: Select all

  SR   PWR  MER
 ==============
 125	-32	19
 250	-32	15
 300	-32	19
 333	-32	15
 500	-32	16
1000	-32	21
2000	-32	21
4000	-32	20

For interest, these are the equivalent readings using the V0.6d software. Power all over the place and MERs generally better.

Code: Select all

 SR   PWR  MER
==============
 125	-27	19
 250	-33	19
 300	-35	23
 333	-36	19
 500	-39	20
1000	-46	25
2000	-52	25
4000	-58	24

73
Ray

[G8GKQ]

Hi Ray and John

The figures quoted on the Filter-Modulator test sheet are measured with a Serit Tuner and MiniTioune V0.6d software. If you are to make any comparisons between your set-up and the test sheet parameters, you need to use the V0.6d software version as Jean-Pierre has changed the MER calibration in V0.7b.

John: Having thought about this and compared your results with Ray's results, I think that everything is working as expected for you. Looks good!

The power measurements in MiniTioune V0.6d had an error of around 6 dB per octave of selected symbol rate as reported here http://batc.org.uk/forum/viewtopic.php? ... 995#p11995. I'm really grateful to Jean-Pierre for resolving this in MiniTioune V.07b.

The filters on the Portsdown Filter-Modulator board are set a little too tight compared with the DVB-S standard and this shows up as a lower indicated clean-signal MER on the Serit Tuners. The problem is not apparent with the older Sharp tuners - and this is why it was not picked up at the design stage. However, I have since conducted tests under simulated weak-signal conditions (using attenuators and injected noise) , and have been unable to consistently measure any improvement in the resolvable signal strength using a wider, corrected, filter bandwidth. So, in short, the only effect seems to be a lower number in the box on the MiniTioune software.

Dave

[gi7ugv]

Hi Ray/Dave,

Thank you for the information and the readings for comparison. I hadn't read about the reading at 333 being down so was a bit confused by that one given the ugly results but it makes sense given what you've said! Verified J02 wiring is connected up okay and measurements against GND on the PI on U20-22 are as in the linked file so all ok there.

I took some readings at the same symbol rates as you did Ray and they are following the exact same pattern albeit all down a bit, the lower readings all have reduced timing lock percentages. I've some work to do on the power supplies and wiring, everything’s twice as long as it will be, and I need some more attenuators for clean tests to the tuner so hopefully with that the gap might close a bit. I'll give 0.6d a whirl out of curiosity to see the differences described.

Guess it's time to get everything fitted properly and move the thinking on to filters/amplifiers, had been putting that off until verified things were at least half right

Thanks for the help again!

John