How to measure channel power of this transmitter? - Page 1
Forum section: electronics / rf-microwave

author: Mad ID
author_rank: Regular Contributor
timestamp: November 22, 2016, 08:22:14 am
content: Hi,I'm trying to optimize a matching/filter network of a LoRa Semtech SX1272 PHY which uses spread spectrum. The PHY cannot be set to transmit unmodulated carrier. I will make adjustment until I get full power with acceptable second harmonic.Here is the first spectrogram.RBW is set to 1kHz. Detector type to RMS. Signal bandwitdh is 120kHz (as is should be). The power at the top is -0.63dBm. There are in total 120/RBW = 120 blocks to integrate. So total power should be -0.63 + 10log(120) = 20.16dBm.The transmitter is setup to transmit 20dB, exactly what I see...but there is a problem.Let's change the RBW to 3kHz, we should get the same channel power..Now the power at the top is 12.56dBm. We need to add 10log(120/3) = 16dBm. In total that's 28.58dBm! I'm not transmitting this much plus it's different.If I set the RBW to 300kHz I get channel power of 15.5dBm.What is wrong with my measurment/calculation? I should also note that the transmitter is not transmitting all the time..I'm using max hold feature to plot the spectrogram.Thanks.

author: Bicurico
author_rank: Super Contributor
timestamp: November 22, 2016, 09:13:49 pm
content: Why don't you just use the advanced channel power measurement available on the SSA3021X?I am not sure your calculations are correct: I am watching TV with my phone in the hand, but without giving it much thought, I would expect you to multiply instead of adding...Regards,Vitor

author: Mad ID
author_rank: Regular Contributor
timestamp: November 22, 2016, 09:15:43 pm
content: Quote from: Bicurico on November 22, 2016, 09:13:49 pmDon't have channel power option.I'm adding log values..i.e. multiplying

author: Bicurico
author_rank: Super Contributor
timestamp: November 22, 2016, 09:20:35 pm
content: You know you can easily activate that function...Anyway, the log values you are adding do not reflect the measured power value. To integrate the power on the banswidth you need to add the measures power on each frequency. I THINK you need to do something like -0.63*120...

author: Mad ID
author_rank: Regular Contributor
timestamp: November 22, 2016, 09:22:06 pm
content: Quote from: Bicurico on November 22, 2016, 09:20:35 pmI'm doing that, and I get correct power. When I change RBW to 3kHz and multiply by 40 (120/3) total power is 28dB..impossible since I'm transmitting 20

author: Bicurico
author_rank: Super Contributor
timestamp: November 22, 2016, 09:25:56 pm
content: With three times the RBW, your integration resolution is less accurate and you end up adding more power than there is in reality. Think of it as higher cordal error.In your first post you were just adding the log of the bandwidth, hence my remark about the need of multiplying.

author: Bicurico
author_rank: Super Contributor
timestamp: November 22, 2016, 09:37:25 pm
content: Hi,I went to my shack and took two pictures of a DVB-T transponder, showing the channel power.First one is at 100kHz RBW, the second one with 300kHz RBW.I am to lazy to do the maths now, but whatever formula you deduce, it should give you around -52dBm in this example.Regards,Vitor

author: Bicurico
author_rank: Super Contributor
timestamp: November 22, 2016, 09:46:44 pm
content: Here is the formula:https://uniteng.com/index.php/2013/07/26/channel-power-measurements/Of course I wanted to go to bed early today, but this got me thinking...Regards,Vitor

author: Bicurico
author_rank: Super Contributor
timestamp: November 22, 2016, 10:05:42 pm
content: Hi,My apologies. Only now, after sitting on my PC and getting all this through my head, did I realize:1) You are using a Rigol and not a Siglent, hence I don't know if Channel Power Measurement can be unlocked or if is supported at all.2) Your calculations were correct (see my attached Excel spreadsheet to calculate the channel power - I get the same results).Sorry!Is the signal constant? Can it be a cumulative measurement error?Regards,Vitor

author: Bicurico
author_rank: Super Contributor
timestamp: November 22, 2016, 10:59:14 pm
content: All this got me thinking so much, that I looked at my SMA Simple SPectrum Analyser software and actually got my Power Meter function to work properly (was on the TODO list for a long time)!Amazingly, the results are quite good and similar to the ones determined by my Siglent.I can go to bed now.@Mad ID: sorry to have filled your thread with so many posts, but I went from tired lazy mode to genius mode (*cough*), which actually made my day. Hopefully there was something in it for you, though the final answer to your question is still do be given...Regards,Vitor

author: w2aew
author_rank: Super Contributor
timestamp: November 23, 2016, 03:38:24 am
content: The problem with the narrow RBW is that the chirp rate of the CSS modulation is too fast for the narrow filter respond properly. And, the pulsed nature of LoRa can also be affecting your result. Why not simply use zero span, with an RBW that exceeds your signal bandwidth? That would show you the total power within that RBW.

author: Mad ID
author_rank: Regular Contributor
timestamp: November 23, 2016, 07:59:57 am
content: Quote from: w2aew on November 23, 2016, 03:38:24 amThat was the answer I was looking for. Thanks very much! Also tanks for sharing your knowledge with us on ytube