DVB-T Crest Factor

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G0LJF
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Joined: Wed Dec 25, 2019 5:28 pm

DVB-T Crest Factor

Post by G0LJF » Fri Nov 26, 2021 2:32 pm

Hi

I found this R&S measurement article most illuminating as it explains the theory and practice of measuring DVB-T Crest Factors.
And its far worse than I thought, having a substantial knock-on effect on any DVB-T PA.

https://cdn.rohde-schwarz.com/pws/dl_do ... S02_2E.pdf

Its a few pages, and it too me a couple of goes, but in essence, a DVB-T signal can have peak powers some +35 dB greater than the mean, but these occur so infrequently that it is difficult to measure.

73, Mike

G8GKQ
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Re: DVB-T Crest Factor

Post by G8GKQ » Fri Nov 26, 2021 6:15 pm

Mike

The Crest Factor of the DVB-T produced by the Portsdown is not as bad as the original studies predicted back in 2007. The software has advanced to include crest factor limiting.

The Crest Factor is still greater than DVB-S/S2 but is manageable for amateur use.

Dave, G8GKQ

G0LJF
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Joined: Wed Dec 25, 2019 5:28 pm

Re: DVB-T Crest Factor

Post by G0LJF » Sat Nov 27, 2021 12:12 pm

Hi Dave,

Interesting to note that DVB-T Crest Factor has already been limited in Portsdown.

Its really illuminating to read the R&S statistical analysis. And the effective 'hopelessness' of any making accurate [what does that mean, its all probability] measurements in OFDM.

73, Mike

SkyVision
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Joined: Fri Jan 24, 2020 1:51 pm

Re: DVB-T Crest Factor

Post by SkyVision » Mon Nov 29, 2021 1:35 am

Hi Chaps

Has anyone measured or calculated Crest Factor or Peak to Average Ratio for Narrow Band DVB-T generated by Portsdown
I was told early on that with wideband DVB-T amplifiers need to be derated 10ish db to allow for the peaks
I don't have an R&S crest measuring device so I can only guess, average can be measured with a bolometer power meter so that's easy
Since narrowband DVB-T has the 2 or 8K carriers compressed into a smaller space I assume the peaks are worse

When I asked the NXP rep which device would be good for narrowband DVB-T PA he reponded with:
"What is the PAR (peak average ratio) of your ham DVB-T"
That was a conversation stopper, which lead me to reading the R&S paper you speak of, not that I understood all of it fully, I'm no Engineer
I live in hope that someone could answer this question or provide some more reading material on the subject

Cheers Roger VK5YYY

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