OE7DBH DVB-T experience

 Here some information and hints regarding DVB-T from my OE7DBH experience:

MER
Shoulder attenuation
Class A amplifier
Antenna
Power measurement

MER (Modulation Error Rate)
MER is calculated in order to put the total signal impairment in a measuring value which is presumably at the input of a receiver.
The MER is the ratio of the average signal power to the average error power .
The MER value is the least of the same or even more importance than a higher transmission power.
A signal in 16 QAM with a MER lower than 8dB can not be decoded.
DVB-T TRX mostly have a MER between 20 and 35 dB, due to amplification this good value will be lost.
Depending on the amplifier or the used electronic the MER will be reduced between 5 and 15 dB.
But be careful if you are going to the limits of your amplifier while you are believing that in DVB mode the
same output power is available as in SSB or FM. If you are making this mistake, your MER will be down to 5 dB.
Your receiving counterpart will notice a signal, but has no chance to get Video and Audio. Less is more.
Let´s compare FM with 16 QAM , same bandwidth and good modulation:
A weak FM signal with good modulation could not be decoded, a strong FM signal is no problem to decode.
A weak 16 QAM with a good MER can be easily decoded, a strong 16 QAM is also no problem.
With a bad modulation both modes are causing problems to decode.

Shoulder Attenuation ( Signal – Shoulder span)
The DVB-T system does not utilize the full channel bandwidth, i.e. some of the 2K- or 8K subcarriers are set to Zero
so that no interference to adjacent channels will be caused.
Due to nonlinearities, there are still outband components and the effect on the spectrum and its shape
has given rise to the term “shoulder attenuation”.
So it is important to keep the attenuation as high as possible to avoid monkey-chatter and also to keep the MER on a high level.
A reduction of 40 % in the Signal shoulder span , leads to a significant reduction of the MER.
The appropriate measurement equipment is a spectrum analyser.

Class A amplifier
The definition of „class A“ from 30-40 years ago is not the same as for 5-10 years ago, the same is valid for the
used technical eqipment. In SSB mode this does not make a big difference, but for DVB signals it is a huge difference.
Let´s take the well known Mitshubishi moduls as example:
M57745 (plastic cover): absolutely not suitable for DVB signals
RA30H4047M (plastic cover) limited suitable for DVB, only for very low output power
RA30H4047M1 (metal cover) suitable for DVB , up to approx. 20% of full power in CW mode

Rule of thumb in the professional broadcast area: 7% of maximum possible power in CW mode can be achived for digital transmission
In this area mostly Push-Pull amplifiers with solid state electronic components of 80 W HF output power are used.
Push-Pull means using 2 amps with 80 watts, so in total 160 watts, combining them at the end with a coupler.
But why they are using a coupler which is only designed for 25 watts in CW?
Because this amp generates only 10 watts in digital mode.
But why to use such high power semiconductors?
This is because they are only working fully linear in the low power range.
Check with the thump rule:. 7 percent of approx. 150 W in CW mode leads to approx. 10 watts in digital mode.
HAMs are taken this professional thump rule not so serious with our amps, our thump rule is rather 10 – 15 %.
In no case exceed this border otherwise your DVB signal will extremely suffer from spectrum and quality.
Please keep in mind: Less is more.
Rather a lower signal with better quality is preferred.
Use an antenna with more gain to achieve more power, but also in this area pay attention.

Antennas
Please use omnidirectional antennas without gain, J antennas, dipols, backfire- or yagi antennas with gain !.
Do not use stacked antennas because those will introduce phase shifting which is very bad for DVB signals....
( Diamond X-200 X-510 X-5000 X-6000 or similar from other manufacturers )

Power measurement
Best choice would be a “channel power meter” a DVB-T power meter, but who can afford this.
Correct would be to measure the carrier power on DVB, DVB-T offers 2k, 4k and 8k.
2 k means 2000 (to be precise 2048) carriers, effectively used are 1704 of them. Measured should only be one of them.
HAMs are measuring across all carriers while using their DAIWA, Bird, Thermal power meter. This is not fully correct but sufficient for us.
A Thermal head power meter shows approx.. 8% less power than in reality transmitted.
From a feedback I know that a Bird wattsmeter shows only 50% of the real transmitted output power.
All other HAM VSWR meters are too imprecise.


From original translated by OM Rainer DG8KD , Thank You

Note:
Bird gehört nicht in der Instrumentenklasse, sondern in der Leistungsanzeige und Einstellanzeige gruppe, aufgrund fehlender Präzision !!.
Dynamik ist mit der Diodedetektoren schwer zu erreichen und keine haben die entsprechende Amplituden und Frequenzkorrektur.
Messungen mit Durchflusswattmetern sollten mit der Reserve erfolgen und sind nicht mit profi Power Meter Instrumenten vergleichbar.






DJ7DA experience: I would like to add one item to your topic *Power measurement*

More than a year ago I bought an "RF Explorer Analyser" which turned out to be my best investment (<200€)
when it comes to RF-measurements of various kinds.
Attached are 2 screen shots of the DVB-T signal as transmitted by our local ATV-repeater DB0QI and received in my shack today.
One shows the actual signal envelope (centered at 436 MHz with 1 MHz bandwidth).
Since the RBW is indicated, already this picture allows for determining the dBm-level.
However, as seen in the second screen shot, the RF Explorer has a mode "PowerChannelMeter" which does all the math for you.