Monday, 30 January 2017

Power measurements of DVB-T transmitters


Power measurements of DVB-T transmitters- first draft

Introduction

I have been puzzled for some time about how power of DVB-T amplifiers were measured relative to other modes. This is an important issue in DATV as DVB-T has been criticized as inefficient compared to DVB-S. It would appear that some of the debate comes down to how the power is measured. Most amateurs use simple diode power meters that do not give an accurate reading for DVB-T.

In this post, I outline the theory and practice of measuring DVB-T power correctly with either a thermal power sensor meter or envelope power using a spectrum analyser. It is important that the correct measurements be used otherwise it is comparing apples with oranges. Different power measurements are accepted in amateur radio, vis, CW (peak power, key down) compared to SSB (peak envelope power).

I wonder if another measurement unit for digital modes may be more appropriate, such as the data rate to the DC power input and spectrum width, bits/sec Watt Hz perhaps?

Theory of power measurement of DVB-T transmitters

Others have written this better than I can, so I use a direct quote:

“The output signal of a DVB-T transmitter consists of thousands of carriers modulated in phase and amplitude. Therefore it resembles a Gaussian noise signal. It should be noted, however, that very high peaks of the sum signal are limited due to effects in the process of generating and amplifying the signal. The only simple way to define the power of a COFDM signal like DVB-T is an RMS definition. It is also closely linked to the theoretical system analysis.

As the number of carriers of a given DVB-T system (either 2k or 8k) is constant and all carriers have defined power, the total power of a DVB signal is the sum of all carrier power values. In practice only the total power can be measured. In principle one symbol is insufficient for assessing the power. With thermal power meters the integration time constant is much larger than a symbol period allowing valid measurements”. Implementation guidelines for DVB terrestrial services; Transmission aspects, Digital Video Broadcasting (DVB); RTR/JTC-DVB-304

The actual number of number of carriers for 2k is 1,705 carriers and for 8k, 6,817 carriers.

Practical power measurement of DVB-T amplifiers

Power measurement is a very complex topic. It is covered in many books and application notes available on the web. It is also very important. Try reading about satellite or cell phone power and efficiency!

Simple diode power measurements used by amateurs in SWR meters are designed for measuring CW or SSB, not complex wide-band digital TV signals, including both DVB-T and DVB-S.

Again, I use a direct quote on the practical measurement of DVB-T power:

“Power measurements on DVB-T transmitters: Mean power measurements
In the case of analog transmitters, signal power is determined by measuring the peak power of the sync pulse floor of the modulated CCVS signal. The sync pulse floor is always the reference in analog TV because this signal component must be transmitted without compression or distortion.
In DVB this is different: the “Sync 1 Inversion and Randomization” block of the DVB modulator... ensures constant mean power of the transmitter output signal. In DVB, therefore, it is not the peak power that is measured, based on the crest factor, but the mean output power. Three methods are available:
1. Mean power measurement with Power Meter NRVS and thermal power sensor (FIG 27)
Thermal power sensors supply the most accurate results if there is only one TV channel in the overall spectrum, which is nearly always the case at the DVB-T transmitter. Plus, they can easily be calibrated by performing a highly accurate DC voltage measurement, provided the sensor is capable of DC measurement.
2. Mean power measurement with Spectrum Analyzer FSEx or FSP
A frequency cursor is placed on the lower and another one on the upper frequency of the DVB channel. The spectrum analyzer calculates the power for the band between the cursors (FIG 28). The method provides sufficient accuracy as in DVB-T normally no signals are put on the air in the adjacent channels.
3. Mean power measurement with DVB-T Test Receiver EFA”
“Measurements on MPEG2 and DVB-T signals (4)” in News from Rohde&Schwarz Number 172 (2001/III)

I have left out the detail of test receivers, as most amateurs would not have access to one. A power meter and the spectrum analyser are pictured below.


The simplest way to illustrate the difference in power measurement is by commercial specification of a modern DVB-T LMDOS power transistor. The CW power is 750 W, while the DVB-T power is 150 W, with an efficiency of around 50 percent. The DVB-T power measurement would have been done with a thermal power meter. The CW (key-down) and DVB-T power represents different modes and different ways of measuring power. However, a thermal power meter can measure any type of signal, including DVB-S, and give the RMS value.

Even the CW figure is misleading as Morse is a digital mode with a low duty cycle, the RMS figure would be much lower.

Operating parameters and apparent "efficiency"

The operating parameters of an amplifier will affect apparent "efficiency". The transistor above, uses a PAR (peak-to-average ratio) of only 8 dB. Amateurs have typically used a much higher PAR (30 dB?) in order to reduce the intermodulation skirts. While this will reduce power input as well as output, there has been much debate over the achieved power out compared to the maximum possible power out, usually using the CW output.

As I have discussed in earlier posts, it is possible to use filters to reduce skirts and achieve higher power outputs, comparable to device specifications. I discovered this based on intuition and experience building, modifying and tuning cavity filters for voice FM repeaters. I have subsequently found that they are routinely used in broadcast DVB-T TV.

Using a thermal power meter and operating parameters akin to broadcast TV, the achievable power efficiency can be better understood.

DATV power efficiency; W/W or something else; b/sWHz?

To a point, power efficiency, no matter how defined or measured, is probably not what we really want to know. Power in, data rate, spectrum band-width and probably cost are what we really want. So a new unit of efficiency is data rate (bits/second) divided by power in (Watts) and band-width (Hz); b/s W Hz! There is probably a unit name for it, but I don't know what it is. I am serious and will discuss it in another post with modes and numbers. 

Let's do it!

So far has been "book learning". The next step for me is to do it. I had sufficient interest in the DVB-T power puzzle to buy a used thermal power meter and a spectrum analyser capable of measuring envelope power. New power meters and sensors are very expensive (~$10K), but I imported a used HP 438A power meter and HP 8482H thermocouple thermal sensor for about $500. The spectrum analyser is a Siglent SSA3021X. Calibrated dummy loads and attenuators are also needed.

I also have a SWR power meter, a cheap AD8307 power meter and access to a Bird power meter, all of which use diode sensors. I should use a HP diode sensor to suit my meter, but I can't afford it!

I certainly don't advocate everyone buying this gear, but I want to investigate the puzzle.

Testing power output from digital signal power amplifiers is not simple. My bench tests will be the subject of another post.

Conclusion

to be done- see introduction









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