Easy and accurate testing of low-power transmitters for digital TV

Using portable TV analyzers for gap filler installation, maintenance, and service measurements

Twenty-five kilograms or more: That is how much the test equipment needed for testing digital television transmitters – a test receiver, spectrum analyzer, and power meter – weigh all together. This weight is not an issue for a transmitter like the one on the Olympic Tower in Munich. It is easily accessible with a van and the elevators in the tower. But what about the many gap-filler stations and transposers? They are usually not put in place until the second phase of the network installation and are intended to close local coverage gaps. They are often placed in locations that are difficult to access, such as on mountains or on house or factory roofs. Service technicians prefer to test such low-power transmitters with a small, portable test device.

As seen in the block diagram of a TV transmitter, a directional coupler with test ports for the forward and reflected power is placed at the transmitter output, followed by a mask filter and finally an antenna. TV network operators define what tests are required in their test specifications. However, the following parameters are always covered by the tests: transmit power, frequency accuracy, modulation error ratio (MER), bit error ratio (BER), and out-of-band emissions. If these parameters are within tolerance, the transmitter is functioning correctly.

Measurement tasks at the transmitter
The output power is the most important characteristic when operating a transmitter because it determines the transmitter's range. Digital TV signals are noise-like signals with a constant average power. A test receiver measures the power with high accuracy by integrating the power density over the occupied bandwidth. An even greater degree of accuracy can be achieved with an external power sensor. The technician has to correct the reading to account for the insertion loss of approx. 0.1 to 0.3 dB from the mask filter as well as the coupling loss from the directional coupler.

Digital TV transmitters can be operated in single-frequency networks (SFN). This demands a high degree of frequency accuracy of the individual transmitters, as they must not deviate from one another by more than 1 Hz. The test receiver must be synchronized with the transmitter to be able to measure deviations in this order reliably. Typically, a transmitter supplies a 10 MHz reference signal for this purpose. Alternatively, the test receiver can be synchronized with a GPS receiver. The GPS signal increases the accuracy of the test receiver's reference oscillator. This has the added advantage that any errors in the transmitter's reference source can be detected.

Measuring out-of-band emissions without bandstop filter

A particularly demanding task is to measure out-of-band emissions, i.e. signal components in the transmitter's adjacent channels. Out-of-band emissions are caused by nonlinearities in the transmitter. Although TV transmitters use highly linear amplifiers operating in class AB mode and a precorrection in the modulator, a certain amount of residual nonlinearity always remains, causing intermodulation products of the OFDM carriers. These appear as shoulders at the edges of the spectrum, and they extend into the adjacent channels. The mask filter flattens the shoulders to below the limit values specified by law. For example, DVB-T requires an attenuation of more than 80 dB relative to the useful signal at a spacing of ±10 MHz from the useful signal.

Modern TV receivers, like the R&S ETH handheld TV analyzer from Rohde & Schwarz, measure power, frequency, MER, and BER automatically in realtime and output results in an easy-to-read display. Fig. 2 shows a measurement for ISDB-T. It displays the MER and BER for layers A, B, and C separately. A power sensor can be connected to measure power with the highest degree of accuracy. A GPS receiver is available as an accessory. The TV analyzer can additionally be equipped with a tracking generator. This makes it easy to measure out-of-band emissions. The R&S ETH stores the measured filter characteristic of the mask filter as a transducer and uses it to automatically correct the spectrum measurement. The R&S ETH can also display permissible limit values and check whether they are complied with (Fig. 3). The included PC software exchanges measurement results, device setups, and transducer files between a PC and the instrument. The handheld TV analyzer combines all the functionality described in a highly compact, battery-operated box weighing no more than 3.3 kg. The instrument is not only light weight, it also lightens the service technician's workload.

Peter Lampel studied electrical engineering at the Saarbrcken University in Germany. He received his diploma in 1995.
From 1995 to 2006, he first worked as a development engineer for radar systems at EADS and then in the development and product management of mobile communications technology at Siemens AG.
Since 2006, he has been product manager of broadcasting and TV test equipment at Rohde & Schwarz in Munich, Germany.

Tags: iss054 | transmitters | installation | gap-filler | sfn | bandstop filter | ofdm carrier | dvb-t | Rohde & Schwarz | N/A
Contributing Author N/A

Read this article in the tv-bay digital magazine
Article Copyright tv-bay limited. All trademarks recognised.
Reproduction of the content strictly prohibited without written consent.

Related Interviews
  • Wireless Transmitters for Boxx TV at NAB 2018

    Wireless Transmitters for Boxx TV at NAB 2018

  • Videosys Wireless Transmitters at BVE 2014

    Videosys Wireless Transmitters at BVE 2014

  • Cobham transmitters at IBC 2013

    Cobham transmitters at IBC 2013

  • Cobham: Mini RF Transmitters at NAB 2013

    Cobham: Mini RF Transmitters at NAB 2013

  • Dejero at IBC 2016

    Dejero at IBC 2016

  • Thomson Broadcast at IBC 2014

    Thomson Broadcast at IBC 2014

  • Thomson Broadcast at NAB 2014

    Thomson Broadcast at NAB 2014

  • Cobham: RF Transmitter at NAB 2013

    Cobham: RF Transmitter at NAB 2013

  • Jampro Antennas at NAB 2013

    Jampro Antennas at NAB 2013

Related Shows
  • BVE 2013 Day 2

    BVE 2013 Day 2

REVIEW Canon UJ111x8_3B UHD Lens
Andy McKenzie Shooting in ultra high definition and high dynamic range has become the preferred option for all television productions with a potentially long commercial life. The hir-ing charge and indeed the outright purchase cost of cameras of this standard are not significantly greater than those of high definition standard dynamic range models. With half the world's news reporters now able to capture UHD video on their mobile phones, why give the production crews anything less?
Tags: iss139 | canon review | uj111x8 review | lens | zoom | digisuper | uj90x9b | hdtv | ccu | Andy McKenzie
Contributing Author Andy McKenzie Click to read or download PDF
The Future of Broadcast Connectivity
Jamie Adkin The use of KVM equipment has been essential to meet the evolving needs of the broadcast industry for many years. Over that time, many in the industry have recognised the importance of using IP-enabled KVM to break down technological barriers and enable real-time access to visuals wherever and whenever they’re needed. These components are vital parts in live production environments in particular.
Tags: iss139 | adder | kvm | ip kvm | Jamie Adkin
Contributing Author Jamie Adkin Click to read or download PDF
Undercover Remote Production
David Bradley

It’s only taken 20 years and it seems that Remote Production is finally coming of age, but it now requires far more cameras than anyone envisaged back in the last century!

The move to remote production is one of the unstoppable trends of 2019 and moving into 2020. Our CamBall cameras pioneered remote working as early as 2003 with sport and wildlife programming using remote production for many years with incredible results.

Tags: iss139 | ptz | br remote | 5g | remote camera | atlantic productions | camball | camball4 | David Bradley
Contributing Author David Bradley Click to read or download PDF
To Remotely Go - TVFutures
Michael Parsons One of my biggest concerns as an academic responsible for the education of hundreds of students is the ‘appropriateness’ of much of the technology we purchase and implement within the curriculum. The last few years have seen tremendous change in all sorts of technologies, and the broadcast industry is just one sector that has seen some significant leaps in innovation.
Tags: iss139 | university | portsmouth | graduation | guildhall | newtek | streaming | pxw-fs7 | ndi | ip | ndihx ptz camera | Michael Parsons
Contributing Author Michael Parsons Click to read or download PDF
Post Production - Keeping Track
Alex Macleod One of the most important things to consider when beginning a new edit, is keeping track of where all your various media assets are - not only their location on whichever storage drive you chose to work from, but also where you file them away in your project once imported.
Tags: iss139 | premiere pro | editing | shotputpro | post haste | Alex Macleod
Contributing Author Alex Macleod Click to read or download PDF