Measurement and testing of broadcast transmission systems

MEASUREMENT AND TESTING OF BROADCAST TRANSMISSION SYSTEMS

By Ampie Greeff – Installation Manager, LS of South Africa (Pty) Ltd.

?1.??BACKGROUND

?Radio and television transmitting facilities are typically comprised of a number of elements which make up the hardware family referred to as the ‘broadcast system’. These components have their own unique set of requirements arising from where the input signal enters the facility through to the final product which is broadcast to its listeners and/or viewers.

?Various infrastructure support systems (such as Uninterruptible Power Supplies {UPS}, Automatic Voltage Regulators {AVR}, standby power generation and even HVAC equipment) complement the broadcast system and their availability is considered critical to the on-air performance of the broadcast system mentioned in the above.

?The primary objective for the commissioning and testing of a transmission facility is for ensuring that the installed equipment achieves the performance specifications as laid down by the network operator, along with ensuring integration with all of the components which comprise the broadcast chain.

?A secondary objective for measurements during commissioning is to provide a record of such testing for future reference by operational/maintenance engineers/technicians. Throughout the remaining operational life of the broadcast system, it is appropriate for operational personnel to conduct ‘spot checks’ on a regular basis on the systems and to compare results against the original commissioning data. This ultimately may determine whether performance degradations have occurred and on what basis will corrective action be required.

?2.???MEASUREMENT APPROACHES

?There have been some significant advances in test and measurement (T&M) approaches for broadcast systems. In the past, it was customary to have fully equipped test equipment trolleys on transmitting stations – particularly for the evaluation and testing of analogue television transmitters. Whilst considered ‘state-of-art’ in its time, these instruments had to be subject to regular calibration cycles to ensure repeatability and dependability of results. In the ‘golden years’ of doing a comprehensive round of commissioning tests on high-power TV transmitters, this was a process which conservatively took two weeks on site to accomplish!

?Test reports had to be painstakingly prepared complete with its Polaroid ‘shots of the instrument displays being inserted into it…

?In the author’s experience, one of the notable ‘shifts’ in the approach to T&M has been the appearance of ‘one-box’ analysers such as the Rohde and Schwarz ETL? - these are now widely recognised by large broadcast network operators as the universal reference T&M receiver. Additional features and ‘plug-ins’ permit the same instrument to be used throughout the entire commissioning exercise for both radio and television broadcast systems.

?Similarly for FM only type T&M applications – the Audemat MC5 has gained popularity for example as an integrated test instrument. Between units such as the ETL and MC5 it is apparent that practically all of the measurements can be automatically undertaken and hard-copies of instrument displays, etc. are easily saved and/or printed out for future reference purposes.

?Despite the automated nature of the measurements being done by the instruments in question – It remains as important as ever to have a clear understanding of the reasons for the measurement/s along with the ability to be able to characterise/interpret the results obtained.

?The convergence of skillsets within the broadcast and ICT industry sectors is continuing unabated – and as almost every T&M application today requires some form of computational ability (on the part of the instrument), this may be offered as software which can be installed onto a desktop or laptop PC. An applicable interface (or ‘front end’) is in turn then connected to the device under test. This approach has led to the appearance of many cost effective test instrumentation solutions which are gaining rapport in professional environments. A further upside is that these test instrument solutions are finding their way into classrooms at Universities and are being shown to students who are able to use the devices.

?Inbuilt test equipment is a standard feature on Nautel’s AM/FM transmitters which serve to aid engineers and technicians with the fault-finding and operation of the hardware. This test equipment complement is accessible via the user interface of the transmitter, which in some cases is via its LCD front panel, or remotely over a TCP-IP link. Having such instrumentation ‘on hand’ (such as modulation analyser, spectrum analyser and even a network analyser!) therefore requires no dedicated CAPEX for its purchasing and subsequent OPEX budgets for future calibration exercises. ?

3.???MEASUREMENTS OF THE BROADCAST CHAIN

?A broadcast chain on a transmitter site normally consists of three main elements, namely the Programme Input Equipment (or PIE), Transmitter and antenna system. In many instances, more than one service is broadcast from a single transmitter site, necessitating a transmitter for each service. In such cases, a RF combiner is added between transmitters and antenna to combine the RF outputs of transmitters into a single coaxial feeder and antenna array.

3.1??????PIE (PROGRAMME INPUT EQUIPMENT)...

?The PIE provides baseband signal feeds (e.g. audio, transport stream, etc.) to the transmitter. Broadcast sites are mostly in remote locations (where normal communications infrastructure is not readily available) and therefore satellite distribution is chosen as the default mechanism to programme distribution. Satellite-receive systems (e.g. dish and LNB equipment) may subject to the effects of external environment and require auditing to ensure these achieve the specified performance norms.

?The baseband signals from the PIE are measured for ensuring the correct amplitude values are presented to the input terminals of the transmitter. As an example, an FM radio audio path may consist of distribution amplifiers, audio processors and audio switching equipment – some of which are able to perform audio level manipulation. These components require evaluation to ensure that the transmitter ultimately respects the legal requirements as is laid down by the applicable Communications Regulatory Authority (such as ICASA, etc.).

?Whilst the monitoring and control of equipment generally forms part of the station telemetry systems, it is considered acceptable to integrate this equipment within the PIE rack and one may find telemetry remote terminal units (RTU) and other hardware installed into a traditional PIE cabinet.

?The author believes it is becoming increasingly seldom to find equipment on a transmitting station which does not offer IP and/or SNMP functionality as standard features. Even the smallest of transmitting sites incorporate some form of LAN and/or WAN – be this for programme feed delivery and/or remote control/monitoring access to the site. The testing of these networks has become a necessary requirement for ensuring reliability.

?3.2??????THE TRANSMITTER...

?The building blocks of a modern radio/television transmitter system consist of a low power exciter/s, the system controller, along with paralleled stages of RF power amplifiers/switch-mode power supplies (configuration and packaging dependant on power level of the TX). Most (if not all) transmitter manufacturers conduct an extensive array of testing with the equipment operating at its rated power into a dummy load. In some instances the end-customer may elect to witness the final acceptance testing (or FAT) of the equipment in person or do so virtually. The FAT data is referenced against the Site Acceptance Test (SAT) data, though for FAT and SAT results to yield similar results – the test instrument complement would in both instances would need to be completely characterised. In many instances the FAT data is accepted as evidence that the system has been tested and upon installation the system is simply placed on the air.

?Probably the most important measurement any broadcast engineer/technician would ever make on a transmitter-under-test (TUT) would be its forward power – providing an immediate indication that all RF power amplifiers are delivering their rated power level/s. ?Reflected RF power will indicate degradation of transmit antenna feeder cable and/or RF combiner. Furthermore, spectrum analysis, for instance harmonics, shoulder attenuation, RF bandwidth and carrier to noise ratios, are measured. For a typical FM transmitter performance attributes such as frequency response (dependent on the audio input mode selected), total harmonic distortion (THD), stereo separation and synchronous AM. Vendor and customer specific tests may be added as/where necessary.

?The approach to cooling of RF power amplifiers has become an important consideration in high RF power transmitters with a number of manufacturers providing options for either liquid or air-cooling. The reasons for their choice for use are beyond the ambit of this paper. Suffice it to say - air flow measurements (in case of air cooling) - and liquid flow and temperature measurements are relevant parameters for each technology and require evaluation/measurement as is required.

?3.3??????RF COMBINERS/ANTENNA SYSTEMS...

?RF combiners are highly sensitive devices as the mechanical design and adjustments of its RF elements are extremely critical. Transportation, specifically on sub-standard roads to transmitting stations may cause these units to suffer from misalignment and deviate from the FAT data which was issued with the item as it was tested.

It is highly recommended to both measure and realign a combiner, before installation, after its transport to a transmitting station. Once aligned, installed and commissioned on site, it is not necessary to measure again under normal circumstances. However, if failures occur due to lightning, ingress of moisture (giving rise to internal arcing) and/or mechanical repairs are needed to be carried out, it is almost certain the combiner will require re-testing. Typical measurements on a combiner may include, insertion loss, return loss and isolation between RF channels.

?Transmit antenna arrays and their associated coaxial cable feeder infrastructure is installed on masts, which are often in excess of 100m in height. This equipment is exposed to the rigours of the exterior environment, specifically lightning, rain and snow, along with wide ranges in temperature cycling.

?High power RF transmitter stations must make use air dielectric feeder infrastructure. These cables are kept under pressure with dry air, supplied by a dehydrator, to prevent moisture entering the cable and as preventing arcing within the feeder. Regular monitoring and recording of line pressure/s along with the operating hours of the dehydrator is therefore necessary.?

?Antenna arrays are designed to cover a specific target area.?Omnidirectional arrays are by far the popular choice (from a cost and performance benefit ratio) however there is may be a requirement for directional arrays. ?After installation radiation patterns need to be verified. Measurement of these radiation patterns are achieved by remote piloted aircraft (or similar devices) with receiving equipment and antenna on board. Alternatively – coverage of the station can be evaluated through doing drive testing. Antenna arrays may suffer long-term damage through the factors already mentioned in the above, in turn negatively affecting radiation patterns causing degrading of reception signals. Measurement of radiation patterns should therefore not only be done after installations, but repeated afterwards from time to time.

?3.4?????? ASSOCIATED EQUIPMENT...

?It was mentioned in this paper that equipment such as UPS's, AVR's, standby generator sets and HVAC equipment have an effect on the performance of broadcast equipment therefore measurement and testing of such devices during maintenance procedures are important.

Some typical measurements on these devices include (but not limited to): phase voltage/s and currents, phase rotation, air flow, temperature measurements and UPS battery health.

4. CONCLUSION

Broadcasters/content providers do not take kindly to signal degradations and delays as these have a negative impact on the revenue stream/s of the station. Signal distributors/network operators need to ensure that appropriate preventative maintenance procedures are followed to the best of their abilities for the sole purpose of maintaining network availabilities. ?Measurement and testing of broadcast systems form an essential part of preventative maintenance procedures.