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Solid-state VHF FM Transmitters SR6..E1 ured or integrated, even using standard equipment, to fulfill specific customer requirements.

Classic VHF sound The VHF FM transmitters can be used as stand-alones, in passive (1+1) or (n+1) at its very best standby configurations and with exciter standby. All modules and units are accommodated in a 19-inch rack for Despite and video broadcasting, forecasts still anticipate an attrac- ease of access. The main modules can tive market ahead for analog FM transmitter technology over the next 15 to be replaced without disconnecting 20 years. Rohde & Schwarz responded to these prospects and revised its highly cables. So a transmitter can be set successful, tried and tested generation of solid-state transmitters. The result is up very quickly and, in the unlikely even more compact transmitters for an excellent price/performance ratio. event of a module failing, it can be exchanged in practically no time.

Solid-state VHF FM Transmitter pact design with easy access to major Design and function NR410T1 [1] was thoroughly rede- components, higher MTBF and opera- signed to create the new transmitters tion up to a VSWR of 3. They also inte- 10 kW VHF FM Transmitter SR610E1 SR610E1 for 10 kW, SR605E1 for grate new remote control standards (FIG 1) is taken as an example to 5 kW and SR602E1 for 2.5 kW. and feature high flexibility in terms of explain transmitter design and func- Compared to their predecessors they system integration. tion. It comprises the following main are superior in efficiency, in their com- modules (FIG 2): • VHF FM Transmitter SU135 (exciter), Characteristics • four VHF Amplifiers VU320, • 4-way splitter, These VHF transmitters for FM sound • 4:1 combiner, broadcasting operate in the frequency • high-power supply unit with two band 87.5 MHz to 108 MHz and gen- transformers, rectifier block, filtering erate a nominal output power of 10 kW, and four DC converters of 3 kW 5 kW or 2.5 kW into 50 Ω at an each, efficiency of over 60 %. They can trans- • fan assembly. mit AF, RDS ( data system) and SCA (subsidiary channel authorization) The exciter generates a frequency- signals, eg DARC (data radio channel), modulated RF signal in the range and can work in stereo, mono and com- 87.5 MHz to 108 MHz at an RF output posite mode. The power is amplified by power of 20 W. AF, RDS or SCA combined 2.7 kW VHF amplifiers. signals are fed to the exciter as mod- ulation signals. An additional module Operation is menu-guided by means allows digital data according to the bit- of control and display elements on serial AES/EBU protocol to be applied the front panel of the exciter. Remote instead of AF signals. control is performed using an external remote control unit as standard or The 4-way splitter breaks the mod- from a PC via the bitbus interface or ulated RF signal down into four identi- an optional parallel interface. Internal cal RF signals of a power of 5 W each communication in the VHF FM trans- and then applies them to the VHF mitter uses a rugged serial CAN bus amplifiers, which boost the signal to (controller area network [2]). This bus is approx. 2.7 kW. The RF output power an open system, ie there are many sup- is set and controlled by varying the pliers of modules and software, proto- operating voltage of the amplifier out- col specifications are fully documented put stages. The harmonic filters of the

FIG 1 Solid-state 10 kW VHF FM Transmitter and no licencing fees are required [3]. amplifiers provide harmonic attenua- SR610E1 Photo 43 077/3 So systems can very easily be config- tion of typically 76 dB. The four signals

18 News from Rohde & Schwarz Number 159 (1998/III) Articles

CAN bus VHF 4-way amplifier 4:1 splitter combiner

UD1 UR1

CAN bus

UD2 UR2 AF VHF FM exciter RF output CAN bus

Bitbus UD3 UR3

CAN bus Commands Parallel remote Status signals control interface CAN bus (option) UD4 UR4 P Forward power RF absorber UR1 to UR4 UD1 UD2 UD3 UD4 P Reflected power Measurement Differential pressure Fan board Power DC DC DC DC T Inlet air supply CAN bus converter converter converter converter T Outlet air T Absorber

70 V AC 70 V supply

FIG 2 Configuration of 10 kW VHF FM Trans- Power supply efficiency of at least 60% even in faulty mitter SR610E1 operation or operation at less than A three-phase line voltage of 230 V or the nominal power. 400 V (50 Hz or 60 Hz) is required to Andreas Buschke supply the transmitter. The high-power are then summed by the 4:1 combiner supply unit consists of two identical to produce the overall output, ie 10 kW modules for voltage conditioning. If one into 50 Ω in the case of SR610E1. of these modules fails, the other will continue to operate. The four DC con- REFERENCES verters connected to the high-power [1] Seeberger, H.: Transistorized 10 kW VHF FM Transmitter NR410T1. News from Rohde supply unit generate the operating Operation and monitoring & Schwarz (1992) No. 136, pp 15–17 voltages required for the VHF ampli- [2] Lawrenz, W.: CAN Controller Area Net- Transmit frequency, RF output power, fiers. The principle of controlling the work. Hüthig, Heidelberg operating and mode, RF output power via the operating voltage [3] Böttcher, J.: Kriterien der Offenheit. Elektro- thresholds, module parameters, inter- of the power transistors yields an nik 25/1997 faces, system control, etc can be set on the exciter under menu guidance. To monitor and control the VHF FM transmitter, various operating states are collected, evaluated by the exciter and can be displayed, eg differential air pressure, inlet and outlet temperatures as well as excessive absorber tempera- Condensed data of solid-state VHF FM Transmitter SR610E1 ture. Measured values of the exciter, ex- Frequency range 87.5 MHz to 108 MHz citer status (eg operating hours, system Output power 10 kW events), modulation values (eg fre- Power supply 230/400 V (50/60 Hz) quency deviation, AF level), RF output Power consumption with integrated fan 16.6 kW power, reflected power and operating Power factor to DIN 40110 >95 voltage of the individual VHF amplifiers Cooling air 15.5 m3/min are also evaluated and displayed. Operating temperature range –10 °C to 50 °C Monitoring functions are supported by Dimensions (W x H x D) 580 mm x 2026 mm x 1000 mm an error table. If transmitter control Weight 700 kg breaks down, the transmitter switches Reader service card 159/07 to a defined, failsafe state.

News from Rohde & Schwarz Number 159 (1998/III) 19