BBC BROWN BOVERI SK 53

GENERAL DESCRIPTION

250 kW Short-Wave Broadcast Transmitters
Type SK 53 C3-2 / SK 53 C3-2P
Type SK 53 F3-2 / SK 53 F3-2P

photo BBC Brown Boveri SK 53

Introduction

The current BBC Brown Boveri 250 kW short-wave transmitter is available in four versions offering maximum adaptability to the wishes and requirements of the user. Flexibility in service is further increased by options such as high-power SSB service, DCC (dynamic carrier control) operation, etc.

The four available versions allow a choice between

fully automatic or motorized manual tuning,
pulse step or standard Class B modulation.

Thus a fully automatic transmitter equipped with a Pulse Step Modulator (PSM) is designated SK 53 C3-2P whereas with a class B modulator it is SK 53 C3-2. When a motorized manually tuned transmitter is involved, the 'C3' in the designation is replaced by 'F3', i.e. SK 53 F3-2P and SK 53 F3-2.

The description which follows covers the features common to all versions supplemented by paragraphs describing special features of the individual versions.

Main features

transistorized preliminary RF and AF stages
choice of PSM or Class B modulation
only two tubes (with PSM) or four (with Class B)
all rectifiers with solid state elements
choice of automatic or manual motorized tuning
unlimited choice of frequency in automatic tuning mode in broadcast bands; on special request outside bands
frequency change time maximum 60 seconds for automatic version; approximately 3 minutes for manual motorized version
sinusoidal and trapezoidal modulation
possibility of high-power SSB service using PSM
easy provision of DCC
protective and automatic sequence circuits ensure safety of the equipment
personnel security system fulfills IEC 215-1
very simple layout of RF circuits ensures easy accessibility to components through doors at front and rear
very high overall efficiency

AF stages and modulator

a. Pulse-Step-Modulator: The transmitter is available equipped with a BBC pulse-step-modulator. This is a solid-state switching amplifier which delivers the AF modulated anode voltage to the RF final stage and in doing so attains a peak power of approximately 1400 kW at 28 kV. It consists of a number of solid-state amplifier stages whose output voltages are added in a summating network. The number of amplifier stages switched on depends on the output voltage required. Thus for a modulation degree of 100 %, all stages are switched on at the modulation peak and all are switched off at the modulation trough. A superimposed low amplitude PDM together with a low-pass filter between the summating network and the RF section of the transmitter smooth out the stepped form of the modulating voltage.

The amplifier stages are controlled by scanning the AF input signal with a frequency of 100 kHz followed by digitalization. A logic module then determines how many amplifier stages are to be switched on or off.

The voltage supply for each stage is taken from a separating transformer with a number of secondary windings. This supply is then rectified in a six-pulse rectifier and smoothed by a large capacitor.

The principal advantages of the PSM system are:

high overall efficiency of the complete transmitter
no costs for replacement of modulator tubes
possibility of high-power SSB operation
possibility of dynamic carrier control
elimination of need for crow-bar protection

b. Class B Modulator: The transmitter may be supplied with a classical Class B modulator whose efficiency has been improved by the addition of a compensator for non-linearity of the final tubes. All stages are transistorized up to the output which is equipped with two CQK 50-2 condensed-vapour cooled ceramic tetrodes. These two tubes modulate the RF output stage via the classical arrangement of modulation transformer, choke and capacitor. Overall negative feedback ensures that harmonic distortion and hum remain within acceptable limits.

Radio frequency circuits

A decadic frequency synthesizer with remote control is required as the driver for the automatic C3 transmitter. Such a synthesizer without remote control can also be used with the F3 versions or may be replaced by one or more crystal oscillators.

The RF voltage (1 V on 50 ohms) delivered by the oscillator or synthesizer is amplified in a broadband transistorized amplifier which covers the frequency range 2-27 MHz, has an overall gain of 40 dB and can deliver 10 watts output power to the grid of the BBC air-cooled ceramic tetrode driver CQL 2. The circuit design is such that tuning of the COL 2 grid circuit is unnecessary and no grid current is taken from the tube. The impedances between the plate circuit of the CQL-2 driver tube and the grid circuit of the final tube are matched by means of a tunable Pi-section.

The output circuit uses a BBC ceramic tetrode CQK 350-1 which is condensed-vapour cooled and like the driver tube it operates without grid current. Its plate-tuned circuit consists of a Pi-section which is inductance-coupled to two further Pi-sections acting as a harmonic filter and having a 50 or 75 ohm output impedance.

If a 300 ohm balanced output is required the 75 ohm impedance is suitably converted by a balun, composed of a motorized variable capacitor and two 150 ohm coaxial line sections.

Tuning system

1. C3 Versions

The transmitter is designed for unattended operation up to the point of inclusion in a totally automatic and computer-controlled short-wave transmitting centre.

Tuning and matching is carried out by means of positioning drive motors which are fitted to all continuously variable elements and range switches. Phase and load discriminators take care of the fine tuning and load matching.

The fundamental control circuit includes an electronic memory, which may be pre-set locally or remotely to any desired frequency. At the desired moment the transmitter is then automatically tuned to this frequency.

The frequency selection and the command to tune can be given locally, remotely or in accordance with the computer programme.

The fundamental circuit also contains a memory in which 100 frequencies may be stored. Selection of any one of these requires only the call-up of the corresponding memory and depression of a frequency change button. However, frequency selection is not limited to the frequencies thus stored.

a. Stored frequency

To change to an already stored frequency, one selects the corresponding store (two digits). The frequency appears in a display window. The command to change frequency is given by depressing a push-button. This command is executed in less than 60 seconds.

b. Unstored frequency

A store is chosen by touch-dialling two digits. If it is already full, the stored frequency is cancelled by a push-button. The desired frequency (7-digits) is touch-dialled and is put into the empty store by depressing another push-button. All this occurs without interrupting the operation of the transmitter. When the moment to change to the newly stored frequency arrives, the procedure is as described under a. above.

Facilities for manually overriding the automatic system are provided so that under all circumstances the transmitter may be tuned and controlled.

2. F3 Versions

To change frequency a band switch is turned to the desired band. Coarse tuning follows automatically.

A selector is then switched to each of eight flip-flop switches consecutively. By moving each switch to left or right the correct position of the corresponding motorized element (coil or capacitor) is set by observing a number (000 to 999) on a LED display. Once all eight elements are correctly positioned depression of a push-button brings the transmitter 'on air' on the new frequency. The time required is approximately 3 minutes.

Power supply

The transmitter receives its power from a medium-voltage network (3-24 kV). An SF₆ circuit-breaker is used at the mains input to control the feed to the separating transformers of the PSM system or to the HT rectifier transformer of the Class B version. Rectification and smoothing takes place in the PSM switching stages themselves. For class B operation a six-pulse thyristor controlled rectifier is used with an associated LC smoothing circuit.

Low-tension (380 V) 3-phase supply for the auxiliary services is taken from a step-down transformer.

Control and safety system

A solid-state control system carries out the operations of starting up, closing down and supervising the transmitter, by local or remote-control. The interlocking circuits of the system are such that it is impossible to run-up the transmitter or its units in any but the correct sequence. All information is monitored on a control panel.

Protective circuits supervise the main criteria during operation. A quick-acting automatic switching and safety system gives full protection against over-current, under-voltage, overloading and mistakes in operation. All vital circuits are monitored constantly. When disturbances occur, the HT is interrupted for a short period and then twice re-applied (so-called three-shot system). If the fault persists the transmitter is definitely switched-off.

An RF directional coupler with reflectometer in the output of the transmitter serves as protective device against excessive VSWR and also supervises and indicates the output power. A special anode dissipation measuring device protects the final RF tube against dissipation overload.

The personnel safety system has been designed to fulfill IEC Publication 215. It comprises:

a. Electric door contacts.

b. Door-key interlock panel which ensures that a door giving access to dangerous voltages can be opened only when an earthing switch for all high-voltages has been operated.

c. Earthing hooks in all compartments. The earthing switch discharges all high-tension capacitors and earths all high-tension points in the transmitter and HV elements. The safety system is integrated in the interlocking system and its condition is signalled by lamps on the control panel.

Physical layout

The RF circuits and the control system of the 250 kW shortwave transmitter are contained in a double BBC MNS cabinet of total dimensions 3800 x 1600 x 2120 mm. This cabinet is extended by 800 mm in length when a balun is added to give a 300 ohm balanced output. The cabinet is completely assembled and wired in the factory, thereby obviating the time-consuming installation of components at site. The remaining parts of the transmitter including either a pulse step or Class B modulator are placed behind the main cabinet. A total floor area of 6.5 x 7.5 m = 49 m2 is required for a Class B installation while the corresponding figures for a PSM transmitter are 8.3 x 8.3 m = 68.9 m2.

Cooling for the power tubes is by condensed-vapour system and the available water is used for cooling the coils in the final RF stage and the GTO thyristors in the PSM unit. The corresponding water-to-air heat exchanger can be conveniently mounted on the roof of the building. General ventilation of the cabinets is provided by a low-pressure blower while a small blower inside the cabinet delivers high-pressure air for direct cooling of contacts and tube sockets.

All elements required to start up and tune the transmitter are accessible on the front of the main cabinet on 19-inch control and supervision racks. Space is provided to take additional instruments or devices desired by a customer.

All coils, capacitors and switches, which have to be varied to tune or adjust the transmitter to a desired frequency are motor driven.

TECHNICAL SPECIFICATIONS (Transmitters type SK 53 C3-2P and SK 53 F3-2P)
Carrier power output	250 kW over full frequency range
Frequency range	5.95-21.85 MHz 3.9-26.1 MHz on request Broadcast bands ITU Radio Regulations, Geneva 1982
Output impedance	50 ohms unbalanced 300 ohms balanced (with balun)
Permissible standing-wave ratio (SWR)	1:2.0 at 50 ohm output 1:1.7 at 300 ohm output
Harmonic radiation	The harmonic radiation meets the requirements of the Radio Regulations, Geneva 1982
Modulation capability	For normal programme service with frequencies between 50 and 5000 Hz with sine-wave modulation Continuous operation with 100% peaks
AF response at nominal output power with m = 70% and referred to f = 1000 Hz	with f between 60 and 3000 Hz +1 to -1 dB with f between 50 and 5000 Hz +1 to -3 dB
AF distortion	at nominal output power, m = 90% and f between 50 and 4500 Hz the distortion factor does not exceed 3.5%
Carrier noise level below m = 100%	f = 1000 Hz unweighted: -55 dB
Carrier shift	with sudden application or removal of 100% modulation the variation in the carrier voltage will not exceed 5%
AF input level	for m = 100% modulation across 600 ohms balanced input impedance: 0 dBm - 20 dBm Recommended 18 dBm
Mains supply	3-24 kV Permissible voltage fluctuation: ±10% Mains frequency and tolerance (other voltages and frequencies if required): 50 Hz ±2 Hz
Mains power factor	not less than 0.95
Overall efficiency (including standard cooling system) a. at 0% modulation b. at 30% modulation c. at 100% modulation d. in programme service	3.9-15 MHz 71% 71% 71% 71%	15-26.1 MHz 70% 70% 70% 70%
Required time for frequency change	60 s maximum (C3 version) 3 min approx. (F3 version)
Specifications may change without notice

TUBE COMPLEMENT (Transmitters type SK 53 C3-2P and SK 53 F3-2P)
RF stages		AF stages and modulator
Number	Type	Number	Type
1	CQK 350-1
1	CQL 2-1

TECHNICAL SPECIFICATIONS (Transmitters type SK 53 C3-2 and SK 53 F3-2)
Carrier power output	250 kW over full frequency range
Frequency range	5.95-21.85 MHz 3.9-26.1 MHz on request Broadcast bands ITU Radio Regulations, Geneva 1982
Output impedance	50 ohms unbalanced 300 ohms balanced (with balun)
Permissible standing-wave ratio (SWR)	1:2.0 at 50 ohm output 1:1.7 at 300 ohm output
Harmonic radiation	The harmonic radiation meets the requirements of the Radio Regulations, Geneva 1982
Modulation capability	For normal programme service with frequencies between 50 and 10 000 Hz with sine-wave modulation Continuous operation with 100% peaks
AF response at nominal output power with m = 70% and referred to f = 1000 Hz	with f between 60 and 7000 Hz +0.5 to -1 dB with f between 50 and 10 000 Hz +1 to -2 dB
AF distortion	at nominal output power, m = 90% and f between 100 and 5000 Hz the distortion factor does not exceed 3%
Carrier noise level below m = 100%	f = 1000 Hz unweighted: -55 dB
Carrier shift	with sudden application or removal of 100% modulation the variation in the carrier voltage will not exceed 5%
AF input level	for m = 100% modulation across 600 ohms balanced input impedance: 0 dBm - 20 dBm Recommended 18 dBm
Mains supply	3-24 kV Permissible voltage fluctuation: with AVR 10%; without AVR 5% Mains frequency and tolerance (other voltages and frequencies if required): 50 Hz ±2 Hz
Mains power factor	not less than 0.9
Overall efficiency (including standard cooling system) a. at 0% modulation b. at 30% modulation c. at 100% modulation d. in programme service	3.9-15 MHz 71% 63% 61% 68-69%	15-26.1 MHz 70% 62% 60% 66-68%
Required time for frequency change	60 s maximum (C3 version) 3 min approx. (F3 version)
Specifications may change without notice

TUBE COMPLEMENT (Transmitters type SK 53 C3-2 and SK 53 F3-2)
RF stages		AF stages and modulator
Number	Type	Number	Type
1	CQK 350-1	2	CQK 50-2
1	CQL 2-1

THIS TYPE OF TRANSMITTER IS INSTALLED IN THE FOLLOWING COUNTRIES
	ITU	Country	ITU	Country
	AUS	AUSTRALIA	BEL	BELGIUM
	B	BRAZIL	CAN	CANADA
	EGY	EGYPT	FIN	FINLAND
	IND	INDIA	IRN	IRAN
	IRQ	IRAQ	KRE	KOREA NORTH
	KOR	KOREA SOUTH	KWT	KUWAIT
	MLA	MALAYSIA	HOL	NETHERLANDS
	NIG	NIGERIA	PHL	PHILIPPINES
	POR	PORTUGAL	RRW	RWANDA
	SRL	SIERRA LEONE	SNG	SINGAPORE
	AFS	SOUTH AFRICA	CLN	SRI LANKA
	SUI	SWITZERLAND	TWN	TAIWAN
	TUR	TURKEY	UGA	UGANDA
	USA	USA