Hyperfix Transmitter
The picture above shows the Racal Hyperfix transmitter station equipment. The three units are, (top to bottom):
- 90512 Power Amplifier/Battery Charger (90512/2 later version)
- 90515 Receiver-Control Unit (90568: later version)
- 90516 Antenna Tuning Unit (ATU)
90512 RF Power Amplifier/Battery Charger (PA/BC)
The receiver-control unit generates RF drive at a low level, consequently it requires power amplification before it is fed to the antenna. The RF PA section of the 90512 amplifies the RF drive signal to either 3 watts, 12 watts, or 50 watts as required. Where the primary power source of the station is the AC mains supply and battery backup is employed, the batteries can be kept continuously 'float' charged by the battery charger section of the 90512. All functions of the unit are under control and supervision of a built-in microprocessor, which is able to communicate with the microprocessor in the receiver/control unit.
The receiver/control unit (Rx/CU) provides 4 inputs to the PA/BC unit, these are:
- Low-level RF drive at either the f1 or f2 chain frequencies
- A logic signal which indicates which frequency is in use either for transmit or receive
- A logic signal which indicates whether the station is in transmit or receive mode
- Communication between the Rx/CU processor and that in the PA/BC
In turn, the PA/BC unit provides the following outputs:
- RF output to the Antenna Tuning Unit
- f1/f2 frequency selection to the Antenna Tuning Unit
- Tx/Rx switching to the Antenna Tuning Unit
Battery Charge function
The mains power supply unit in the 90512 is able provide enough power to run its internal circuits, run the other station units, and also to charge from fully discharged the 28V DC standby battery. A maximum of 200 watts is drawn from a 240v AC mains supply under these conditions.
The unit also provides power distribution to the other units employed on the station, and as such enables a variety of configurations of primary power source. Where the primary power source is the mains supply and a standby battery is provided, no break whatsoever in power supply to the station units will take place when the mains fails. In order to reduce the peak consumption of the unit, the battery charger is switched off during transmit periods.
RF Power Amplifier function
The input from the RX/CU is initially buffered and amplified in a variable gain pre-amplifier using a single VN66AD FET device. The bias of this stage is under control of the microprocessor, and can be used to set the RF power output of the unit. This requires the operator to set the RF power level via the keyboard on the RX/CU, and the information is communicated to the PA/BC over the communication link.
The RF output of the pre-amplifier is fed via a wideband coupling to the push-pull driver stage, which uses VN66AD FET devices. The driver output is coupled via a further wideband transformer to the PA, which uses a DV28120-V device which contains the two power FET devices in one encapsulation. Teh output from this stage is coupled to the antenna output. The PA stage is protected against overvoltage conditions caused by antenna mismatch and also against overheating. Either condition will result in the PA being shut down. The PA stages are only switched on for tyeh period of a transmission. This is to prevent the noise that may be generated in the FETs from affecting the received signal.
As stated above, wideband coupling is employed between all the RF stages. Although the Hyperfix system is specified to work in the badn 1.6 to 2.5 MHz, the bandwidth of the PA unit is at least 1.6 to 3.5 MHz. The output of the PA stage is fed via a substntial RF filter, to reduce the level of harmonics present in the output.
Control Processor
In addition to the internal 'housekeeping' functions and monitoring referred to above, the microprocessor controls the Tx/Rx relay in the Antenna Tuning Unit (ATU), and also the f1/f2 relay which ensures that the antenna is correctly tuned according to the frequency of transmission. The relay is timed to ensure that its contacts are closed before the start of RF power generation, and don't open again until it ceases. The f1/f2 control is also used in the PA/BC unit, as it is possible to set different RF power levels for each frequency.
Construction
The unit is contructed in two parts, with a dividing screen in between. The front panel carries the RF PA assembly, attached to the front panel heatsink. The rear panel carries the power supply unit, again attached to a heatsink. An interface assemblyconsistsof two PCBs, one of which contains the microprocessor.
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Last Update 14/5/05 prev 23/4/05 AC