GB3TM Digital
 

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GB3TM adds Digital Transmit                            by John Lawrence GW3JGA

 

GB3TM came on air in July 1994  and has remained in continuous operation on 23cms FM (1249MHz in, 1316MHz out) for a period of over 15 years. There have been a couple small failures during that time, a capacitor and a fan.

 

Members of the Arfon Repeater Group decided early in 2009 to add a digital ATV transmit function to the existing analogue transmitter at GB3TM. As it is reasonable for TV amateurs to purchase a relatively inexpensive DVB-S receiver (e.g. Maplin A94FJ  Comag SL25/12 or SL30/12) but not an expensive DATV transmitter, it was decided to just add a digital transmitter at this stage, whilst retaining the existing analogue repeater unchanged.

 

Independent DVB-S Transmitter

 

Therefore, the intention was to install an independent digital transmitter alongside the existing analogue one and feed the same incoming video and audio to both transmitters. Because of licensing regulations, both transmitters would have to be on the same frequency. Changeover from one transmitter to the other would be controlled remotely by a DTMF tone on the incoming audio channel.

 

Analogue transmission would be the fallback condition, This would allow suitable captions to be displayed while in analogue beacon mode. The switch-over to digital by a DTMF tone would give 15 minutes of digital transmission after which the transmission would return to analogue. Change to digital or return to analogue could be made at any time by a suitable DTMF tone.

In the final arrangement the two transmitters would be switched by relay to a common transmit aerial. For the initial testing and operation, (February 2010) the digital transmitter would be connected to an independent Alford slot aerial.

 

Technical description

 

                       

 

Fig.1. GB3TM DATV DVB-S Transmitter

 

The GB3TM DATV DVB-S transmitter, shown in Fig.1, is a self-contained Digital Television Transmitter operating on 1316 MHz and based on the DVB-S specification. The continuous output power is 10 W into a load of 50 ohms.

The transmitter is powered from the 230V mains supply and consumes 115 W. It consists of two 19 inch rack, 3U high, units housed in a 19 inch cabinet.

 

Encoder & Modulator

 

The Encoder and Modulator are built on two Eurocard size (100mm x 60mm) PC Boards. The two boards are designed and built by the AGAF organization in Wuppertal in Germany. The boards were originally built into a metal enclosure for portable use. Shown below in Fig.2. Also see CQ-TV221 February 2008, page 9.

 

Fig.2.             

 The encoder accepts a PAL video signal and stereo audio signals. These are digitally encoded and then applied to the modulator.

 The encoder ‘jumper’ settings follow the guidance given in the AGAF instructions.

 JP5     7          CVBS

            8          PAL

            5          3.0 Mbit/s       Data rate

JP9     4          3.375 Mbit/s   Transport stream

JP7     2          QPSK

            3)

            4)         No jumpers   ˝ FEC

            5)

 The output from the modulator is a QPSK modulated signal on 437.25 MHz. This signal is fed to the up-converter. The enclosure is built into a 19 inch, 3U high, rack frame, shown below. This is fitted in the upper part of the rack cabinet.

                        

 FIG.3. Encoder, Modulator and Up-converter in the upper 19 inch Rack Frame

 Up-converter

 The up-converter, shown in Fig.4, is also designed and built by the AGAF organization in Wuppertal in Germany. The input frequency is 437.25 MHz and the output frequency is 1316MHz. The output signal from the up-converter is at +3dBm (2mW) level and is fed to the Driver Amplifier.

 

                       

 Fig.4. Up-converter. Input 437.25MHz to Output1316MHz

  Main Power Amplifier Unit (Lower Enclosure)

 The main power amplifier unit is built into a 19 inch, 3U high rack enclosure which is housed in the base of the main cabinet. It contains both the driver amplifier and the output amplifier, also the power supply for both sections. The Main Power Amplifier is connected to the Up-converter by an SMA - SMA cable

 The main power amplifier unit is fitted with a panel meter and range switch. As shown in below. Circuit is shown in Fig.11.

 

 Fig.5. Front Panel of Main Power Amplifier

 Driver Amplifier

 The driver amplifier consists of a Mitsubishi RF Power Module type M68719.

The module has a gain of 27dB. An input of +3dBm provides an output of +30dBm (1W) at 1316MHz. The Power Module is run at this level to minimize gain drift with temperature. The RF Power Module is housed in a small die-cast box fitted with a heat-sink, shown in Fig.7. This is located inside the main amplifier rack frame.

                        

 Fig.6. Driver Amplifier M68719, with box cover removed  

 Power Amplifier

 The Power Amplifier consists of a Mitsubishi RF Power Module type RA18H1213G (from G.H. Engineering). The module has a gain of about 23dB but the input signal is attenuated (on the PC board) so as to provide an output of +40dBm (10W)

The module is capable of at least 16 W output and is under-run to provide good amplitude linearity. It is fitted to a custom PC board mounted on the inside of the large heat-sink at the back of the amplifier rack frame, shown in Fig.7.

  

                       

 Fig.7. P.A. Power Module RA18H1213G on Heat Sink (Thermal switch to right)

 Power Supply

 The power supply is a modular unit capable of providing 12V at a maximum of 12A. It is built into the main amplifier rack frame. The supplies to the driver and

main amplifier are individually fused at 5A and 10A respectively. The supply to the output amplifier is fed through a ‘transmit’ relay which allows the output stage to be controlled remotely.

 Cooling

 The transmitter has two cooling fans.  A 90mm 12V axial fan mounted in the top of the cabinet. Shown in Fig.1. This is powered from the Driver Unit power supply and is operational when the Driver Unit is switched on.

 A 60mm 12V axial fan is mounted on the main Power Amplifier heat-sink at the rear of the amplifier. This is powered from the 12V supply to the Power Amplifier output stage and functions only when the output stage is operational.

                

For adequate cooling, when the transmitter is operating, both fans must be working. A thermal switch is fitted to the Power Amplifier output stage heat-sink (to the right of the Power Module shown in Fig.8) and this is connected in series with the transmit relay circuit. This switch opens if the heat-sink temperature exceeds 40deg-c and turns off the output stage. The switch closes again when the heat sink temperature has fallen to about 35deg-c.

 

DTMF Decoder and Switching

 The DTMF decoder, timer, test-tone generator, video and audio distribution amplifiers etc are housed in a 19” 1u unit, shown in Fig.9.

 The Decoder uses the usual MT8870D decoder IC and high-speed CMOS is used in the timers and logic control. The test tone generator is described in Circuit Notebook No.103 and the distribution amplifiers use conventional video and audio op-amps. The video switching allows for an external test card or caption (and test tone) to be switched by DTMF and transmitted for test purposes. The system falls back to normal through-video and sound after 1 minute

            

            Fig.8. DTMF Decoder and Switching unit

A general view of the equipment at GB3TM is shown in Fig. 9.

      

 

 

            Fig.9. Equipment Rack at GB3TM

Fig.10. Off-air picture from GB3TM Transmitter under test.

Fig.11. P.A. Circuit Diagram


 

Fig.12. GB3TM DATV Transmitter under test  (Chris – filler picture - optional)

 Design and Construction by GB3TM Technical Group.

David GW8PBX, Brian GW4KAZ, Barry GW8FEY & John GW3JGA.

 

 
   
 

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