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
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
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 minimise 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.
Figures.
Fig.1. GB3TM DATV DVB-S Transmitter
Fig.2. Encoder & Modulator
Fig.3. Encoder, Modulator and Up-converter in the upper
19 inch Rack Frame
Fig.4. Up-converter. Input 437.25MHz to Output1316MHz
Fig.5. Front Panel of Main Power Amplifier
Fig.6. Driver Amplifier M68719, with box cover
removed
Fig.7. P.A. Power Module RA18H1213G on Heat Sink
(Thermal switch to right)
Fig.8. DTMF Decoder and Switching unit
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
Photographs
Figs. 1 to 7, 10
&11 by GW3JGA
Figs. 8, 9 by
GW8PBX