There are three parts to be dealt with:
Standart conversion.
There are a lot of COTS products doing this job very well with prices starting from a hundred dollars.
However, we found it cheaper and simpler to use video drivers performing quite well. These drivers simply change the line and sync frequencies of VGA to CCIR or NTSC. This one is freeware : VGA2TV06.ZIP.
Other drivers can be found such as GAME ZAPPER or VIP50 but they are limited to NTSC.
The drivers are operationnal on DOS and W31 but are quite tricky with some apps. For the purpose of pattern generation, they suit well to our need.
(VGA line and frame 31250,60 ; CCIR 15625,50 ; NTSC 15750,60).
Here are the diagram, the p.c.b.
and part location of a small VGA signals to composite video adaptator.
It's B&W only!!!!. (but for that price ......)
The patterns have been created with Paint Shop Pro in 640*480 then converted to .exe with Graphic Work Shop to be easily spawned from the main program.
There was a need for a very good front end selectivity, high gain, and unconditionnal stability in the temperature range of -10 to 50 deg c.
The diagram and pcb
gave the predicted results. the gain is 31.5 dB and amplitude/freq curve is satisfactory. CURVE
The input circuit is made of epoxy stripline (not microstrip), the picture is self explanatory.
If you wish to have a closeup view, send me a note.
The filter is designed to give additionnal filtering after the LNA. The drawing is simple and the picture shows pretty well the mechanical layout. As is, the insertion loss is 0.6 dB and the tuning not critical at all. The filter has been designed with a program published in HAM RADIO a lot a years ago translated in AMSTRAD basic. (if somebody wants to translate it in QBASIC : welcome!).
The photos (438
/1249
) are self explaining. Drawings (1Mo). Measured gain is around 11 dbi. Horizontal main lobe width is 50deg at -6dB for the 1249MHz antennas and 60deg at -6dB for the 438.5MHz antenna. Side lobes levels are under -22dB.
The antennas have been made by F1CJQ
The repeater power supply is made of two separate parts. The first one, transformer, diode bridge, filtering capacitors is located in the rack containing the P.C. The 12.5v regulator is located inside the repeater box to cope with the DC lines losses. The temperature requirements where the same as the other repeater parts. This regulator is able to supply current in excess of 13 A under 12.5V with an input voltage as low as 15.5V in the -10deg 40deg celsius temp range. The schematic given here satisfies the requirements. Output voltage vary between 12.3V for 13A and 15.5V input voltage and 12.6V no load.
A 5V input voltage controls the tx relay.
There are two of them feeding one antenna each. The PA are built around MITSUBISHI hybrid circuits M57762.
The temp requirements where also the same as the other parts of the repeater. The cooling path has been designed to keep sufficiently low temp on the hybrids during sustained periods of transmission in hot summer directly under the sun. This has been achieved as follows :
Firstly, no attempt has been made to overdrive the hybrids, they are fed 'small pitch' and deliver only 14W.
Secondly, thermal transfer has been carefully designed by mounting the hybrids in an Al U 10cm large and 5mm thick. The mating faces of the hybrid and the U have been adjusted together using polishing paste. The same has been done between the Al U and the Al case of the repeater. The external radiating face of the repeater has been north oriented.
The power dissipated in the repeater case amounts to about 195w of which 95 are for the hybrids alone.
The photos show the mechanical layout. A 48deg cent. thermo switch completes the protection of the hybrids.
The schematic and pcb artwork are quite simple. Attention has been paid to RF decoupling paths.
For more infos, send me your 'TAG':
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