1220-1300 MHz ATV
Notes for Version 3.2 by David Minchin VK5KK, Copyright 1997
The original Version 1 & 2 designs were produced, as kits, to satisfy a limited need for both A5 (AM) and F5 (FM) TV converters for use at 1250 MHz. With the release of the 1270 - 1285 MHz frequencies formerly used in some states for Airport Radar, there are now three VSB A5 and two F5 ( 1250 + 1283 MHz ) TV Channels in the 1240 - 1293 MHz segment. This increase has warranted the complete ground up redesign of the converter to cope with the wider bandwidth as well as improvements in out of band and strong signal handling.
For Kits and/or PCB's please Email me or look for details on the Equipment Supplies Web site.
The converter is a single conversion design using a tuneable 740 - 820 MHz Local oscillator and a standard 480 MHz nominal IF frequency. All components are mounted "trackside". The PCB has plated holes to bond the groundplane areas on both sides. The first RF stage, uses a MGF1302 Gasfet enabling an overall approximate 2 db noise figure. Particular attention, in this design, has been given to rejection of signals below 1 GHz, e.g. an uplink transmitter on 400 MHz in close proximity. The RF stage is followed by a PCB Interdigital filter providing an approximate 80 MHz Passband, far wider than the original ¼ wave filter design. A MAR-6 MMIC amplifier then provides additional RF amplification before the RMS11X Double Balanced Mixer. The mixer is followed by a 3db pad and a 525 MHz Lowpass filter to remove unwanted mixing products and residual local oscillator energy. A MAR3 provides IF amplification to compensate for losses through the previous mixer and attenuator. The typical conversion gain is 27db. Improved design, layout and shielding limits local oscillator feed-through to the IF. LO feedthrough to the IF must be reduced as much as possible to assure proper operation of typical broadband FM TV IFs.
|Input Frequency||1220 - 1300 MHz|
|IF Output||479.25 MHz|
|Noise Figure||2.0db typical @ 1280 MHz|
|Conversion Gain||27db typical*|
|Image/Out of Band signal rejection . . .||Better than 50db|
|Supply Voltage||10-16 VDC|
|Supply Current||80 mA|
|Physical Size||100mm L x 67mm W x 20mm H|
*This gain can be increased, see application notes.
The converter has provision for "power down the coax operation" for masthead mounting. It can also be used as an IF for future 10 GHz ATV down-converters, as per current 12 GHz Satellite DBS technology. The new Version 3.2 has several enhancements with regard to IF filtering /gain as well as mixer drive levels over Version 3.1.
THIS IS AN ADVANCED PROJECT. Before embarking on this project, it is assumed the constructee has built some UHF type projects before and has the ability to work with SMD COMPONENTS.. You will also need access to a Digital or Analogue Multimeter, possibly a 1 GHz frequency counter and a signal source on 1200 MHz. The construction notes have been written, assuming medium level construction skills. If you are not confident of being able to complete this project enlist the help of a local club or amateur with the expertise and/or test equipment!
CONSTRUCTION & TESTING
The PCB is double sided with one side being an uninterrupted groundplane. The PCB is solder plated with "plated through" holes bonding the groundplane to the upperside ground areas. Remember to use the shortest possible lead length when installing all components with the lowest profile above the PCB as possible. Attention must be paid to the correct mounting of the Antenna connector with absolutely no exposed centre pin (e.g. butt soldered directly to the PCB).
After completing the PCB assembly, check the DC resistance across the input/ output of the 78L08, checking that there are no shorts to ground. Connect 13.8 VDC via a Multimeter set to 0-200mA range. Check that the current drain is around ~ 70mA. Check that the 8 Volt rail is within tolerance. Measure the voltage from the output of each MMIC to ground. The MAR6 should read ~3.6V, the MAR3 should read ~5.0V. Check that you have MINUS 5 V between Pin 5 of the 7660 and ground.
If all is Ok, disconnect 13.8V and install the MGF1302. NOTE that the MGF1302 source leads do not go through the PCB. Connect a 50 ohm load to the antenna connection. Connect 13.8 Volts and check for ~1.6 V drop across the 150 ohm resistor near the MGF1302.
Loosely couple a 1 GHz frequency counter to the oscillator output and confirm that the oscillator is working and has a range of approximately 740 - 820 MHz. Alternatively a UHF TV tuned above Channel 60 may be used to monitor the oscillator. Check the orientation of the BFR91A should the oscillator not work.
Connect the IF to a TV set tuned to Channel 20 (478.25 MHz) or use a 480 MHz FM TV IF and connect a ~1250Mhz signal source. Alternatively use an off air signal to confirm operation of the converter. Disconnect the converter and remove all temporary connections. Solder the hobby brass cover over the Oscillator. This cover should not be omitted. Install the PCB in its final enclosure.
Confirm that the converter is stable with the in place. If the converter has been constructed correctly, no sign of instability should be evident. If you do run into trouble Microwave Conductive foam will help kill enclosure resonance. Not everybody can find this stuff, IC conductive foam is a poor substitute (it sought of works...). Would you believe truck tyre rubber has been successfully used as it has a high carbon content. You will also find that if you put a small shield over the gate circuitry of the MGF1302, this will solve any instability due to a poor or no antenna being connected.
If Mast head mounting the Converter, use a suitable waterproof container. If you are powering a masthead mounted Converter via the IF coax, simply feed 13.8 VDC via a RF Choke/ bypass decoupling network at the shack end of the coax. If not using this feature, L7 can be deleted.
If you need to increase the gain of the converter you may substitute a MAR6 inplace of the MAR3 (converter gain will increase 6 db to 33 db) or an ERA3 (converter gain will increase 11db to 38db). This extra gain should only be required where the converter is feeding the IF via lossey coax. The standard converter gain should be ample when used with the current ESC FM IF.
If you want to get the last db of noise figure out of the converter you may replace the input PCB inductor with a small hoop of 0.7mm TCW to about the same dimensions. The Noise figure is then optimised by varying the height of the hoop above the PCB. The measured NF after optimisation was ~1db on a HP8790B.
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