E-mail: Chuck, WB6IGPP
This month I would have liked to cover the synthesizer that I used for the 1296 Mhz transverter that I presented last month. However I haven't completed the conversion documentation in sufficient detail. I have on the other hand made the unit function as described with in this application but notes and other details must be re-confirmed as to accuracy. Will get the details out to you as soon as possible and the local oscillator side of the synthesizer is concerned.
In the continuing endeavor I would like to cover some of
the tuning procedures in greater detail, specifically the
methods that Kerry N6IZW and I use in converting micro
stripline circuitry. These procedure were developed by Kerry
and are presented in a general format application basis
suitable to all microwave bands of interest. Follows is the
text from Kerry's paper on Re-tuning Surplus Microwave
Microstrip Circuits For The Amateur Bands.
In recent years, considerable quantities of surplus
microwave equipment have become available to the amateur
community. The advantages of adapting surplus equipment
rather that building from scratch often include, cost, time
and performance. The cost for a typical piece of microwave
surplus purchase by an astute Amateur will be far less that
the new cost for a few key components. The time saved is
often in terms of many hours otherwise spent in locating
components, fabricating boards, and building enclosures.
The performance obtainable is usually quite good due to
the commercial processes used to fabricate the units in
quantity. Most surplus microwave equipment is tuned for
commercial or military frequencies making them unusable on
Amateur frequencies without modification. This fact is what
causes the same piece of equipment to b so much junk for one
person and a treasure for someone equipped for retuning.
Microwave circuit fabrication techniques have progressed from waveguide to microstrip printed circuit boards to ceramic substrate hybrids to large scale monolithic integrated circuits. Wave guide type equipment still finds favor among those beginning on the amateur bands at 5.7 Ghz and above but microstrip circuits are prevalent to those building medium to high performance amateur equipment through 24 Ghz. The good news is that the microstrip surplus is typically available due to the industry moving into higher and higher integration technologies such as the hybrid or MMIC.
The bad news is that the newer technology equipment often contains integrated components which are internally matched or tuned for a specific frequency and are not practical for most amateurs to modify. An example of a non modifiable piece of surplus is the typical VSAT terminals becoming available. These units contain >1 watt transmitters at 14 Ghz and low noise receivers around 12 Ghz but are so highly integrated that very few of the microwave components are usable for amateur purposes.
It may be that the conversion of microwave surplus equipment may eventually dwindle as the modifiable technologies disappear but this should not be a problem for some years to come. Another good side of the newer technologies is that they provide high performance with low cost and ease of application for those assembling their own microwave circuits boards using new components.
My experience has been that the current limited supply when properly adjusted will prevent device damage should the gate bias become accidentally removed during the tuning process. Always remove power when making connections and soldering tuning stubs. Make sure the amplifier output is terminated before applying power. The power supply output should be connected to earth ground.
Everything must be grounded to power (earth) ground including the soldering iron tip. The typical FET's in microwave amplifiers will self destruct with more than 5-10 volts on the gate.
Apply only as much input RF power as required to get usable output measurement. This reduces the chance of damage to higher power devices prior to getting the output matched. Also this prevents saturation of a stage which then appears to not respond to tuning. Applying more than about +10 Dbm directly to small FET's may cause damage.
Use current limited power supplies set to limit slightly above normal expected operating current. This will in most cases prevent blowing up the FET's if the negative gate bias is missing or something is accidentally shorted with the tuning wand. With this approach, sequencing of the power supplies is not usually important.
Place attenuators directly at the input and output of the amplifier. This removes the effect of poor cable, source and power detector matching. Always remove power when making connections and soldering tuning stubs. Make sure amplifier output is terminated before applying power.
Prepare tuning wand and tuning stub material. Cut about 1" or 2" strips of .080 wide (not critical) of about the same width as the main 50 Ohm microstrip lines in the amplifier from thin copper or brass stock (.003" to .010"). Tin both sides of the strips and flick off excess solder. Make several tuning wands by cutting one end of a wooden toothpick square at the largest diameter. Using superglue, attach a square (.0 80" x .080") of the prepared tinned copper or brass to the cur end of the toothpick. wipe off excess glue from the exposed side of the square and let dry.
Remove existing tuning stubs. Use a Exacto (tm) knife, make a deep enough cut to disconnect tuning stubs from the main 50 Ohm line. Be careful not to cut the thin brass lines. If you are unsure of possible damage to the bias lines, carefully check continuity or use a magnifier to do visual inspection before applying power. In some cases it pays to go through the agony of removing the stub completely as the correct new stub placement may overlap and cause problems.
Connect the amplifier to signal source, attenuators, power detector and power supplies. turn on power and adjust input attenuation for as low of input as can be readily detected on output. Start at output and slide tuning wand along (in contact with) the main 50 Ohm line watching for an increase in output. Note the maximum output reading obtained with the wand. Remove power and solder a square of the prepared material in the same position as noted by the wand.
Do not add solder. The tinning is normally sufficient to allow the new tuning stub to be held in place with the pointed end of a toothpick and than just touched with the soldering iron to reflow the solder. Turn on power and verify that the output is as high or higher than obtained with the wand. Move the tuning stub if required to obtain results equal to or better that the wand.
Slide the wand over the previously attached new stub and if improvement can be made, attach another square. Continue this for the entire length of the main 50 Ohm line until no further improvement is found. Increase the input power if working with a power amplifier and retune the output stage for maximum power. Be careful here so as not to mismatch the output so bad with the wand as to damage the FET.
The process can be very slow with some stubs only gaining a fraction of a dB. In most cases it will take all of those small increases to get good results so don't expect to see major improvement necessarily with a few stubs. It may take 4 stubs per stage sometimes to get the maximum output.
Direct grounding of a microwave power device emitter/source is essential for proper operation. Any form of insulating grease can prevent full gain/power output form being obtained.
When mounting boards into enclosures, care must be taken to ensure that the entire perimeter of the board is connected to the enclosure to prevent oscillations.
For more information and supplies: Chuck, WB6IGP
For comments: Rein, W6/PA0ZN