Construction Notes for the 30W 1296MHz Amplifier by brk18073

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Instructions Rev #4 (Construction Notes)
01/05/07


               Construction Notes for the 30W 1296MHz Amplifier
Amplifier design, parts, and inventory: The amplifier is designed around an Infineon PTF 10021, LDMOS
FET. The device is rated: VDD = 28VDC, IDQ = 360mA, POUT = 30W, Efficiency = 50%, GaindB = 13dB. The final
design and board layout closely follows the „Test Circuit‟ that was suggested by the chip‟s manufacturer.

The schematic of the amplifier is shown in Illustration 1. All the parts required to assemble the amplifier are
included with the exception of the input and output connections [SMA, N-Type, etc], the heat sink, and
enclosure. These parts were not included to allow the builder to customize the amplifier to fit his specific
needs.

It is assumed the builder has experience in assembling SMT parts, including intimate knowledge of Murphy‟s
11th Law of Electronics i.e.: The speed a SMT component disappears across the workbench is directly
proportional to its replacement co$t. The use of de-magnetized tweezers is recommended as some of the
components have a ferrous base and placing the component is a pain if you can‟t let loose of it. To assist you in
eliminating „escaping‟ parts, the components have been affixed to an inventory sheet. It is suggested that you
remove a part from the sheet only when ready to solder it to the PCB.


       ------- Please do not open the FET anti-static bag until instructed to do so -------

Suggested Construction Procedure:

1.     PCB preparation- Using a diagonal cutter; trim away the area where the FET will be positioned. Use a flat
file to remove the PCB material until the pads for the small vias are totally removed; see Figure 1 and 2. Use a
VOM to determine there is no continuity between the upper two large FET pads (GATE/DRAIN) and the lower
clad ground surface.

2.     Heat-sink preparations- The heat sink must dissipate approximately 55W of heat during operation.
The heat sink, Figure 3, used for the photograph is classified as: aluminum, flatback, 5X8”, 0.375” base plate,
24 X 1” fins. Consult the Internet1 and/or E-Bay to obtain proper sink material. Insufficient sinking will allow
an elevated operating temperature, resulting in thermal damage to the FET. 2 If you happen to find a piece of
0.250 aluminum plate the size of the PCB in your junk box, leave it in the junk box- it won‟t dissipate sufficient
heat to protect the FET. The volume of your junk box 0.250” „sink‟ is 9.5 cubic In, the volume of the sink in
Figure 3 is 136 cubic In. Forced air cooling (12V computer muffin fan) can be used to reduce the volume of the
sink, however, there is a risk, fan failure may result in FET failure.

Using the layout dimensions on the engineering drawing, Illustration 2, establish the layout of your heat sink.
Drill and tap 10 #4-40 PCB mounting holes, 8 for the PCB board, and 2 for securing the FET. You may also
desire to include taped holes for the 28V/Gnd standoffs, and other special needs for your application.




         1
             Also see: http://www.communication-concepts.com/heat_sink.htm
         2
             www.electronics-cooling.com/Resources/EC_Articles/JUN95/jun95_01.htm
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There are a number of different ways to mount the PCB/FET/Sink combination. Due to the physical dimensions
of the FET, and the thickness of the PCB, the DRAIN and the GATE tabs do not physically contact their
respective pads on the PCB. Figure 6 shows a possible solution to this problem.


3.   Mounting the parts- Prior to soldering any components on your PCB, it is recommended that you use
rubbing alcohol and a Q-tip to remove any surface oil that may be present. The component silkscreen on the
board will assist you in locating the parts.

L1, consisting of a small piece of brass, is cut to the size of soldering mask which is labeled L1. The actual
dimensions should be 0.1W X 0.2”L (2.5 X 5.0mm) but are not critical. The brass jumper is shown in Figure 5.

L2, a ferrite bead is place as shown in Figure 7. The ends of the wire supplied with the bead need to be
scraped to expose the copper. Once the leads have been tinned with solder, they are soldered approximately
0.25” (7mm) from the end of the VCC rail.

Due to their size, C6 and C10‟s position is not included in the silkscreen (see figure 4). Additionally, C6 and
C10 are polarized so proper indexing is critical. It is recommended these two components be installed last.
When all the components are mounted, except the FET, take a few minutes to double check all connections for
cold joints, solder splashes, correct parts in locations, polarity reversal, etc.


4.    Adjusting the Bias Level - Using a VOM, check the resistance between the board‟s Vcc and GND pad,
it should be greater than 20K. Assuming no shorts were found, the Bias pot can be preset at this time. The
pot is a single 270o rotation pot and is fragile. Use a proper sized screwdriver and a gentle touch. With the
PCB oriented so that the RF IN terminal is on the left, adjust the R1 pot fully CCW (Counter Clockwise). Now
apply a low current (100 mA?) variable voltage supply to the Vcc and GND terminals. Slowly bring up the Vcc
voltage to 8 V. As the input voltage nears 7 or 8V you should note the following:

                The voltage at the output of IC-1 (junction IC1-R1-C11), 7805, to GND. should be 5V.
                The voltage at the GATE of the FET should be near 3.5V.


       ----- Leave the pot in its fully CCW direction until instructed to re-adjust it -----

5.   Assembly of the PCB-FET-Heat sink: Use alcohol and Q-tips to remove any oil film or flux residue
from the top surface of the heat sink. Place the PCB on the sink and temporarily secure it with four mounting
screws. Attach a properly grounded anti-static wrist strap to your wrist. Open the bag containing the FET,
place the FET in its PCB opening making sure the DRAIN (which has a notch) is toward C4 and the PCB‟s output
terminal. Temporarily mount the FET in place using two mounting screws. Align the PCB, FET, and other items.
Solder the GATE and DRAIN tabs at this time. Install the remaining screws and tighten all screws until snug
without stripping the holes.

6.    Tune-Up - If this was a perfect world, you could connect the amplifier to 28 V and squeeze the PTT . . . .
and BINGO the wattmeter would pin out at 30W. However, to avoid the embarrassment of seeing the „smoke
escape‟ it is suggested that the Tune-Up be made in three steps: (1) at 12V which will allow you time to
familiarize yourself with the device, and then if all operates as expected (2) at 24V and „full output power‟, and
lastly for those who demand the maximum….(3) go for it- Vcc =28VDC.

        (1) Preliminary Tune-Up- Prior to connecting any voltage, double check the position of the pot, is it in
the CCW position? Connect a suitable 50 ohm load to the amplifier‟s output. To avoid an impedance error being
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reflected back into the DRAIN, the load should ideally be a 50 ohm resistive rated at 30+ watts at a minimum
of 1296 MHz. Connect a signal generator, or 50 ohm impedance to the input. Apply 12V to the Vcc and GND
terminal. The IDQ (Current, Drain, Quiescent) should now be increased by SLOWLY turning the bias pot CW
monitoring the IDQ and VG . You should be able to ramp IDQ upward stopping at 200 mA. Apply +10 dBm of drive
at 1296 MHz, the IDQ should increase as the RF drive is applied, some RF output should be noted.

         (2) Final Tune-Up- Assuming an output signal was obtained in the above preliminary tune-up, turn off
the RF drive and 12V to the amplifier. ***** Return the Bias pot to the fully CCW position-----
Apply 24V to the Vcc and GND. With no RF drive, SLOWLY advance the bias pot until the I DQ of 300 – 360 mA
is obtained. The FET is rated at IDQ = 360mA Max. While monitoring the P OUT, increase the RF input drive to a
maximum of 33 dBm., the output should be approximately 25-32W, 44-45dB. The current should be about 2A
at 24 VDC.

          (3) Final Tune-Up PLUS one  - At this point some of you may be asking, “Why are we only operating
the Vcc at 24VDC, isn‟t the FET rated at 28V?” Yes this is correct, and if the amp is operating properly in
your estimation, go for the 28VDC. However, when you observe the P O you will see there is only a modest
increase when pushing the device to 28V. The specification sheet on the FET predicts 29W at 24V and 33W
at 28V (less than 1dB).


Warning: Even though the FET is rated at VD = 60V , keep in mind that when operating the FET at 30VDQ,
the VD swing will be from 0V to 60V resulting in device failure. Do not exceed 28VDQ.

Wrap Up: Mount the amp in an enclosure of your choice and start microwaving the ether. 
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Parts Provided-

C1               ATC 100B 33 pF                L1      Strip thin brass Approx .1 X .2” (2.5 X 5.0mm)
C2               ATC 100B 1.2 pF               L2      Ferrite Bead, J.W. Miller FB64
C3               ATC 100B 0.5 pF               IC1     7805 Voltage Regulator
C4               ATC 100B 33 pF                R1      10K Adjustable
C5               ATC 100B 33 pF                R2      2.2 ohm
C6, 10           10 uF                         R3      24.9K ohm
C7               0.1 uF                        Q1      PTF10021 FET
C8               ATC 100B 33 pF                        PC board
C9, 11, 12       0.1 uF
C13              68 uFd

Note: C1-C5 are special microwave capacitors and should not be substituted. C13 is a simple filter and can be
50 or 100 uFd. R3 can be 25K




Suggested tools and hardware needed-

Drill, drill bits and taps
Flat file
Small side cutters
Small insulated screwdriver or tuning tool
Anti-static wrist strap
Grounded soldering iron and rosin core solder
Tweezers, Q-tips and rubbing alcohol (Jack Daniels not recommended)
Signal source with output up-to 4W at 1296 MHz
50 Ohm load (50W recommended)
Power meter with attenuator rated to 50W
Adjustable power supply with current limiting (0-28VDC, 0-3A)
18 AWG hookup wire (for connecting amp to power supply)
Screwdrivers for the pc board and RF connectors mounting screws
4-40 Pan head screws (10 required)
RF connectors (2 required) and mounting screws (Users choice)

								
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