Caution: This RF deck looks
easy to construct, and it can be, but if you do not have extensive experience
working with surface mount parts on high power SSPA's at 23cm, your results
can vary widely. Even if you are an accomplished builder of other things, the
assembled/tested RF deck may be your best choice instead of the kit; it could
save you a lot of time, trouble and expense. |
The assembly instructions are next; please read the entire document, and
carefully
This is what the
properly assembled RF deck will look like before mounting to your heat sink; A
high resolution photo
is here to assist you with parts placement.
The boards in your
kit will may be the ones without solder mask (shown in the next photo), but for the
purposes of placing components in these kit instructions, I'll be using the
green or gold boards as examples because those boards
have markings
with better visibility.
The RF deck assembled from
the kit will look like this one. The solder mask and two of the outer trimming
pads on the input and output have been eliminated, yet both green and gold PC board revisions
perform the same, there is no operational difference at all. The boards are
identical in thickness, material, trace design and copper weight.
OK, let's get
started...
Locate the output
board; referring to the
schematic and the reference designators on the board, install the 6 high
voltage microwave chip capacitors.
The output board is
the shorter one.
If your kit was supplied
with the seven 24pf high frequency RF capacitors instead of the two 24pf ATC
capacitors, use the layout in the next photo for reference:
Because 24pf ATC capacitors are not always available, your kit may be supplied with 24pf high frequency RF capacitors. To get the same current carrying capacity and bypassing performance as the ATC types, double them up in pairs and mount them together laying on their thin sides as shown here.
This will consume four of the seven 24pf capacitors supplied (the remaining 3 are used on the input board later in the assembly procedure).
Note: save the
trimmed-off capacitor leads in the next step, they will be used later.
Trim the leads on
the electrolytic capacitors to 6mm length and bend them as pictured, about 2mm
away from the body of the part. Note the position of the negative lead on the
body of the parts, and the direction of the wire bends on each (they are not bent
the same as one another).
Install them like
this, positive to the bypassed end of the VDD trace, negative to the ground
foil. Leave enough room for the two mounting screws and their flat washers to
be placed at the top of the board; note the position of the negative polarity
marking on the bodies of the parts, they face inward toward the center of the
board.
Now, using a
cut-off piece of one of the capacitor leads you saved
in the previous step, jumper in one of the trimming pads at the output as
pictured here.
How many pads you
will need to jumper in to trim the amplifier output properly is going to depend
on how it plays when tested; starting with one is a good place, I find them to
require as little as none to as many as three. Most of them play best with one
or two (at 1296).
The output board is
finished.
Locate the input
board (the longer one) and install the remaining parts. Checking the schematic and the high resolution photo
will help you put everything in the correct place, and orient the Zener diode
properly.
The input board is
finished.
If you purchased
the copper spreader and then mounted your own LDMOS using the flow-solder
method (see the video here),
make certain the area under the LDMOS tabs is free of flux or other material
which may hinder good electrical contact.
Slip the input
board under the transistor gate tabs. Use it as a lever to bend the tabs upward
a bit as you see it done here. Do the same for the tabs on the drain side.
You probably
already noted there is no room for mounting screws near the transistor tabs;
with high power devices at microwave frequencies, it is very important to get
good reliable contact between the PCB and the copper spreader, so we'll be
arranging a very tight fit as the boards are finally set into place. You won't
be soldering the PC board to the spreader using this procedure, as there are
serviceability drawbacks to that method.
The copper
spreaders I have available are machined with a .055 trench depth, and that
positions the transistor tabs about .005 above the board surface when all is in
place. There is some variation (+- .002 or so) in the machining of the trench,
so we'll need to fill that gap with a small spacer placed between the pcb and
the copper spreader (next step). The spreaders for this RF deck are shipped
with spacer material the correct thickness for the
spreader provided. If your kit was supplied with the PCB clamp system these
spacers will not be supplied with your kit, nor do they need to be installed.
If you are making
your own copper heat spreader from the drawing available here, you can eliminate the need for
these spacers by changing the trench depth from .055 to .060 as long as you can
hold the tolerance to +-.002 (.058 to .062).
Slip the spacer in
place partly under the LDMOS tabs, but not far enough forward to climb up on
the LDMOS footing.
You may need to
insert the PCB at a slight downward angle to get it into place, it's going to
be a tight fit. If you can't get it set into place properly, with the edge of
the pcb in alignment with the edge of the trench, the spacer may have slipped
onto the transistor footing.
Check to see if the
mounting holes are centered where you see the two 3/16 screws and flat washers
installed. If everything looks OK, install those 3/16 x 4-40 mounting screws
with their flat washers.
Install the output
board with it's shim in the
same manner.
Observing proper
ESD safety methods, bend the transistor tabs back into place (level). Using
just your clean fingers, push the tabs flat against the PCB.
Place a drop of
liquid flux at the center edge of all 4 transistor tabs, so it flows a bit
under the tab when it is applied. Not too much, just a drop will do. Solder the
transistor tabs to the PCB as pictured here, making certain of a complete and
even flow as you see it pictured here. Clean up the flux residue with iso
alcohol and a cotton swab.
Once the copper
spreader is mounted to your heat sink, tightening the 8 outer mounting screws
will apply a slightly (and beneficial) higher pressure yet to the
board-to-spreader contact area.
You're all done and
ready to test; here is
the recommended test procedure.
Note: the
installation of a PCB clamp is optional, but
recommended.
Extra mounting screws could not be used for clamping due to the width and positioning of the PCB traces, so a Teflon spacer clamping system was developed. The sheet metal parts of the clamp system are made from flexible aluminum sheet, and will self-adjust to provide the correct pressure during heating/cooling cycles. This PCB clamp can be installed to an existing amplifier in about 5 minutes with only a screwdriver.
All kits, rf decks and 600w 23cm amplifier systems shipped after September 2021 contain the clamping parts. For those purchased prior to that date, a drawing with instructions for making your own clamping parts is shown below.
Here's how to install the clamp: