After constructing 30+ RF decks so far, these are some of the observations I made:
All of the amplifier decks were able to
produce at least 645 watts (saturated) with some of them able to
produce 700w. That's a variation of 0.3 to 0.4 db, and is normal considering
manufacturing differences from one device to another, even within the same
lot. All of the RF decks were able to achieve 600w with no more than 10w
drive. From the NXP data sheet, this is what you can expect from the best of
It should be noted this data was taken with IDQ set to 150ma, which is OK for CW, but too low for linear operation. With that in mind, I set IDQ to 1.5 to 1.8 amps, which produces good linearity.
The IDQ setting affected the input match; the higher the IDQ, the lower in frequency the input match moved. VDD had negligible effect on input match (from 35 to 51v).
Total VDD current drawn at full output varied, with the average around 25 amps (50v VDD). Most drew a bit less than that.
When mounting the deck, use all 8 of the supplied 4-40 machine screws and flat washers, or your metric equivalent (the washers will help distribute the mounting forces). Get them firm, but not so tight you damage the board material, which is soft and thin. Use heat sink compound between the copper spreader and the heat sink...not too thick, just a thin layer will do.
All assembled/tested RF decks have already been set up in the manner described below; no additional adjustments are necessary for operation at 1296 MHz.
The power supply used to set up the amplifier was current limited to 3 amps, and that's important when setting IDQ. If you are doing this yourself with any LDMOS device on any band, and it gets away from you, that current limiting will save the device. Fuses are not fast enough. The termination on the output is a 30w cellular type (no drive power for this test, of course).
While making the IDQ setting at 50V VDD, the input match is observed with a scalar analyzer and optimized at 1296 MHz (where most of us will be using this). The two parameters affecting the input match were IDQ and trimming pads. Usually, only one or two of the input trimming pads were needed with IDQ set between 1.6 and 1.8 amps. The inset below shows the measured optimization.
On the output side, most of the decks required
just one or two of the output trimming pads to be connected.
Both VDD traces must be connected together when routed to your 50v rail. Wire size to each VDD trace should be at least #16 AWG passed twice through a ferrite (Laird 28b0562-100 is recommended).
bias is configured for 12v (30ma current is needed for the bias regulator on board). The resistor you see from the VDD trace to the bias pad drops 50v down to 12v for testing. Normally, the 12v for bias would come from a control board or other source, but can be derived in this way from the 50v rail as long as the 50v is switched off when not transmitting.
The output coax should be able to handle the power; SM250-50 is recommended...this is the conformable 0.25 diameter cable you see in the photo, and is available from www.communication-concepts.com .
The input coax can be most anything with good return loss at 1296; rg316 is not very good for this, so the recommendation is RG402 (conformable), shown in the photo above. This is available on EBay and other places at reasonable cost. .086 or smaller conformable coax is also OK, but usually costs more than RG402.
are a couple of ways to connect to the input and output pads. The first is
by direct solder of the shield to the ground foil, which is difficult to do
because of the high thermal conductivity of the PC board material...the heat
is drawn away very quickly into the copper spreader, making soldering
difficult if not impossible. If you choose to do it this way, the method
shown in this photo works well. Prepare the coax so the center conductor
bends down, but does not quite touch the board when the coax outer is laid
onto the ground foil.
Temporarily remove the 4 screws at the end of the board, but leave the two next to the transistor in place. Slip one or two sheets of paper between the board and the spreader, and solder the shield to the foil, but do not solder down the center conductor at this time. That paper will help keep the heat from being drawn away too quickly into the copper. Remove the paper and replace the mounting screws. Now solder the center conductor in place. This same method can be used for the input coax connection.
This alternate method shows the coax pre-mounted to .020 tin mounting strips, and because I must install coax when I set up the decks, and then remove it, it is the method I use most of the time.
Cut your tin strips to .375 by 1.375 (3/8 by 1 3/8). Mark and drill the hole locations for pass-through (for the screws) by comparing the strip to the board hole pattern.
Prepare your coax and solder the shield to the tin strip, aligned as shown in the photo. Now you install your coax using the two mounting screws at either the input or output, whichever you're working on. Once the assembly is fastened in place, you can solder down the center conductor.