RF Deck Information

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 the lot:
  
   

• 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.

Initial testing of the deck (see test setup to the right):

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.
 

Making the connections to the deck

• 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.

• Here 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.

 

 

 

The next page contains an important warning about the presence of flux residue around the output connection

 

 

The output pad has considerable microwave power on it when the amplifier operates. It is absolutely essential to keep this area clean. The area shown in this photo is the output pad and its trimmer pads and is the only area on the boards requiring this special care.

Smoke may be the first warning as flux residue heats up, followed by small sparks and then by a larger arc. Stopping the transmission immediately at the first hint of trouble will prevent damage to the board, giving you time to clean the area with isopropyl alcohol. Flux trapped under the trimming pad jumpers has been reported to cause this kind of problem, so use just a small amount of the no-clean type flux and clean it up afterwards.

This also applies when you solder the output coax to this area; make certain there is no flux in the area between the output pad and the ground plane, or around any of the trimming pads.

Installing the PCB clamp on the RF deck

Some digital EME operators report that repetitive full power heating and cooling (1 minute on, 1 minute off) of the rf deck in these modes will eventually cause the thin PC board material to lift away from the heat spreader just enough to affect the match. This is noticed when the gain and output power drop down significantly from their usual levels, sometimes by as much as a third. Clamping the boards securely to the heat spreader solves this problem and restores normal function to those units affected.

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 semi-rigid aluminum sheet and will self-adjust to provide the correct pressure during heating/cooling cycles. The installation procedure is located here:

https://www.w6pql.com/1296/pcb-clamp.htm