Technical Articles:
Comment? email to:
The
XRF-286 is a great 26v LDMOS part for use on 1296. The amplifier
components used in the 600w project
were developed when I came across some surplus devices; unfortunately, they are
no longer made by Motorola, but can still be found in PCS amplifiers. A link to
a copy of the data sheet is here.
I have a limited supply of these parts, harvested from surplus Spectrian PCS amplifier boards. They have been remounted to more robust heat spreaders, fully tested, and will be included with the two experimenter kits (75w and 150w) that will be offered in late August this year ('08). The 75w kit should be a good match for a 3w transverter, and the 150w kit for a 10w radio. Please check the kit section for availability at that time.
On the web, one can find amplifier designs developed by F1ANH and F6DRO, implemented on FR4. The basic design is very good, though I did make some changes to the bias circuit and the input and output matching transformers. And since FR4 can be problematic, I also ported it over to two low-loss substrates: Rogers 3006 for the prototypes, and Rogers 4003c for the later versions, which are the versions in the kits. Typical performance results for the single, and the double-device amplifiers are shown below.
This is one of the 2-device prototypes implemented on 4003. The performance
of this amplifier is shown at upper left.
It's basically two of the single-device designs, combined using a pair of branch-line hybrid couplers, all etched onto the same substrate.
The hybrid couplers offer isolation and protection for the individual amplifier units; if one should fail, the other is protected by the 100w termination at the dump port of the output coupler.
If you look at the input matching stub on each device, you'll see some rectangular "snowflake" trimmer stubs. Those are there to fine-tune the input match. However, in this case, a small 1-3pf high-Q trimmer capacitor was used instead of the stubs. It's up to the individual builder, but I find it easier to adjust the trimmer than to fuss with the stubs; one can do it either way.
The board is mounted onto the heat spreader shelves using the 4-40 screws
shown, and spaced above the heat sink using 1/4" aluminum bar stock spacers. The
100w termination also requires a 3/16" spacer for proper positioning, and these
parts will be in the kits. Here are the drilling templates for the
heat sink and the
spacers. The schematic is
shown below. Additional construction information will be available soon.
