Current Status: Boards are back and one is assembled. Qualification is proceeding, but
so far the results are excellent. Pics are in the photo gallery.
The board design (done in ExpressPCB's free design software, available
from http://www.expresspcb.com/ and last updated on June 8th, 2005) can be downloaded from here .
I have uploaded the results of the initial
qualification of the filter board. See the following files for details on each band. This data was
gathered using a small 50 ohm non-inductive resistor as the high pass dump resistor, and an HP Vector
Network Analyzer. The VNA output was dumped to a text file then processed in Excel.
We have also run high power tests, with excellent results.
Each filter was subjected to 1500 watts key down solid dits (resulting in a 50% duty cycle) for three
minutes. Any component failure was noted (there were none), and temperature of the cores, caps and
wire was checked. Then for the three low band filters, the high pass filter was subjected to 100
watts key down solid dits for three minutes at two bands above the fundamental (so the 3.5 mHz filter
got 100 watts on 14 mHz). Again, we checked for any issues, and again, we found none. While it is
quite possible that we will find issues in actual contest use, our initial high power tests revealed
Notes on output filters:
We need output filters, or some kind of output filter, because solid state amplifiers
generate a lot of high energy harmonics.
In a tube amplifier, the output impedance of the tube is matched to the antenna (50 ohms nominal)
through a tunable output tank circuit of some sort (usually at pi-L circuit, I think). This does
two things - it does the impendance matching, but it also serves as a fairly narrow
pass-band tuned filter. In that capacity, it knocks down the harmonics generated by the tube to acceptable
But in a solid state amplifier, we
typically use a broad-banded impedance transformer rather than a tuned circuit. That transformer
doesn't have the same band pass filter effect as the tube amps output circuit, and so we have
bad harmonics in our output. We need to get rid of those harmonics if we want to build a clean
amplifier that meets the standards of good engineering practice, never mind FCC regulations!
So in essence we have to options. One, we can build a set of
low pass filters (because the harmonics are always higher in frequency than the fundamental that we
want, we don't need band pass filters) and switch them in using some kind of
(hopefully automated) relay arrangement. Or alternatively, we could build a tuned output circuit
similar to that used in tube amps, and motor drive it to auto-tune.
Now the idea of an auto-tuning output circuit is very attractive. In essence, we would be building
a 50 ohm input, variable output antenna tuner. We could do it with a fairly simple pi-L
circuit design - WC6H has designed a 50 to 1000 to 20-200 ohm circuit using motor driven vacuum variable
capacitors and switched tapped coils. There are, however, a couple of problems. First,
the parts are expensive - expecially the vacuum variable caps. Second, building such a beast requires
both mechanical and electical design skills that are beyond my capability. So at least for now,
I am focusing my energy on the switched filter bank idea.
The design of the filters is one major question, what to do about the third harmonic is the other.
By using a push-pull design for our RF section, we get rid of the
second harmonic. Well, we don't get rid of it, but we knock it down to the point where it doesn't
require special consideration. But the third harmonic, now that is a problem. The third harmonic
in a broad- banded amplifier is going to be around 14 db down worst case (see the
application note for details). 14 db is roughly equal to a factor of 25, which means that
if our fundamental is 1500 watts then our third harmonic is going to be 60 watts or so. That is quite
a bit of power to disappate in a filter.
In Mechanicals of the amp from the front of the QST that included Helge's article,
we can clearly see the mechanical construction of the filters that he used. Each of
the filters uses the same schematic, and the same mechanical construction. In fact, I think he used
the same PCB for each filter, and just adjusted the component values.
We built a set of filters using
Helge's design, and they worked perfectly. Unfortunately, they were not designed to handle 1500
watts of output in a contest situation - and they proved unable to handle that power level. The iron
powder cores and copper wire in the inductors heated up so much during testing that we feared disaster
in a real world situation. Back to the drawing board.
Back in 1999, William Sabin, W0IYH wrote an excellent article in QEX describing what he called a "Diplexer" filter for use in solid state amplifiers. Sabin noted that a standard low
pass filter design has a very high SWR in the stopband. This causes the harmonic output of the amplifier to be reflected back
to the RF section, possibly causing unexpected intermodulation distortion and other issues. More recently, Steve Friis (WM5Z)
designed a set of these diplexer filters, based on the QEX article, for use in his 600 watt EB-104 based solid state amp.
He was very satisfied with the result, and published a very nice web site describing his design. The Project Gamma team decided to adapt this design to 1500 watts for our
purposes. We designed a printed circuit board to hold all six filters, and selected components that
would handle the expected power level. We applied our learning from the first set of filters to this
one to ensure adequate power handling capability.
The ExpressPCB design file for this board.
Pictures detailing the assembly of the filter board.