Notes on project Gamma - The RF Deck
Introduction and commentary
One would think, if one didn't
know anything about solid state amplifiers (and I don't, or at least didn't), that this would
be the hard part. In a tube amp, the RF deck is the easy part. There are only a few components (tube,
socket, input choke), they are fairly easy to understand, and quite robust (both
mechanically and electrically). But with a solid state amplifier, this just isn't true. Transistors
are very finicky. Designing a new solid state RF deck is hard. Luckily,
it turns out that you don't have to design a new RF deck! A very capable RF engineer at Motorola
named Helge Granberg did the heavy lifting about 20 years ago, and wrote it all down for
us in a series of application notes and engineering bulletins. Even if you don't want to use Motorola
parts (or, to be more precise MA-COM parts - since they now have the rights to the
old Motorola transistors), you can still use the bias circuitry, board layout and input/output circuitry
from those old Motorola documents.
So the question on the RF deck boils down to a fairly easy set of questions:
- Do you want to use Motorola parts or not?
- If no, which parts are you going to use, and how are you going
to get the boards designed?
- If yes, which part ? The obvious choices are the MRF-150 or the MRF-154.
My sense after some discussion and thought is that the answer to the first question is "yes".
Even though the MRF-150/151/154/157 design is an old one, it still offers the best performance
in several dimensions. First, and perhaps most importantly, these parts have the best IMD characteristics
and linearity of any commonly available HF amplifier. Second, the board designs are done, and tested,
and have been proven in battle. Third, 48 volt DC power supplies are available used a good prices,
which isn's true for the higher voltages required by some alternative parts.
On the third question then, the real issue is this: do you use MRF-150's
(150 watts each, requiring either 12 or 16 transistors for our project), or MRF-154/157's
(600 watts each, requiring 4 for our project)? Despite the obvious benefits of using the MRF-154's
(only two boards and many fewer parts required, and simpler splitter/combiner), the collective wisdom
of the solid state crowd seems to be in favor of the smaller parts. The issue that everyone talks
about is heat. Apparently, the MRF-154 has major heat management issues. You are
making a lot more heat in a small space - getting that heat into a heat sink under contest conditions
is apparently quite difficult. I think that if you are willing to go with a water cooled design,
using the MRF-154 would be ok - but if you want an air cooled design, sticking with the MRF-150's
makes more sense.
There is also a secondary
issue of cost. Because the MRF-154's are much less frequently used, they are much more expensive.
MRF-150's can be had today for $188 for a matched quad (see below). Advanced Power Technology
is about to releast their plug compatible VRF-150 (parts should be available in Q1, 2005),
and they are quoting $38 each in quantity 5000.
MRF-154's on the other hand, are hard to find and priced at over $850 per pair. So even if our
design uses 16 MRF-150's (we may get away with 12, but that is a subject for later), and even
if we pay the current price of $188 per quad, our power transistors will cost $752. Four MRF-154's
will cost us the princely sum of $1,700 - and if we blow one up we will be
Notes and commentary
Communications Concepts makes and sells boards and parts kits based on various Motorola application notes. It appears that
either the board based on the EB104 (600 watts, 4 x MRF150 ) or the board based on AR347 (1000 watts, 2 x MRF154) would be the best bet.
A potentially interesting source for MRF-150's
From: David Gilden [mailto:David.Gilden@rfmw.com]
Sent: Tuesday, January
04, 2005 8:31 AM
We can supply the MRF150 in any fashion you wish.
Singles units are $41.00 each
Matched Pairs are $88.00 each ( 2 transistors)
Matched quads are $188.00 each (4 transistors)
RFMW, Ltd/RF Power Exchange
I had a nice chat with Joel Levine
at RFMW. He urges against the MRF-154/7 on heat management grounds. He also
suggested parts from IxysRF
, which I am looking in to right now. Other potentially interesting transistors
- The 2SC2879
, which is used by lots of folks for 100 watt PA's (e.g. in the Elecraft
K2/100). This part operates at 12 volts, and is quite a bit worse on linearity/IMD than the
Motorola parts are. Also, the idea of 12 volts at 4000 watts is a bit scary.
- The Advanced Power Technology
parts. These are plastic packaged (and thus lower cost) RF transistors. Most interesting I
think is the ARF463A/B, which are 125 VDC parts rated at 100 watts output.
Notes: Sand Labs makes RF boards (and possible power
supplies). http://www.sandlabs.com/pa.htmlunder products/RF