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A design technique for Power Attenuators Without Power Resistors.
By
K0CQ
Techniques for attenuator design have been around for a long time.
When I was a kid, ½ watt resistors were as small as practical, ¼ watt were not even a dream. In my
dad's time resistors were carbon rods with the wire leads wound around the ends, color coded body,
end, dot. And usually 20% tolerance, though by my time 10 and 5% were available. It seems that
carbon composition resistors come in a continuous distribution of values (perhaps around a center
target value) and so are selected and marked after making. The RETMA standard 5%, 10%, and 20%
values have been chosen so in the selection process, there are NO rejects. 1 K + 10% just meets 1.2K 10% and 1 K – 10% just meets 829 + 10%. Same thing for the 20 and 5% resistors, and today for the
1% series, even though modern film resistors are carved to resistance with abrasion or laser to cut the
spiral path in the cylindrical film on quartz or ceramic.
So long as signal needing attenuation were not strong, the available 1/2, 1 or 2 watt carbon comp
resistors served adequately. Indeed the design formulae and tables in the 4th edition of ITT “Reference
Data for Radio Engineers” neglect to mention power dissipation.
I began to analyze attenuator circuits for power dissipation sometime after I built my first computer for
me to use, after building a couple for others. Mine runs CP/M with 8” floppies, a Z80 CPU. I found that
in a T with three resistors that the dissipation in the shunt resistor was about twice the dissipation in the
input series resistor and much smaller in the output series resistor. So for maximum dissipation for a
given resistor power rating, its beneficial to use one resistor for each of the series elements and two in
parallel for the shunt element. And for RF purposes, two resistors in the opposite direction to the
ground plane are a better approximation to a disk resistor than one resistor. So the power dissipated in
the attenuator is primarily in those three resistors and close to equally distributed in those three
resistors.
I created a design program MULATT.BAS that designed for power dissipation. And achieved the
desired total attenuation by a combination of cascaded attenuator. The program uses standard 5%
values and shuffles the shunt resistor parallel combination to achieve the theoretical shunt value which
otherwise would not often be a standard value. In some cases it goes to three resistors with the third
usually a significantly higher value, just for trimming to the precise value, presuming the resistors are
precisely the marked value.
The program computes power dissipation in each resistor and chooses attenuation that for the applied
power limits the power dissipated in each resistor to rated power. Then it computes another stage until
the sum of the stage attenuations is less than the desired total attenuation and computes the last stage to
provide the total attenuation so long as the power dissipation limit is met. This makes an asymmetrical
attenuator for power dissipation. It has its high power and its lower power ends. This asymmetrical
attenuator uses the minimum number of resistors to handle the power dissipation.
At the cost of using more resistors the program can be used to design an attenuatorof ½ the total
attenuation. Then build two copies with the low power ends connected in the middle. Both ends will
handle applied power, but only one set of resistors will be dissipating most of the input power.
MULATT.BAS shows the details on the screen for each stage and then shows a summary of the design.
Back in DOS days, a CTL-P would echo that to the printer. In Win XP, I'm unable to get the Turbo
Basic compiled version to save the screen to a file. May have to rework it in C to allow compiling for
windoze, OS/2, DOS, or Linux.
The program was originally written in CBASIC, a compiled business basic for CP/M. Then sometime
when I had a PC that I could use as a terminal for the CP/M system I was able to capture it to PC and
somewhere about 1990 converted it to compiled Borland Turbo Basic. I've not used Basic for work in a
long time because one a project where I needed large integer calculations that were accurate I found
several Basics moved 8 and 9 digit integers to 6 digit precision floats for multiplying and dividing
which took me months to realize and that cost me that job.
I suppose I'll have to convert it to C to get more features, like a combination of file and screen output. I
still have a DOS computer running with TBASIC so I'll look at that first.
Insert MULATT.BAS or MULATT.C here. And several useful designs, like 100 watts to 1 watt for
transverter service using 5 watt metal film power resistors.
73, Jerry, K0CQ