Steve’s Article about his linear amplifier |
Last
update was Remember, you must be the holder
of an Amateur Radio license to operate this equipment on the air! If you
have Ivex Software, the schematics
can be downloaded here. I recommend you right-click on this link, and select
“Save Target As…” |
|||||||||
Introduction After much contemplation,
and longing to QRO, I determined that I wanted an amplifier. The conclusion
to build or buy was decided when I found that there was not much offered
meeting the criteria that I set for myself. These limitations were that it
should run on standard house current and not require 240 VAC. This limit was
set by the problem that I live in a very old neighborhood where many of the
houses only have 120 volts AC available. The power limitation was set at a
maximum of only 15 amps available to the radio shack. Approximately 500 to
600 Watts PEP output maximum was chosen. Whether to use tubes or solid state
was then considered. I decided on using transistors, as that would eliminate
having to deal with the high voltages needed to support the anodes. I also
didn’t want filaments heating up the room that I use for the shack plus the
power consumption required for making tubes glow and is a waste of energy.
Transistor amplifiers are broad-banded, eliminating the need to tune the
amplifier each time I QSY’ed. Another goal was to
have this amplifier automatically change bands, along with the Yeasu FT-757 HF rig I now use. Picture of EB104 board Then the choice for which
part to use had to be made. After searching the Internet, I found that most
of my work was done! Communications Concepts[i] has several kits. One of them caught my eye. It is
a kit based on the Motorola MRF150 part, and copies the amplifier in EB-104[ii], a Motorola Engineering
Bulletin. This design uses four devices in push-pull/parallel. It has
individual bias adjustments, and an on-board regulator for the bias. Some of the other features
that I desired are to have complete metering capabilities. That must include,
drive power, power out, reflected power, drain current and drain voltage. This amplifier is to use
switching provided by the transceiver. Although there are many amplifiers on
the market that can be RF sense keyed, I prefer to have the rig control this action
either through push to talk (PTT) or VOX. For my personal choice, it would
seem that there would always be some clipping of the first syllable or Morse
character if the amplifier had to wait for exciter power to cause the
amplifier to initiate. Another reason is the extra stress on the relays
trying to make contact while live with 160 or more volts RMS at nearly 4
amperes of RF current. Also, during this time period there would, no doubt,
be short periods of time (best case of 15 milli-seconds) where there would be
no load on the transceiver, and the power amplifier module, which could
destroy an expensive set of transistors in either the amp itself, or the
exciter. Another feature I want is that
this accessory be as small and compact as possible. The house where my family
lives is small, and to consume a large portion of real estate with a
physically large amp doesn’t seem prudent. Something else I wanted was
to be self-contained. It must have the power supply, amplifier and the
filters built into the same cabinet. Power Supply I began by finding a
cabinet. I found one that looks very similar to the LMB CO - 1 series. (See picture of cabinet) It measures almost
10” wide, 12” deep by 6 inches high, not counting the extra 1” for the feet.
This is a used cabinet, but LMB does make one that is very similar. The only
difference I could see was that this one has a top that opens with a hinged
lid. The LMB series allows the whole top to be removed. Chassis Layout I then went shopping for
the power transformer. This component needs to supply 50 volts at peak
currents of up to 20 amps peak. I found one that suits this need on an
Internet auction. It was designed for a 500-watt stereo amplifier, and has 70
volts RMS center tapped (35 volts RMS per side) at 14 amps continuous. This
is close to 900 watts input available.
(35 * 1.414 = 49.5 Volts DC, 14 Amps continuous * 1.5 for ICAS service
= 21 Amps Peak, 49V * 20A = 1030 Watts Input, at 45% efficiency (Worst case)
gives 465 watts output minimum. At 55% efficiency this gives about 560
Watts.) Then I also found some
33,000 uF capacitors, and a couple of 50 amp
diodes. So far, so good. I saved a couple of hundred bucks! Caps, $1 each,
new would have cost $20 apiece. Power transformer, $10. New? Over $30. Diodes
$5, new are about $15 each. Cabinet, $0, new $50 to $60. I next constructed the
power supply, fitting the transformer into the far left rear of the cabinet.
The capacitors squeeze in between the transformer and the front panel. I
mounted the power diodes on the left of the cabinet. (See Cabinet picture) A word about safety! Even
though there is only 50 volts on the capacitors, this is enough to give a
very bad shock. Also, keep in mind that you are looking at over 1200 watts of
stored energy on the caps, and rings can melt at the currents this supply can
produce. Surge amps can approach 200 to 300 amperes of current! Welding rods
melt with as little as 30 or so amps. I recommend that while working on this
power supply you remove all jewelry, including rings and watches. I built the power supply (See Schematic) on the left side of
the cabinet, keeping as much of the components crunched as far to the side as
possible. I saved some room by not using the customary wrap around capacitor
mounts with feet, but rather made home-made ‘L’ brackets to hold them against
the cabinet side. This method saves almost ¾ of an inch in both horizontal
directions. As can be seen in the photo, this is a very tight fit, and both
of the 3-inch diameter 33,000 uF capacitors take up
even more room than the transformer. The 50 amp diodes are mounted just above
the transformer. All wiring is done with 12-gage wire. I recommend stranded
wire here. All of the wire ends are terminated in ring lugs, soldered on
where the wires connect to screw terminals, such as the power diodes and
capacitor bank. Cooling In full power operation,
the transistors will be operating with close to 1200 watts PEP input power,
and with 500 to 600 watts out will have to dissipate at least another 500
watts as heat. This must be removed from the active devices. I had two
choices. I could mount the heat sink on the outside of the case, and allow
convection cooling, or I could mount the heat sink inside the cabinet. In
this picture, the heat sink and fan are mounted together in the long metallic
object just to the right of center in this picture. The fan is located on the
right side between the cabinet wall and the heat sink box. In this picture,
the copper heat spreader is not yet installed. and
install a blower or fan. My choice is to mount the heat sink and the RF
amplifier module vertically and attach a small muffin fan to blow cooling air
through the cooler. Although this adds extra sheet-metal and machine work, it
keeps the hot surface away from accidental damage or obstruction and keeps
fingers away from potentially hot surfaces where one could get a burn.
Remember, a hotplate consumes nearly the same amount of energy. (See Picture
2) I found mine on an online
auction, and cost me less than $20 bucks. (New ones run around $50 to $60)
The one I bought was 10 inches wide, 1-3/4 inches high and 13 inches long. I
cut mine to fit so that it would stand on its side and length-wise so that
each end can be bolted to the front and rear panels using 4-40 screws. I also
attached the device to the floor of the cabinet with the same size screws.
This was done to keep the cabinet rigid, and help keep vibration noise down.
My heat-sink was used, and did have a few holes drilled and tapped in it. I
decided that it would be prudent to re-use these holes to minimize drilling
extra holes, so a template was made by taping a sheet of paper to the device
side surface and tracing the location of these holes. This will be used to
punch the holes for the copper heat spreader described below. Once the heat sink was cut
and trimmed and tested for fit, I mounted a slab of copper .325 inches thick[iii],
using screws and put heat-sink compound between the two parts. Aluminum is
actually a poor conductor of heat compared to copper, so the copper sheet
acts as a heat spreader increasing the effectiveness of the heat sink. I
estimate that the heat sink alone would only be able to dissipate about 150
to 200 watts, while with the copper and forced air cooling, it should easily
handle the required thermal requirements with some safety. My typical
operating style is only about 30%. A hole was cut at one end
of this assembly to mount the fan. The idea is that the fan will blow through
this hole and the sheet-metal cover will duct the air around the fins,
circulate past the band-pass filter and then past the power transformer,
carrying the heat out the rear of the cabinet. I suggest that the fan be
mounted in the front end of the heat sink to minimize re-circulation of the
hot air, though this brings the fan closer to the operator. The fan noise
isn’t so high in level as to be objectionable and is somewhat attenuated by
the cabinet. I chose not to temperature control the fan since the noise level
is low to begin with. If you find that the noise level is objectionable, you
could add a thermostat to the fan, and only turn it on when the heat rises
above a preset level. I wanted to minimize the amount of construction, and
decided that the fan running anytime the amplifier had power on was something
I could live with. Low-pass filter assembly Solid-state amplifiers have
several problems that have to be handled correctly if the amplifier is to be
legally used on the air. First, it must be stable and not become a power
oscillator. Transistor amps also are inherently rich in harmonics. This is
because of the non-linear junction that all transistors have. Diplexer Schematic Part of the problem is
solved with the design used here. Push-pull amplifiers are not prone
to developing even harmonics if the devices are reasonably matched for gain.
In this amp, it was found that second and fourth order harmonics are down in
excess of 40 dB from the output, so anything added by the filter is gratis.
However, the third harmonics are only down a measly 10 dB to 13 from the main
signal. With potentially 600 watts PEP, this implies that on the order of 50
to 60 watts of harmonic energy is available to radiate. Obviously this is a
problem that has to be taken care of at the amplifier. The requirements are
that for 500 watts and less, the harmonic energy must be below –40dB, and for
600 watts the requirement becomes slightly more stringent. I chose the goal
of –46 dB as the maximum harmonic content allowable post low-pass filter with
a 50 ohm load. The filter has to present a
load impedance that the amp was designed to operate into. In our case, this
is 50 ohms. Since the filter has to be designed to resist passing the
harmonics that is not going to happen for those byproducts. Harmonics in a
typical low pass filter circuit that are higher in frequency than the cutoff
of the filter are either absorbed by the filter components, or are reflected
back into the amplifier where it adds to the heat dissipation problem of the
transistors and also contributes to other byproducts of the amplifier such as
inter-modulation distortion. Since my goal was to build
a linear and not spend hour upon hour of design work, I searched previous articles
and publications and found an excellent design by William Sabin,
W0IYH[iv] where he passes the lower desired signal to the
antenna port using a low-pass filter, and sends the higher frequency
by-products via a high-pass filter into a termination. (See his article) This arrangement of low-pass/high
pass filters is called a diplexer. This keeps the load to the amplifier very
close to ideal at all times, adding to the stability of the amp. It also adds
additional attenuation to any harmonics simply because the harmonics are
absorbed and not allowed to bounce back and forth between the transistors and
the filter. It also reduces the mismatch presented to the transistors. One
other aspect is that since the harmonics are absorbed, there is less energy
to mix within the active devices used in this amp, thereby lowering
inter-modulation causing distortion products. I made a few component changes
to his design. First, I used larger toroid
forms. I used capacitors rated for higher current and voltage than was used
in the original design. I moved the dump resistor from the board, and mounted
it to the bottom of the chassis. I also chose one that can handle the full
power of the harmonics Mine is rated at 50 Watts. Go to this page
to see the coil data I used, and here to see the parts
list. These are starting places, and you will probably have to remove a
turn, and stretch or compress the turns to tune each coil. Use the method
shown below. It works very well, and isn’t affected by stray capacitances or
load much. It should give you very satisfactory results. The board layout was done with a program called
“IVEX WinBoard[v]. It is
available at many parts suppliers. The program details are available at
http://www.ivex.com. You can also download a demo version here, but it has
very limited capabilities. The version that I use can be purchased for
$29.95. It allows you to print full size templates. You also are able to
produce Gerber files, if you have a vendor that can make the boards for you.
One advantage of this is that the vendor will silk-screen the board for you
if you want. If you have Ivex Software, the schematics can be downloaded here. I
recommend you right-click on this link, and select “Save Target As…” The inductors for the
low-pass section of each filter are wound with two parallel 18 gage wire. My
calculations showed that the filters had to carry three and a half amps at
the operating frequency. This called for a minimum of 16 gage wire. The first
attempt to wind a coil on the toroid form using 16 gage proved to be a
formidable task. I discovered that I could not bend the wire around the form
without fear of breaking the toroid form. I came upon the idea to wind a
double winding of 18 Gage. This gage is much easier to wind, and even winding
2 strings at the same time proved to be much easier. (Picture of 80 meter coil) The 10/12 meter, 17/15
meter and the 20 meter bands were wound on T130-0 forms, and the 40/30 meter,
80/75 meter, and 160 meter bands were wound on T130-6 cores. The reason for
the choice of material was decided on because of the huge re-circulating
currents expected at this power level. Using higher permeability cores would
cause excessive heating in the form. Also, temperature stability is a must,
as large excursions in the tuning of the coils would degrade the filter
response curves. I felt that this would exacerbate the losses in the core,
and might cause more heating, causing a vicious cycle. The few extra turns
needed on the form caused no problem other than adding a few extra inches of
wire. (See pictures of complete coil set) MAKING the Boards EI9GQ[vi]
did a wonderful article about making your own printed circuit boards. It is
located on his web page. The following is a quote of how he does it; To make smart looking PCB's, all
you need is: A computer, a laser printer, copper clad board, etching
solution, a clothes iron and some Epson glossy photo paper. You can buy
special film for making PCB's, but I have found that the Epson paper gives
better results. I use Epson photo quality glossy paper for inkjet printers. I have used this method to make
PCB's for both DIL and surface
My laser printer is an old Apple
LaserWriter II NTX (300dpi.) If you don't have access to a laser printer, use
an inkjet printer to print the layout on ordinary paper, then copy the image
to the Epson paper, using a photo copier. I haven't tried this method but it
should work. When building the filter
boards, you will find that there are areas on the ground planes that need to
be drilled and a small wire fed to both sides, then soldered. This is to give
continuity to both sides of the board. This should add to VHF and UHF
stability of the amp. If you have the boards made at a circuit shop, and the
holes are plated through you will not have this chore. Do not forget to
include these. It is important to maintain ground continuity on both layers.
This ensures the minimization of ground loops. It also helps to insure
shielding.
Having low VSWR to the amplifier
transistors is important. In this respect, the accuracy of the inductor
values should be reasonably close to the predicted values. I used a very
simple method to measure the inductors. I found a capacitor of known accuracy
(I used a 2000 pF that measures 1986 pF on a bridge with a .1% accuracy for
the lower value inductors and an 820 pF that measures 798 pF for the higher
values), and connected it in series with the inductor I was making. Picture of test setup)
I then bridged the output of a signal generator with these, and tuned the
signal generator frequency to where I saw a dip in the output. Tune the
frequency for a dip measured on the indicating device. I used an oscilloscope
for my display. If you have an RF volt meter, this might be a better choice,
as it could give better resolution. The dip indicates resonance. This method
requires an accurate way to measure frequency. My signal generator, an
HP-8640B, has a built in counter, but if yours doesn’t you can bridge the
circuit with one as well. Stray capacitance doesn’t seem to affect the
results very much. The formula for resonance is “=sqrt((25330.3/LuH
X CpF” This is easy to use in a spread
sheet program such as Microsoft Excel or works. That way, you won’t have to
keep re-entering the variables in a calculator, and re-calculate your
findings. Simply plug in the Values and the
spreadsheet program will do all the work. See the example below. Where the
answer 2.154138 is, is where the formula resides. Your value of test
capacitor will go where I have 1985, and the desired inductor you are trying
to tune goes where the inductor value is shown. If the test setup produces a
frequency lower than what your target shows in the program, then either
spread the turns out on the form, or remove a turn to tune the coil. You want
to get within 1% of that produced with the program. This is only a rough tuning,
and when the filter is built, you will want to fine tune the inductors for
lowest SWR with the filter terminated with accurate loads both on the output
of the low pass filter and the high pass network. I started off with finding
the self-resonance of my test fixture. This showed that I had a stray
inductance of approximately .056 uH which has to be taken into
consideration while pre-tuning the inductors! Another method available
would be to read the article written by Bill
Carver[vii] which gives a description of a simple L/C tester.
If you don’t have an accurate method of measuring inductors and capacitors,
you might consider building this unit. It will give you the ability to
construct inductors to within 1% or 2% of the required values. This is
necessary to achieve a return loss of better than –22 dB, or a VSWR of less
than 1.17:1 to the power transistors. All of the toroid inductors
wound on size T130 forms use two 18 Gage wires wound together as one wire. I chose to use this method as 16
gage is very stiff, and difficult to wind on the form. 18-gage wire is much
easier to work with, and with two parallel windings, gives ample current
carrying capacity. Each individual filter is
constructed on its own board; there is provision for 6 boards covering all of
the MF/HF frequencies except 60 meters. Since the current FCC regulations
only allow 50 watts PEP-ERP, there seemed no logical reason to include this
band. These filters mount to a “mother board” which contains the relays to
switch in the proper filter for the desired band of operation. Using this
construction technique allows compressing the real estate so that a more
dense construction can be achieved. The filter modules mount perpendicular to
the motherboard. If fewer bands are needed or wanted, then just delete those
band modules. The filter selection is
controlled by 18 relays. The first group of relays steers the RF output of
the amplifier into the proper filter for the band of operation. The next
relay directs the output of the harmonic filter to the dump load, a 50-watt
non-inductive resistor. The final relay on the motherboard sends the output
of the low-pass filter to the antenna changeover relays. Control Control for the band
switching comes from the HF radio. Many HF radios have BCD band data output
that can be used to control band switching data for this linear. If your
radio does not have this capability, then a six-position switch can be used.
The BCD to decade decoder IC decodes the BCD data. The output of this circuit
drives the MOSFET switches that turn on the proper filter. They also drive
the band indicator LED’s mounted on the front panel. Because the rig gives
the data out as BCD for each band, and the filter sections in many instances
cover multiple bands, there are diodes that further decode this information.
For example, the 6th filter covers the 12 and 10-meter bands. The
5th filter covers the bands 15 and 18 meters, etc. It would not be
prudent to build filters for each band when one filter can easily cover the
needs for multiple bands. The cost of building the filters, plus the real
estate taken up by the extra-unneeded filters is wasteful. It would require
twice the cost to build for nine bands as oppose to only six! A separate board that
contains 2 single pole double throw relays controls antenna changeover. In
the un-energized position, the RF in jack is connected directly to the output
connector, with an RF sensor mounted in between the output relay, and the
output connector. A small time delay is
added, and is used to enable the transceiver after the time delay to prevent
the transceiver from generating any RF until the changeover occurs in the
amp. Keyer The keyer circuit consists
of a pair of inexpensive relays that switch the input/output through the
amplifier board. (Schematic) Q2 senses the
key line from the transceiver and inverts that signal, and sends it to Q4, an
emitter follower that amplifies the current to drive Q1 and Q3. Q1 goes low,
enabling the relays. Q3 enables the transceiver transmitter section only
after a small time delay to prevent the exciter from producing any RF until
the relays in the amp have had a chance to switch. It typically takes 15 to
20 micro-seconds for the relays to energize fully, so 60 milli-seconds should
be adequate. If the time delay is to short for your relays, then increase the
value of R4 accordingly. The transceiver will not be enabled until the
voltage on Q3 reaches 5 – 6 volts. Diode D1 enables fast turn
off of Q3, while D2 drives the FET and C1 through R4. Although this circuit
is a little slow for QSK at high speeds, it does protect the exciter and the
power amplifier from seeing an open while the relays are switching. It also
prevents the contacts from seeing any RF power while transitioning. If you wanted a lamp or LED
to indicate that the amplifier was keyed, the circuit for Q3 could be
duplicated on your board to do this. Metering The meter is a 1 ½” square faced meter that was
picked up surplus for next to nothing. I am using one that has a 0 to 100
Micro-Amp scale. Simpson manufactured it. You can use any sensitivity you can
find, just change the resistor values for the scale you use. Mine is built
for 10,000 ohms per volt. If you find a meter with 1 Ma. Full scale, use
1,000 ohms per volt. If you should be so lucky as to find a 50 Micro-Amp
movement, then use 20,000 ohms per volt to figure out what resistors to
substitute here. Meter Circuit The scale was removed, and
a new one was made using a really nifty program called meter129[viii].
See the article printed in the October issue of QST. Read and follow all the
links at the web page shown in the footnote, as several updates have been
added since the article was published and several needed utilities are also
available to help you get your printer to interface with the program. This
program helps you make a really professional meter face with the appropriate
scales. It has the ability to make linear, logarithmic and anti-logarithmic
scales. You can even draw up to four scales on one meter like I did. You can
choose to draw the arcs or not. The meter face that I made came out as good
as a professionally manufactured meter, with exactly the legions that were
needed. Mine reads 0 to 600 watts on the top scale to represent power out, 0
through 25 amps indicating drain current for the next scale, 0 - 60 volts
showing the supply voltage, and 0 to 40 watts of drive power. The meter scales I drew are
referenced to two arcs. The tick marks are then drawn to indicate from both
the upward and downward directions from the arc. The top arc is used for the
power out, and amp scales, and the lower arc references the voltage and drive
scales. The software intelligently knows where to draw the tick marks, and
place the arcs and scale readings. You only have to enter the data via
windows that clearly show you what will happen to the meter scale. The meter is switched using
a six position double circuit rotary switch bought at Radio Shack. This part
cost me $3.98. I wired it up to provide readings of power out and reflected
power by connection to the RF power sensor connected to the antenna relay
board module. Current is read off a home-made series shunt. This is constructed of a
short piece of brass tubing that is approximately 6” in length. I used a lab
grade power supply and a 1 ohm resistor as a load. I set the lab power supply
to 12 volts, and set the current limiting to 10 amps to calibrate the point
where the panel meter read 10 amps. At this point, I soldered the meter lead.
If needed either you can move the tap wire, or a notch can be filed in the
center of this to raise the resistance until it provides the proper reading
to the meter. Be careful not to file the tubing to small, or it may act like
a fuse! Drain voltage is read
directly from the power supply through a 600K resistor, and drive watts is
read from the power sensor on the amplifier input. The remaining position can
be set up to read other monitored points if desired. I chose at this time not
to. Maybe some day I might add another point of interest. Some suggestions
might be heat sink temperature or bias voltage. Maybe even AC mains voltage.
Since you are reading this with the desire to build your own amplifier, feel
free to personalize your creation! My knob also is a Radio
Shack item. There are four of them in a package. I do recommend filing a
small flat spot on the shaft so that the knob cannot slip on the switch. I
did this to prevent the knob from slipping on the switch shaft. More to come soon! |
[i] Communications Concepts web
page. Check here for parts
http://www.communication-concepts.com/default.htm
[ii]
Motorola Engineering Bulletin EB104, by Helge Grangeburg. 1983 Available for reading and
download
from
http://www.communication-concepts.com/default.htm
[iv]
QEX magazine, July/ August, 1999 pg 20
[v] Ivex software can be found at http://www.ivex.com.
[vi] Home Page of EI9GQ can be found at http://homepage.tinet.ie/~ei9gq/
[vii]
The LC Tester, Communications Quarterly, Winter 1993, a copy of the
schematic can be found
at http://www.qrparci.org/Img00001.gif
and the parts layout at http://www.qrparci.org/Img00002.gif.
[viii] Meter 129 can be found at http://mywebpages.comcast.net/tonne/meter.html.