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ABOVE & BE"YOND~F-- k ~"
C. L Houghton WB6IGP
San Diego MiClowal'l!! GfOLt?
6345 Badger Lake
San Diego CA 92119
Mlc:l'OW1Ive Circuits Designed
From PUff
last month we went into the application
programs necessary 10 set up
PUFF and gel it running. I wanted 10
establish an easy path 10 get PUFF up
and running lo r the first lime, compared
10 the start-up experiences both
Kerry NSIZW and I encountered. After
a Ilnle use this Is all elementary, but in
the beginning It can be a liltle defeating.
In addillon 10 basic design, I want to
cover the features needed to complete
the amplilier circuitry. T his includes
the positioning 01 feed resistors, bypass
capacitors and the actual toealion
0 1 the RFC leedllnes to ou r
slripline CiiCUitry. The final prodl.lCl will
cover the complete design for the device
N6lZW and I picked 10 use , an
NEC04583 GaAsFET. The design was
lor 10 GHz operation.
In the design as oovered last monlh
!he strlplines lor both input ana output
networks were designed using the
component sweep portion of PUFF.
These parameters determine the exact
omenslons lor the ~nesat Ihe
frequency specifoed. At !his point, the
circuit has malching stlbs on both Inpul
and outpul striplines, as well as
connections (in PUFF) to COIV18CIOr 1
and 2 as shown on PUFP s screen
(F11. What is needed to make ltIis a
COfT"4)lete circuit is the DC leed RFCs'
and bypass capacitors' connections,
as well as the DC isolation capacitors
on the InpuVoutput of meamplifier.
Alter all tnese ilems have been
added , the fi nal step is 10 add th e
ground foil covering most of the edge
01 the circuli board area. A note 01
caution: Alter the ground loll is added
10 the circuit artwork In (F1), do not
run any plotting of param eters cecause
ltIat can cause the program to
hang or lock up your co mpuler. I
guess what is happening is that there
are so many unrelated items in ltIe artwork
PUFF gels conlused. The program
wiD not hang when plotting pa ­
rameters with the RFC circuitry or bypass
capacitors added to the circuit.
This can be quite useful fOf seeing if
your added compouerts have any etteet
on the design. When you have
completed your design, save !he basic
l ile eerere doing ground loil operations.
Then save the final ground foil
circuit in a different I~e name lor artwork
output. Tha i way, if any errors
are encountered you can go back and
use ltIe previous file.
let'a start with ltIe AFC (RF dloke)
lor the amplifier and cover how PUFF
operates with RF chokes. We found a
smell erro r In the length of the RFC
when specifying a 90 degree (quarter
wavelength) long RFC. The program
gave you a 90 degree AFC but its final
length was not 90 degrees but actually
something shorter. (The irregularity we
noticed was that PUFF calculaled me
90 deg ree une OK b ut when we
placed II on ltIe PC board it was positioned
from the cenler of the st""line
VHF and Above Operation
tors fo r power and
th e coa ~ia l connec­
.....
t.e t's gel back to
the 10 GHz repeater
concept. Th e amenuere
Just described
ware co nnected in
tandem-that' s lour
ampilliers in series
p rov iding about 40
dB of gain. The final,
filth amplifier, a
broadband commercial
unit capable of
anot her 10 10 15 ea
01 gain with a maxlmum
of +20 dB output,
was us ed to
drive the output omnito
!he center 01 the comeding pad lor
DC bias. This made ee actual length
01 !Ijs RFC something less that 90 degrees
in actual artwork generatiorl.)
The remedy lor this p roblem is 10
lTIaJte lhe RFC longer by han !he width
in electrical degrees 01 both the
stripline and the COI'WleCting pad width.
T his would be a n actuar lenglh 0 1
about 120 elect rical degrees, making
the actual RFC length muctI closer to
a quarter wavelength long and a batter
RFC. Both sho rt and normal RFC
functioned Quite well in actual use, Ihe
longer (pure 90 degrees) acting slightly
better.
A etnpnne 90 degrees long at our
frequency of interest is the same as a
quarter wavelength of transmission
line and presents a high Impedance 10
the RF frBQuency. We make the resistance
of this line in PUFF 140 ohms
and it functions well as an RFC.
A new part is added 10 me parts list
wh ich is a t-une of zero o hms
impedance and l mm wide. This gives
a space in which to place the input
and output coupling capacitors on ee
S1~. To put these in the circuit, go
to !he end 01 the circuit and erase Ille
COMBCIiOn 10 the ooonedor by doing
a -shift," (eilller a 1 or 2) lor the Irllut
or output comection. Th&n place the
break on Ihe st~ine ends for our capacitor
and reconnect the other end 10
the I/O connectors.
Placing the RFC on the boaro can
require a little juggting. It you wan! !he
RFC 10 be al the inpuI Of output of the
stnpline it's no problem, but usually '1\'8
want them placed somewhere midi»"
slUon on the stripline. To do this we
have 10 reassign a IractiOnal value for
our input or output stripline and construct
it bac k logether with the treelional
components equal to the original
single part. In this way we can now go
to any of the transitions between parts
and place an RFC at those junctions.
This Involves lots 01 juggling, but it's
not bad at all compared to making artwork
on a CAD sy stem wllh all lis
oomple~ities .
At the bottom 01 the RFC I·line we
can again connect striplines lell and
rlghlior \tie DC bias feeds and bypass
eecec neee 10 ground. Don't forget 10
use another zero ohm 1mm break in
the DC capacitor coupling point. the
same as in the input output circuitry
dBSCf1;ltion lor the COt()Iing capaCitor.
When al this is done you can define
ano!her short section 01 transrNssion
line to be used in making the gl"Ol.Kld
perim eter a nd bypa ss ca pacitor
ground ing terminations. This part of
the cllCUit should be done on a copy of
your near final circuitry. As I sta led
earlier, if you attempt a plot your 0Qm.
puter will hang up ancllhat will be that
Save copies and use mem and II you
encounter problems as the design erec
resses: you can always retreat back
to the previous saved copy, saving
you from any error.
10 GHz Amplifier Application.
The first project use of the amplifiers
constructed to tesl the perlormance
of PUFF and actual operation
compared Quite well. Kerry N6 lZW
constructed and tested a unique an-
tenna system 10 serve as a microwave
repealer lor bolh sse and WBFM at
10 GHz. Preliminary tests show lhal
this system worked q uit e well and
proved lhe reliabilily of me amplilier
design 10 be used in our 10 GHz repeater.
Firsl , a little about the "repeater.­
ThiS repeater is noc standard In that II
consists 01 only an amplifier and two
antennas. The design fOf this system
was the in spi ralion of Kerry N61ZW
wtIo has worked at this method by first
demonstrating it on a spectrum analyzer
on his workbench. Bench tests
showed that about 60 10 80 dB of Iso­
Iatiorl could be obtained between two
armi slot antennas lor 10 GHz placed
on opposile ends 01 an eighl-loot section
of stercare waveguide. One antem
e is pointing upwaros and the ctner
antenna is pointed downwaros. At
one antenna an amplifier with SO to 60
dB 01 gain is inserted between the two
slot antannas separated by tha waveguide
section. See Figure 1 for details.
The amplifier consists 01 four each
01the amplifiers that wa designed and
buill using the PUFF program. The eevc
es we used were the same NEC­
04583 GaAsFET devices as shown in
Ihe examples (10 dB per stage and
aboul a 1-2 dB noise figure, hopeful·
ly-we r ever rt measured them yell.
The amplifi ers mat we constructed
worked well and reneet very well on
the PUFF design program. M8aS!Jred
results compared Quite closely 10
PUFP s prediction 0111 dB per stage.
KerT)' was able to obtain about 18 cS
01 gain per stage wlItI custom adjus!·
ments with v ery s mall copper
-snowflakes." These were attached 10
a loothpick and used 10 position lhe
copper pieces (about 25 to SO thou·
sandths square) on !he prinled Circuit
boa rd to tune Ihe amplifier lor me ~ l ·
mum gain. AI the 18 dB gain level the
amplifier started to break into oscillations
and was very unstable. Reducing
the single stage gain to the 10-12 dB
limits allowed Ihe stable design needed
to tacnuate packaging II rntc a
housing.
Th e amplifiers Karr y cc nstructec
were wrapped on the edges with 1/2­
mcn-wrce. c.cas-mcn-tnrce copper
Ilashing 10 form the case sides, and
very short grounding lor the amplifiar
edge ground conn ections. Copper
sheeting about 0.008 inch thick was
used 10 form a case cover. The ground
loil side 01 the ampl ilier lormed the
bottom 01 ltIe amplifier. Both sides 01
the printed circuit board ground loil
were soldered to the copper flashing
and form a very sold case lor \tie PC
board, as wei as lor
feed-through capaci­
, 10GH> Slot AnI""".
" • _ lftlong
"
F..,. 10 GHr AmI>l'r..n
,,-"'112