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Pete Putman KT28
3335 Fieldstone Drive
Doylestown PA 18901
In Praise of 13cm
This month 's column goes
above, beyond and way out with a
look at a nifty Iransverter kit for tne
13 centimeter (2304 MHz) microwave
band. n'e the LMW Electronics
2304TRV1K, manufactured
in England and imported lor
the USA by Down East Microwave
of Troy, Maine.
LMW manufactures a variety of
UHF products, including assembled
transverter units lor 903,
1296 and 2304 MHz, with power
outputs in the 2-6 wan range. The
opti ons are endless. inc lud ing
outboard preamplifiers and internal
sequencing boards. I elected
to build up one of the kits (A) to
save a few dollars and (B) to try my
hand at some microwave co nstruction.
NumbM21 onyour FHdback c:.rd
ABovE AND BEYOND
Transverter Schematics
The heart ollhe LMW transverter
is the Universal Local Oscillator
board, or ULO (see Figure 1). This
is a stable LQ for microwave use
that uses relatively few parts, is
easy to fire up, and delivers plenty
of output.
Both the 1296 and 2304
transverters use the ULO. For
1296 operation, lhe c rystal frequency
is 96 MHz, which rrnnnplies
12 times to 1152 MHz (1152
+ 144 '" 1296). For 2304 operation,
a 90.667 MHz crystal multiplies
12 times to give 1088 MHz.
This is doubled to give 2176 MHz
(2176 + 144 = 2304) 00 the transmit
and RX mixer boards .
VHF and UHF Opera tion
Figure 2 is the schematic ot the
RX mixer. T1, a 2N9 18 is used as
the oscillator powered from an
8-volt regulator. T2 is a 2N5 179
which works as a buffer/doubler
stage and drives T3, a BFR9 1
tripler. Another BFR91 works as a
doubler and the output is fed to yet
another BFA91 Class-A amplifier.
The outputs are derived from a 3
section filter at two points, providing
both low and high level LO
injection.
I bought the U LO , tra nsmit
mixer, receive mixer, IF amplifier
and a chassis trom Bilt Olson
of Down East during the Pack­
Rats Hamarama in October 1987.
Each individual board came in a
lipLoc bag with instructions, a
schematic and several bags of
parts.
Parts only Partly There
The first thing I did was to check
lhe parts against the parts list and
found a number of items missing
from each board. Forexample, all
five low-level amplifier stages and
a mixing diOde were missing from
the TX mixer. Some capacitors
and a mixing diode were missing
from the AX mixer as well. On the
other hand, the ULO kit contained
two 2N5179 transistors instead of
one which turned out to be a
happy accident, as the 2N918
osci llato r transistor simply would
not work.
Bill said there were problems
with kit packing in England, and
he began paCking the kits on this
side of the Atlantic. The missing
parts shOwed up promptly from
Maine via First Class mail , and I
set about to work .
PhOtoA. Top view ofthe ULO board. Note the crystal oscillator to the far
left. D(/AXoutputs lie on the right.
TheULO
This took little time to assemble.
It shouldn't take more than about
2 hours to wire one and briefly test
it. As mentioned earlier, not detecting
output from the 2N918, I
replaced it with the spare 2N5179,
and the output shot right up! All
stages are easily tunable with a
diode probe and VTVM. One construction
note : The interstag e
coupting capacitors after Q3 are
extremely small and fragile chip
capacitors , which ca nnot withstand
much heat. The instructions
suggested heating and tinning the
PC board pad, allowing solder to
flux over the ends to make a secure
connection.
The pictorial diagram shows the
approximate position of the trimmer
c apacito rs for full output.
Setting them as prescribed, with
just a bit of tweaking, yielded just
that. I connected each output to
my Boonton 92 AF Millivoltmeter
lor the final alignment-not necessary
but very helpful. The manual
claims 40 mW at point " Y"
and 10 mW at point " X", which
seems like quite a bit 01 LO injection
at urst glance!
There is, however, a good rea-
Figure 1. The Universal Local Oscillator schematic-the heart 01the LMW transverter.
son for it. Both the TX and RX
mixer boards double the LO frequency
before injection. In the
case of the AX mixer, that is
accomplished by a pairol Howlell­
Packard HP2817 hot-carrier diodes-certainly
not a scheme with
lots of gain, but effective. The TX
m ixer board also uses two
HP2835 diodes as a ring mixer,
with an NEC 85637 transistor as
the doubler.
I measured nearly 50 mW from
the "Y" port and 12 mW from the
" X" port, so the manufacturer's
specifications were up to snuff.
The output was also checked with
a frequency counter and trimmed
to 1080.00 MHz after siUing for 30
minutes. The display still sat on
1080.00 MHz 30 minutes latervery
stable indeed.
RX Mixer, IF Amp
Photo B shows both these
boards in the completed transverter
case. LMW describes the
AX board as a " l ow Noise Preamp,
Filter and Mixer" stage. The
design is very simple (Figure 3). 11
uses just four active components:
a Hewlett-Packard HXTA3645
bipolar transistor (T1) , an NEC
85637 bipolar device (T2), and the
two HP2817 diodes mentioned
earlier.
T1 fu nctions as a low-noise
(2 dB NF) amplifier stage driving
T2, which also has about a 2 dB
noise figure. The 2300 MHz signal
is then mixed in the interdigital
lifter with the doubled LO signal to
provide output at 144 MHz, and
not much output at that. Total
conversion gain is only on the
order of 8 dB with a 4-dB system
noise figure, which is the reason
for the IF amplifier staqe.
This is nothing more than a
z -rneter low-noise preamplifier
using a BF981 MOSFET to develop
nearty 20 dB gain with about a
1.5 dB noise figure. The total sys-
73 Amateur AadiO • April, 1988 63