Mosfet-Converter-for-220-MHz.pdf

(502 KB) Pobierz
mosfet
converter
for
220
mhz
I
z
You must still
select
2
-
7
these semiconductors
2
,.
d
0
D:
for the higher frequencies-
but it's worth
it
28
january 1969
J,
w
L
-
Superior gain characteristics
of the metal-
oxide semiconductor field-effect transistor
(MOSFET) have been exploited i n ham-band
converters for six and two meters in
ham
radio's
June and August 1968 issues.ls? Al-
though its gain diminishes slightly between
200 MHz and 300 MHz, the MOSFET-partic-
ularly the dual-gate type-is still a fine per-
former at 220 MHz. The deluxe combination
of low noise, high gain and low cross-modu-
lation are packaged in the following solid-
state converter, which
i s
reasonably easy to
construct and tune.
circuit considerations
The 1'14-meter band
is
more demanding of
semiconductors than are the lower frequen-
cies. Only by selecting high-quality compo-
nents was it possible to achieve the desired
level of performance. Much to my surprise,
the oscillator and multiplier transistors are
equally as critical as those used in rf stages.
The first rf amplifier uses an RCA 3N159
dual gate MOSFET, which
is
a low-noise ver-
sion of the RCA 3N140. I n using the 3N159,
you achieve the lowest possible noise figure,
about 2.5 dB. Very low feedback capacitance
e
2
N
m
2
-
0
r'
c
0
-less than 0.02 pF-allows operation with
no neutralization. The second stage, being
less critical, uses the 3N140 operating at low-
er gain to prevent overload.
Two rf stages may be stagger tuned if wide-
band operation
is
desired. Unlike the two-
meter converter, which had to be stagger
tuned, this converter can have both stages
peaked on the same frequency. This added
utility
is
made possible by using dual-gate
MOSFET's in both rf stages, with Teflon sock-
ets, to minimize feedback capacitance.
Only one change
i s
made in the 3N141
mixer circuit from that in the 6- and 2-meter
converters. It
i s
the addition of a neutralizing
circuit from gate 2 to ground.3 The
L
and
C
resonate at 14 MHz, effectively bypassing
gate 2 at the intermediate frequency. (Values
must be changed if output other than 14 MHz
is employed.) Incoming signals will be im-
proved one S-unit or more with this circuit.
The technique may also be used on the two-
meter converters.
Birdies and "garbage" on 220 have not
helped to make it a popular band. One way
to improve the situation
i s
to use a high-
frequency local oscillator as was done here.
A 2N3478 drives a 103-MHz fifth overtone
crystal whose frequency is doubled in a sec-
ond 2N3478. Cheaper transistors did not de-
liver enough injection voltage for good gain
in the mixer.
I
find that the use of back-to-back diodes
in the antenna circuit
is
quite controversial.
Some feel they are essential to prevent burn-
out of the front-end transistors, while others
say this
i s
not so.* My friend Ed, WZDMR,
uses a 3N140 preamplifier with no diodes on
ten meters with a kilowatt. His only isolation
is
a DK60G relay. He operates within
'14
mile
of another kilowatt on the same band with-
out problems.
I
have removed the diodes in
all my converters without mishap, but 10
watts of rf i s the greatest power I've generated
on any band. Diodes are shown in the sche-
matic for those who care to use them.
Sometimes rf burnout has been traced to
slow decay of the transmitter's output, rather
than the lack of isolation of the relay. In this
case, large amounts of rf may load the con-
verter during coaxial relay switching. A very
good solution to this problem
is
found in
delaying the relay, or by using two sequen-
tially operated relays-one for the transmit-
ter; one for the receiver.
construction
As with the 6- and 2-meter converters, all
components were mounted on copper-clad
printed-circuit board according to the layout
shown. The board
is
fastened to a Bud
CB1626 chassis. Satisfactory operation
is
once
again possible without shields. For those who
may have forgotten the rules for handling
MOSFET's, the following should be observed:
1
Keep MOSFET leads shorted until ready
.
to use. (These devices are shipped this way.)
2.
When cutting leads, grasp lead and case
with fingers to reduce possibility of electrical
and mechanical shock.
3.
Do not solder or change components
with MOSFET's in their sockets. (Such a prac-
tice may be acceptable if you use a soldering
iron with a grounding system.)
4.
Never insert or remove transistors when
power has been applied.
performance plus
A comparison of this converter was made
with my Nuvistor converter whose noise
Depends on the transistors. Quality control and in-
spection mean different things to different manufac-
turers. Better use the diodes and be safe.
Editor.
january 1969
29
30
january
1969
YIrtmAL,
T H N
tDrCI)
OI
COCCC(I-M
. M
O
fig.
2.
Chassis layout for the 220-MHz mosfet converter; this chassis designed for Bud CB-1626 chassis:
figure
is
about
5
dB. The noticeably quieter
MOSFET's are music to my ears. I find that
the gain is comparable, which
is
not an easy
achievement with the best of the current
JFET's.
While 220 i s an orphan band in many parts
of the US., the Mt. Airy VHF Society has kept
it alive in the Philadelphia area. Only one
station has managed to overload my MOSFET
converter. (He
is
hard on the tubes too!) Ad-
mittedly, this
is
no test for comparison, but
I
feel the MOSFET
is
at least as good a the
s
proven two-rf-stage 2-meter converter-ith
respect to cross modulation and dynamic
range.
not all are interested in the depletion-mode
types for vhf such as the RCA transistors used
in this series of three articles. Stability prob-
lems of the oxide layer which once plagued
the MOSFET have been largely solved, giving
time for curing the
ills
of static burnout and
frequency limitations.
MOSFET amplifiers at
500
MHz are prac-
tical now, but high-volume production
i s
not in effect on these units. Some feel the
rnosfet's
and
the
future
Only a few of the s~miconductor manu-
facturers arc manufacturing MOSFET's, and
'Bud CB-1626 chassis available from Allied Radio
Corporation,
100
N. Western Avenue, Chicago, Illi-
nois 60680.
Order
catalog number 42E7812,
5.55
plus
postage; shipping weight, 12 ounces.
january
1969
31
Under-chassis construction of the
220-MHz
mosfet converter.
upper frequency limit of the MOSFET
i s
1000
MHz,
but science may soon disprove that
notion. Even now, Motorola has incorporated
a
layer of silicon nitride to eliminate static
burnout of the oxide layer.
Fairchild has integrated additional circuitry
in the MOSFET package to reduce the burn-
out hazard, while other manufacturers are
experimenting with still different techniques.
Presently, there
is
no simple solution com-
patible with all MOSFET's, but the fact also
may be changed in the near future.
The metal-oxide semiconductor field-effect
transistor answers the challenges of high in-
put impedance, extremely low feedback ca-
pacitance, high gain, low noise, inexpensive
construction, wide dynamic range and low
cross modulation. It is so distinctive that it
invites comparison in all rf and many dc ap-
plications.
references
1. D. W. Nelson, "The WR2ECZ Six-Meter MOSFET
Converter,"
ham
radio. june,
196R,
p. 22.
2.
D. W . Nelson, "The Two-Metcr Winner,"
ham
radio, Augu5t.
1968,
p.
72.
3.
H.
M . Kleinman, "Application of Dual-Gate
MOS
Field Effect Transistors in Praclical Radio Receivers,"
IEEE
Transactions on Broadcast
and
TV Receivers,
July,
1967.
ham radio
next month in ham radio magazine:
signal detection in the presence of noise
programmable repeater identifier
stable transistor vfo
universal solid-state preamp
power supplies for ssb
analyzing incorrect dc voltages
sloping dipole antenna
plus many more..
.
32
R
january
1969

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