Solid-State-RX-for-160-Meters-DO-ODGAPIANIA-NIEKTÓROŚCI.pdf

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solid-state
receiver
for
160
meters
I
r.
Although this receiver
was designed
for the
1.8-MHz
band,
it
contains
.
r
Recent changes in the
FCC
rules governing
amateur activity on 160 meters have
prompted renewed interest in 160-meter op-
eration. Since many ham-band-only receiv-
ers do not cover 160,
1
felt there should be
a
lot of interest in a high quality, solid-state
receiver capable of tuning 160 meters. The
receiver presented here uses many of the
g
=
ra
$
-
-
w
r
a
-
d
many interesting
4
circuits
3
;
that could
be
applied
to higher frequency
units
3
latest semiconductor devices, and many of
the design ideas and circuits would be equal-
ly applicable to higher-frequency receivers.
A
block diagram of the 160-meter receiv-
er i s shown in
fig.
1
Note that there is no
.
rf stage ahead of the mixer. This omission
is not an attempt to simplify the receiver
but to improve its over-all performance. Sev-
eral years ago recent receiver developments
were reviewed,' and the authors pointed out
that to minimize spurious responses, no rf
stage i s needed, or desired, at frequencies
below 15 MHz.
In the receiver shown in
fig.
1
a
455-kHz
i-f was chosen so
1
could try the Murata
CF-455
ceramic filters.2 These filters are com-
pact, exhibit good shape factors and are easy
to use with simple resistance-coupled tran-
sistor circuitry. Collins mechanical filters
could be used as well.
a
y'
v;
variable
attenuator
A
schematic of the variable attenuator and
high-pass filter
is
shown in
fig.
2.
The at-
tenuator can be varied from zero to
40
dB
in 10-dB steps3 and
i s
used if you are listen-
"
-
6
Q
december 1969
ing to local stations or
i f
you want to drop
out weak signals and hear only the strong
ones.
The high-pass filter
i s
used to eliminate
cross modulation and intermodulation from
local broadcast stations. Although the mos-
fet mixer has excellent square-law character-
istics, you still need some front-end rejec-
tion of the extremely strong broadcast sig-
nals present on a long 160-meter long-wire
antenna.
down corner frequency
is
1800
kHz.
At 1604
kHz, response is down 45 dB. An excellent
discussion and application of modern net-
work synthesis to the design of low- and
high-pass filters
is
given by Wetherhold.4
The serious filter designer will also take a
look at the work by Geffe.5
I
used 5 percent mica capacitors in my
filter, but performance closer to the theoret-
ical would be obtained if more expensive 2
percent mica capacitors were used. Both
fig. 1.
Block
diagram
of
the solid-state 160-metar
4
VMIAaE
AND
m
m
YXEC
AIRlFlER
"fr'-T%Ss
Plr
m
OSOLLAlVR
AND
M I 0
AMRIFER
MbRUER
(LENERATOR
The high-pass filter
is
an elliptic function
type with response zeros at 918 kHz, 1586
kHz and 1415 kHz. The zeros are the fre-
quencies where the theoretical attenuation
becomes infinite. The theoretical passband
ripple
i s
less than 0.2 dB, and the 0.2 dB
Receivar front panel.
types are available from Allied Electronics.
The
Q
of the inductors should be at least
100;
1
used Ferroxcube 4C4 pot cores. They
have the advantage of being adjusted ap-
proximately 2 6 percent. In addition, they
have a negative linear controlled tempera-
ture coefficient that compensates silver-mica
cost about $2.50 apiece in
capacitors. T h ~ y
the 14 mm size. Write to Ferroxcubea for
more information.
dual-gate mosfet mixer
A scheniatic of the mixer front end
i s
shown in
fig.
3.
The data sheet for the
3N141 shows a mixer biased to 3 mA with
both gates at about the same potential. In a
companion application note,a H.
M.
Klein-
man mentions two requirements for spuri-
ous response elimination: clean oscillator
waveform and properly adjusted operating
conditions and signal levels. This means that
biasing voltages are important. Also, local-
oscillator injection should not be too high.
december 1969
7
LATTE~~~!ON]
I
P
169
/I
very important. As noted above, the stage
should operate in class A for minimum
spurious response. Fortunately, class A op-
eration
is
also important to frequency sta-
bility. In simple oscillators an important
source of frequency instability i s associated
with the intermodulation of harmonic fre-
quencies which are produced by the limit-
ing that takes place if agc
i s
not used." The
Clapp oscillator in this design has been
linearized by adding an agc circuit; fre-
quency stability, with variable supply volt-
age,
i s
improved by about
15
times.
The oscillator circuit is shown in
fig.
4.
Diode
D l
linearizes the oscillator as follows:
fig.
2.
Variable anenuator and high-pass filter for
the solid-state receiver. Input and output impedance
of the filter is 52 ohms. With Fenoxcube 1408C-A25-
4C4 pot-corm assembly. 1408HD hardware and
1408FlD bobbin. 13 turns no. 24 enameled was used
for the 3.94 p H inductor, 16'1s turns for the 6.Z
p H inductor. and 17 turns for the 7.08 p H inductor.
Higher
0
could be obtained by using 19/46 L i b wire.
As a compromise between conversion gain
and spurious-response generation,
I
used
a
2.5
volt p-p signal to the mixer.
I
used electronic tuning for the input cir-
cuit so
I
would have complete freedom in
the placement of the mixer module. If you're
interested in using tuning diodes in the place
of mechanical tuning, Motorola has two
helpful application notes.oJ0
High-pass filter uses high-a pot-corn coil forms.
local oscillator
The stability of the receiver
is
determined
by the local oscillator, so circuit design
is
when power
i s
applied, the gate
i s
at the
same potential as the source so forward
transconductance
is
maximum. As oscilla-
tions build up,
Dl
clamps the gate below
ground; this charges up capacitors
C1
and
C2 and results in a negative dc voltage at
the gate, and the forward transconductance
of the device
is
lv#ered until equilibrium
is
reached.
fig. 3 Dual-gate mosfet mixer. L1 is 115 p H and consists of 411/a turns
.
no. 24 on Fenoxcube 2213C-A8MC4 pot core, tapped 1 turn from cold
end.
8
a
december
1969
,
Without D l in the circuit, the 3N152
would have to provide its own limiting, and
this could only be done with the mosfet op-
erating in a nonlinear mode. W2YM uses a
diode in a similar circuit and reports that it
stabilizes the oscillator.12 Capacitor C3
is
se-
lected to provide the desired injection level
at the mixer.
For best operation, the
Q
of
L1
should be
high. I used two Micrometals T94-2 toroid
cores in series.' The tuning capacitor
is
from
a surplus ARC-5 transmitter. This capacitor
i s
an excellent choice for any vfo. Since the
capacitor
i s
quite rigid,
I
mounted all the
vfo components on the capacitor frame.
The gears that are an integral part of the
ARC-5 capacitor are used for the dial.
Vfo board is mounted on top of the
ARC-5
tuning ca-
pacitor. The series toroidal coils are mounted on a
'14''
brass bolt which is soldered to the capacitor
frame. Teflon pads are used to clamp the coils.
fig. 4.
High-frequency oscillator. L1 is two Micrometals 194-2 toroids in series,
41
turns no.
22
on each.
a-m
i-f
strip and detector
The a-m i-f strip shown in
fig.
5
uses a
unique integrated circuit, the LM372 manu-
factured by National Semiconductor.'3 This
IC
is
an i-f amplifier, a-m detector and an
audio amplifier with self-contained agc.
This gem will take an amplitude-modulated
*Micrometals toroid cores are available from Amidon
Associales,
12033
Otsego Street, North Hollywood,
California
91607.
rf signal at frequencies up to 2 MHz and
give you 0.8
V
p-p audio output. The agc
provides an essentially constant output as
the rf input varies over the range from 50
microvolts to 50 millivolts.
In the photo of the a-m i-f strip the 3-
and 6-kHz Murata ceramic filters are on the
left. The 6-kHz filter
i s
used with the a-m
i-f system, and the 3-kHz filter
is
used for
ssb. Resistor R1
is
adjusted so that maximum
fig.
5.
A-m i-f strip uses
a communications-type
integrated circuit, the
National Semiconductor
LM372.
R1
is adjusted for
200
mV output.
december 1969
a
9
MHz. However, since this is a "first-genera-
tion"
IC,
agc detection and
a
ssb product
detector must be furnished externally.* One
of the big advantages of using the LM371
i s that you don't have to worry about inter-
nal feedback. No neutralization is needed,
even at
100
MHz. However, proper shielding
must be used between input and output.
The LM371 data sheet shows the input
and output impedances of the device over
its complete operating range. However, for
my application on the relatively low fre-
quency of
455
kHz,
I
didn't worry about a
A-m
1-1
board.
band i-f system
m-
quires an external
age input from the
age circuit shown
in
fig.
9.
MI[
I
FR
w.31
om
audio output
is
200 mV (200
mV drives the
audio-output stage to full output).
S S
i-f
strip
~
~h~ ssb
i.f
strip shown
in
fig.
6
is
based
on
the
use
of
an
LM371
integrated
circuit,
The
LM371
is
quite
versatile
and
may
be
used as an amplifier from dc to over
100
perfect match, and
I
used readily available
i-f transformers.
'A
newer device, the National Semiconductor
LM373
i-f strip, is
a
broadband communications I C capable
of
per10rmin.g the diverse functions required in a-m,
fm and ssb receivers and transmitters. Simple external
connections convert the i-f strip from one mode to
another. The
LM373
has a self-contained audio agc
system with fast attack and slow release.
fig.
7.
Product do-
teetor and audio
amplifier.
10
m
december
1969

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