CW-Selectivity-with-Crystal-Bypassing.pdf

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If
you
are
like
most
amateurs who own
commercially built equipment, chances are
you skip over modification articles no
matter how good the circuit improvement
might be. But if the modification can be
added without drastic changes, then perhaps
you might be interested.
This article describes just such a circuit.
By using existing controls on your ssb
transceiver or receiver you can enjoy the
benefit of added selectivity for cw recep-
tion. The circuit is simple and effective.
All you will need are a crystal, a switch-
ing diode, and a couple of resistors and ca-
pacitors. No holes to drill, no extensive
digging into the existing equipment, and
no worries about defacing panels or chas-
sis. Although primarily intended for tran-
sistor or IC stages, this selectivity circuit
will work with some sets using tubes.
cw selectivity
with
crystal bypassing
You can improve
selectivity in
your ssb transceiver
with
this simple filter-
a no-holes
modification scheme
adding cw selectivity
Adding
a
crystal filter in the i-f section
of a transceiver or receiver, particularly to
improve cw reception, often presents prob-
lems. The conventional approach is to
break the signal connection between the
i-f stages to insert the crystal filter. This
requires some means of switching out the
crystal filter when it's not desired. Also, un-
less the i-f strip has sufficient reserve gain,
the insertion of the filter may cause a
serious decrease in sensitivity.
The crystal bypass filter presented here
was developed to overcome most of these
problems. It's particularly applicable to
ssb transceivers, which require better selec-
tivity on cw than provided by the ssb
filter. The ssb filter provides good skirt
selectivity so all that's needed for good
cw reception is a single crystal filter whose
frequency
is
centered in the transceiver's
filter passband. The crystal bypass filter
and the ssb filter work in tandem.
This method won't give quite as good
results as
a
six-pole,
400-Hz
cw filter, but
i t will provide better cw reception under
crowded band conditions than is possible
by using the ssb filter alone.
The only other simpler way to provide
cw selectivity would be with an outboard
audio filter. The one great disadvantage
52
june
1969
of this method is that the agc can't be
disabled in any present-day transceiver, so
strong signals entering the ssb filter still
control agc action, although the audio
filter makes them inaudible.
crystal bypassing
The simple idea of crystal bypassing is
shown in
fig.
1A.
The emitter swamping
resistor bypass capacitor provides no se-
lectivity and, in fact, is chosen so the
emitter is grounded for ac through an ex-
tremely small reactance at frequencies the
stage
i s
expected to amplify. If the swamp-
ing resistor were not bypassed, increased
signal input would produce bias across
the swamping resistor opposite to the for-
mode of the crystal will ground the emitter
for ac at only one frequency. At all other
frequencies, the stage will be highly de-
generative and operate at reduced gain.
The degree to which the crystal grounds
the emitter will determine the loss intro-
duced by the crystal. With crystals of high
Q,
which have very low equivalent resis-
tance at their series-resonant frequency,
the loss
is
small. In fact, it's conceivable
that if an i-f stage were inadequately by-
passed to start with, the crystal bypass
might cause the stage gain to increase at
the crystal frequency. O n the other hand,
if i-f stage gain
i s
independent of the by-
pass capacitor, the crystal bypass wouldn't
provide significant selectivity.
fig. 1. Capacitance and
crystal bypassing of a
common-emitter stage
(A).
Equivalent elec-
trical circuit of the
Q
ward emitter-base bias, and stage gain
would decrease considerably. This degen-
erative effect is used sometimes to stabilize
a stage, and also is the basis for many
compressor circuits.
practical application
The bypass capacitor should first be
lifted from ground to determine whether
the crystal bypass method might be effec-
tive with a particular amplifier stage. The
reduction in gain will indicate the broad-
ness of the crystal response. If there is little
gain reduction, signals outside the crystal
passband will come through at about the
same level as those in the passband. If a
considerable reduction in gain occurs and
the bypass capacitor is fairly large
(0.1
pF
or so), indicating the circuit
i s
of low
impedance, crystal bypassing will work
well.
With
conventional
grounded-emitter
transistor circuits, crystal bypassing will be
most effective. However, other bypass ca-
pacitors can be tried until the most effec-
tive gain control action is found.
equivalent circuit
The equivalent electrical circuit of a
crystal is shown i n
fig.
18.
It has two
resonance modes, one formed by the par-
allel LC combination (high impedance)
and one formed by the series LC combina-
tion (low impedance). The two resonant
frequencies are very close together. While
the series-resonant condition can't be ex-
ternally influenced, the parallel-resonant
condition
i s
easily affected by wiring and
crystal holder capacitance.
If the emitter bypass capacitor
is
re-
placed by a crystal, the series-resonant
june
1969
fl
53
Fig.
2A
shows how crystal bypassing can
be added to an existing i-f amplifier. The
crystal is inserted between the existing
emitter bypass capacitor and ground. The
capacitor should be increased to 0.1 pF if
it's not this value already. An spst switch
shorts out the crystal for normal operation.
Although the coax cable
(fig.
2A)
should
be as short as possible, it's capacitance
the diode forward current to about
500
pA.
The considerations outlined above also
apply to crystal bypassing i n vacuum-tube
i-f stages. However, the problem here is
that voltage levels are usually high enough
to damage most crystals. The series ca-
pacitor between crystal and emitter in
fig.
2A
will act as a dc blocking capacitor
when inserted between the i-f tube plate
456
kHz
I-F
STRIP
-NICAL
I
fig.
2.
Simple switch-controlled bypass circuit
(A). A
diode-switched crystal bypass for an '38-34 transceiver
is shown in 8.
doesn't affect the circuit greatly. Cable
capacitance primarily affects the parallel
resonant mode of the crystal, since the
cable
i s
shunted across the crystal.
Fig.
2B,
a variation of the circuit i n
fig.
2A,
was tried on an
SB-34
transceiver.
Diode switching eliminates an rf-cable run
from the emitter and takes advantage of
a switch position already provided on the
front panel. A similar scheme should be
possible with many other transceivers.
Switch 51 transfers the meter from a I - o h m
shunt in the amplifier plate lead to an rf
output-level circuit. The diode i s connected
to the switch arm that goes between
ground and the plate-voltage line; thus for-
ward bias is either applied to or removed
from the diode. When forward-biased, the
diode shorts the crystal for normal recep-
tion. The I-megohm series resistor limits
circuit and crystal. If the capacitor is
large enough, the crystal frequency won't
be affected. A value between 0.1 and 0.25
r F
should be about right.
a
final
word
If you would like to add better cw se-
lectivity to your ssb equipment, by all
means consider the crystal bypassing
scheme presented here. The circuit won't
work i n all ssb gear, but you can deter-
mine this easily enough as described pre-
viously. Careful examination of the front-
panel control functions on most equip-
ment should reveal one that will perform
the auxiliary job of a crystal switch. A
no-holes modification can then be added
to provide real cw selectivity at the ex-
pense of only a few components.
ham
radio
54
june
1969

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