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I MODEL 4OO TRANSCEIVER
A.
Circuit Theoiy (Cont)
this signal is leceived at a level of 20 db
over 59. the unwanted sideband will be
pre6ent at a leve1 oI approximately 55.
T h e s a h e i s t r u e o f c a r r i e ! g u P P r e s s i o n .
W i t h c a r r i e r s u p p r e s s i o n o f 5 0 d b , a n d
a s i g n a l l e v e l o f 5 0 d b o v e r 5 9 , c a r l i e r
will be present at a level of approxirnately
53 to 54.
In the Model 400 Transceiver,
the single
s i d e b a n d , s u p p r e s s e d c a r r i e ! s i g n a l i s
generated by the crystal lattice filter
method.
Refer to the schematic diagram'
a n d t o F i S u l e 6 I a n d 2 , B l o c k D i a g r a m s
SIGNAL CENERATION
In the TRANSMIT position (i. e., when the
pu6h-to-talk switch on the lnicroPhone i3
pressed ot when the Function Switch is
moved to TRANSMIT), the transmitter
portion of the transceivel is activated,
and generates a sinSle sideband,
s u p p r e s s e d c a ! ! i e r s i g n a l i n t h e f o l l o w _
i n g m a n n e r : C a r r i e r i s S e n e r a t e d b y V l 5 ,
C a r r i e r O s c i l l a t o r , w h i c h i s a P i e r c e
oscillator,
with the crystal oPerating in
parallel resonance.
This stage operates
in both th€ ttansoit
and leceive rnodes.
When transmitting,
the RF outPut oJ the
oscillator is injected into the control grid
of the Balanced Modulator, VI4.
This
balanced modulato! is a beam deflection
type, and opeiates 5imila! to a cathode
ray tube in that the elect!on bearn frorn
the cathode i6 deflected to one outPut
plate or the other by the charge aPPear_
ing on the deflection Plates.
The RF
energy fed to the cootrol Srid of the
balanced modulator appeals on both
plates oI the output, in the absence of
signals to the deflection plates.
The two
output plates feed the carrier
to Trans_
forrner Zl40I
in push PuU, and the two
RF signals cancel each other out in the
output ot the transforn:rer.
The defl€c -
tion plate reference voltages are adjusted
bv means of the ca!rier
balance control
60 that with no audio, the RF being Ied
to the output plates will cancel out' and
the output lror'j. Zl40l
will be zero,
Audio from Microphone Amplifier
Vl6
is superilnposed on one deflection Plate,
thereby unbalancinS lhe modulator, and
the two sideba$ds resulting froh the 3uln
and diff€rence frequencies of the audio
and carrier
appear as a double sideband,
suppressed carrier
si8na1 in the outPut
o { T r a n s l o r m e r Z l 4 0 l .
T h e c a t r i e r
suppression is aPproximately 50 db.
The double sideband, suPPressed carrier
siSnal is then couPled to the crystal filter'
which euppreeees one sideband, and
permit6 the other sideband to be fed to
the First IF Amplifier,
V8.
The carrie!
frequency crystal alrd the filte! crystalg
are selected so that in the LSB poBition
on 4O and 8O hete!s,
the sideband siSnal
i s S e n e l a t e d w i t h a c a l r i e r f r e q u e n c y o f
5l?2. 8 kc, and this siSnal \till fall within
the bandpass of the filt€r 6uch that the
lower 6ideband will be attenuated by at
least 40 db.
See Figure 3. On the USB
p o s i t r o n o f 4 0 a n d 8 0 m e i e r s , t h e c a r r i e r
c l y s t a l i s 5 I ? 6 . 8 k c , w h i c h p o s i t i o n d t h e
double 6ideband signal on the other side
of the response culve oJ the filter,
attenuating the upper sideband by at
I e a s t 4 0 d b . I n l h e s i n g l e c o n v e r s i o n
mixing proces6, these sidebands become
inverted.
O n Z 0 , 1 5 , a n d l 0 m e t e r s , w h e r e o P e r a -
tion i6 generally on upPer sideband, the
s i B n a I i 6 g e n e r a t e d w i t h t h e s a m e c a r r i e r
crystal used in generating the lower side-
band on 40 and 80 meters.
The five
crystal filter used in the tranaceive!
r e s u l t s i n a n i m p r o v e d r e a P o n a e
characteristic
on the low frequency end
of the bandpass, and advantage is taken
of this eflect to provide better sideband
suppression on the lnost used 6ideband
for each frequency band,
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