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1
0.2.2.7
Log-Amp
Gain and Offset Adjust
To
con vert
t he Jog-amp
detector's output to the
required level for the Scope Amplifier's
input,
an
operational amplifier
(U206A)
is
configured
as
an
inverting amplifier to perform the
overall
gain
and
off-
set
scaling.
T
o compensate
the T
C
of the
logging
IC,
the offset
voltage
is
temperature -tracked
via
CR204.
The output voltage
is
50
m V /dB with
-80
dBm,
pro-
viding
0.0 V
de
out.
The
absolute output depends
on
t he
losses in
the Al7.
1
0
.2.2.8
Base-Line Limiter
The
spectrum-ana lyzer
display
is
base-line limited
to prevent
the
display from dropping
below t he
bot-
tom
of
the screen.
A
precision
rectifier circuit
(U206
and
CR20~)
clamps
t.he
log-amp
output whenever
it
~oes
below
-2.000 volts.
1
0.2.2.9
Offset Buffer
Amplifier
The
Jog-am p's
calibrated
output
from U206A
is
am plified and offset to
conform
with t he System
Ana-
lyzer's
internal
DVM
.
This
DVM
requires
a unipolar
input
with
a 0 to lOV range.
1
0 .2.3
FM/SSB/WB
S IGNAL-PRES
ENT
DETECTORS
10.2.3 .1 General
In
addition
to
AM
detection,
which is
described in
the linear
IF
section 10.2.1.8,
t he Receiver incorpo-
rate::;
a
pulse-counting
FM
discriminator, a product
d
etector/
BFO
for
SSB/CW
demodulation, and
a
wideband
signal-present
detector. The latter tests for
the presence
of a sufficiently strong
signal
for
the
Scan
Lock function
a
nd
flags
the CPU
when a
signal
of
suf-
ficient amplitude
is
present.
A block diagram
of the
FM/SSB/WB signal-present
detector
is show
n
at t
he
end of
the
section
in
.Figure
10-ld.
1
0 .2.3 .2
IF Limiter
The
IF
output
is
hard-limited
by
Q310/Q311
con-
figured
as an emitter-coupled
amplifier
with
positive
feedback. A high-speed,
CMOS
hex-inverter
(U310)
buffers the
limiter's
output to the
pulse-counting
dis-
crimi nator and
to
the
IF/BFO
frequency-counter select
gate.
The
limiter will provide an accurate
frequency
to
the counter
in
the
presence of as
much
as 90 percent
AM.
10.2.3.3
Pulse-Counting Discriminator
The
pulse-counting
(averaging)
technique
is
used
for
FM demodulation.
That
is, a constant width and
;lmplitudc pulse is generated for each
zero-crossing
of
10-3
the
IF
signal.
The
pulses are
then
averaged in a
n
RC
low-pass filter to generate a voltage
proportional
to
the
inpu
t
frequency.
The output
is then
a
de voltage pro-
portional
to the average frequency,
with
the
FM-
demodulated
signal
riding
on
top. The Receiver uses a
dual
-edge,
triggered monostable (U3ll/U312)
to
pro-
vide a pulse with each zero-crossing,
thereby doubling
the carrier frequency and
easing
t he filtering requi re-
ments.
Difl'erential
amplification
by
U313
removes
the
de
offset
and
increases
detection-gain by
6
dB. The
pulse width
is set
for a
50
percent duty
cycle
at
700
kHz, the nominal
IF-center frequency. U314A
pro-
vides
additional
filtering
and
FM-gain
calibration.
1 0
.2.3.4
Product
Detector
Reinjection
of the carrier in a
product detector
demodulates SSB and CW.
An
active
mixer
(U317)
performs
t he
detection;
it requires
a low-level
BFO-
injection signal minimizing
shieldi ng requirements.
Q:ll:l
and
Q:~14
form a feedback filter amplifier to
I
I amplify the detected
SSB/
CW output, 2)
filter the
IF/
BFO
components, and
3)
provide a low output
impedance
to
the audio filters.
1 0.2.3 .5 BFO
The
BFO
(Q306)
is
a modified Colpitts-type oscil-
lator with gate-bias AGC.
The
oscillator
nominal
fre-
quency
of700
kH
z is adjusta ble
±3
to
5
kHz
from the
front
panel.
Select-in-test C377
provides
centering of
C:l94
's
adjustment
range.
As
a
buH'er,
Q307 isolates the
BFO from
the product
detector
and the frequency
count
er.
Under
software control, Q315
disables the
HFO
when the
Receiver
is
not
in SSB
mode.
1
0
.2.3.6
Wideband
Signal-Present
Detector
The wideband
signal-present
detection circuit is
used only for
the
Scan Lock function. U307
provides
an amplified (40 dB)
signal
to
the
frequency
counter
via counter-output
select
gate U315, which,
in
con-
junction wit
h
the 20-MHz
low-pass filte
r
followi ng
the
first mixer
on
Al
7 A2,
allows the System
Analyzer to
determine the frequency of a
signal
within
20
MHz
of
the
L.O.
frequency. U308 mon itors the signal
strength
of
t he incoming
signals.
When the input signal is
of suf-
ficient
lP.vel
and
the L.O.
is within 20
MHz,
U308
flags
the microprocessor, which
then
determines
t he fre-
quency.
lJ:308 is
set
to
trip at a
nomi
nal
level
of
-50
dBm
into
A8-ll
.
the
Receiver
input.
10.2.3.7 Counter-Output Select Gate
The
BFO, the
limited-IF
output,
or the
wideband
signal-present
output
are selected and
routed
to the
internal
frequency cou
nter
under
software control
via
U~J!).
The
Processor hoard then determines the error
frequency
rr lat
ive
to
t he
programmed
L.O.
frequency
that is displayed
on
the CRT
in
receive
mode.
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