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1
0.2.1.6
Second
Mixer and Local-Oscillator (L.O.)
Filter
The
IF signal
is
down-converted
to 700
kHz
by U302
(an
active AGC'd
mixer) for
two
reasons:
1)
to trans-
late
the
IF
into
a
range
usable
by
the
scope's
IF
dis-
play,
and 2)
to distribute the
IF gain between
two
ditferent
frequenc
y-a
mplifier
strips
in order
to ease
isolation
and
shielding requirements.
T
o
maintain
freq uency accuracy, t he second
L.O.
is generated by
ti
lt
ering the
10-MHz
system
reference.
10.2.1.7 70-kHz IF Amplifier
The
remainder
of the
IF gain
is
provided at
700
kHz
hy
A(;(;'d
amplifier
U303 and
Q303-Q304;
these
form
a
feedback ampl ifier
for
low-output
impedance,
to
dri
ve
the
detectors
and buffer
amplifiers.
1 0.2.1.8 AM Detector
U204A, CR307, and CR308
form
a
biased-diode
peak
detector
for
maximum sensitivity
and
linearity.
U304B
fo rms
a
2-pole,
low- pass,
gain-of-10
active
filter with
3
function~:
1)
removal
of
the
residual
RF from
the
detected
baseband,
2)
baseband
amplification,
and
3)
low-out
put
impedance
to
drive the
audio-filter
strip
and
associated outputs.
The
average detected
de
from
the
AM detector
is filtered and then
routed
to
the
AGC
ampl
ifier
for level-control.
1
0.2.1.9 AGC Amplifiers and Overload Detector
U:~O!)
and U306
form
the AGC
and delayed
AGC
(
DAGC)
amplifiers,
respectively.
T
o
provide
the
level-
accuracy necessary
for
the
AM
modulation
measure-
ment
s.
the
AGC
loop is
a
single integrat
or. AGC
to
Q:lO!)
is
delayed
to
prevent
de
terioration of the
Receiver's
noise
figu re
under
initial
gain-reduction.
l/:!J6
monitors
the
DAGC output to
determine
AGC
saturation and
to
signal the computer
of
overload
con-
ditions
in
AM
and
SSB.
1
0.2.1.1
0
IF
Buffer
Amplifiers
The
IF
out
put
is
buH'
e red and
amplified
by
Q309 and
Q:H2
before
it
is
applied to
the
Scope
Amplifier
and
the
FM limit
er.
10.2.2
LOGARITHMIC AMPLIFIER
10.2.2.1
General
The
log
IF
section
consists
of a
3-stage,
synchron-
ously-!
uned amplifier
operating at
460
kHz with
taps
to
a
logging I('
at
each
stage's output.
The
linear
IF
10-2
provides an
output
after
FL301
for
the log
amp.
This
output signal
is
down-converted
via
U201
and
the on-
board
,
10.24-MHz,
XTAL
oscillator, Q204.
The
out-
put
of
the logging
IC
is amplified,
peak-detected,
and
then
gain-
and
offset-scaled
to
provide
the
log-amp
output
to
the
spectrum
analyzer
and
to
the
DVM.
A
block
diagram
of
the logarithmic
amplifier
is
show
n in
Figure 10-lc
at the
end
of the section.
10.2.2.2
Down-Converter
and XTAL Oscillator
The
IF signal
from
Q302
is
down-converted
by
active
mixer
ll201.
A
thermistor at the
mixer
input (R205)
provides
temperature
compensation
for the linear IF
amplifiers. The
10.24-MHz
L.O.
is
generated
by
XTAL
oscillator Q204.
Freq uency-calibration
of
the 10.24-
MH
z
oscillator
is
done
via
C236.
1
0.2.2.3
Synchronous-Tuned
Amplifiers
A
:~-stage,
synch
ronous-tuned
amplifier
provides
the
inputs
to
the
logging IC.
T he amplifier
consists of
three
series-
I
uned, interstage
bandpass filters- C208, C213,
C2 1R
are the associated trim
capacitors-
and
three
am
plifier
stages. The
first
stage
is a
buffer
amplifier
driven
by
the
active mixer.
The second
and
third
stages
are
differential
amplifiers with
cont
rolled limiting.
H2 I
9. R226,
and
R229
balance the
gain
between the
stages;
R229 is
an
attenuation.
1
0.2.2.4
Logging Integrated
Circuit
ll:Z04
perfo
rms the logging function.
It
consists of
four
pairs
of
diH'erential
amplifiers
with
paralleled
out
puts.
Each
successive input
is
d riven by a
signal
l
5
dB
lar~er
than
the last. (Each of the four
inputs
drives
two
stages separated by
a
15-dB
internal attenuator.)
As t he
input
level increases,
successive stages
satu-
rate. thereby piece-wise
approximati
ng
the
log
func-
tion.
1
0.2.2.5
Log
Post-Amplifier
The log-amp
output
is
amplified
and
buH'ered
by
the
post-amplifier to provide
a
high
-level,
low-impedance
dri
ve
to
a
biased-diode detector.
For gain stability,
Q20:Z forms
a
common-emitter
amplifier with ac
and
dC'
emitter
-degeneration.
Emitter-follower
Q203
buH·-
er;; Q20:Z's output
to
provide
a
low-impedance
output.
1
0.2.2.6
Log-Amp Peak
Detector
A
biased-diode peak
detector
converts the logging IC
out
put
to a
baseband
dr
level.
Buffered diode
bias
minimize;;
t he quiescent diode-current requi
rements,
and
allows
equal
bias
curren
t in both
diodes
to
mini-
mize
offset
volt
age
and
tempe
ra
tu re
drift.
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