Advertisement
4-1
GENERAL
SECTION 4
THEORY OF OPERATION
The monitor is basically a special;
FM superhet
ver.
It has
been s
ifically
signed to operate in a
high
RF environment and as such
does not
an RF preamplifier.
It
s also been designed with a wide
IF filter
in
to meet FCC requiremen
for FM modul
on monitors.
Because of these
ints
it
is neither as sensitive nor as selective as
a normal
FM receiver.
However,
it
will operate in close proximity to antenna
farms and mas
r antennas
i
rmodulation and overload problems
normally encoun
red in these areas.
Un1i
691 is
other monitors/RF ampli
er systems, the signal path in the
same whether the monitor;s
ven from
transmitter or
0
air
«
transmi
dation
within
only difference is a -40dB signal sampler when driven from the
input (RF IN) port.
This means that
is
no signal degra-
used off-air above the mute level.
This 1
is set for use
70dBu contour with
antenna prov;
Refer to Block Diagram
12001.
RF signal ;s fed to either the Antenna
or RF inputs.
The
si
1 then passes through
voltage tuned
i
on board
(AI3)
then to
A4 receiver assembly.
The output of
e
locked 1st Local Oscillator is also fed to
A4 assembly producing a 10.
IF.
The L.O. phase lock loop circuitry is 1
on the A2 assembly. The
IF
is
fil
red, demodulated, and
resulting composite signal is
ampli
ed and fed
the various filtering, stereo and SeA demodulation and
metering circuitry required.
A portion of
10.7MHz IF is also fed to a
mixer a10ng
a swept 2nd L.O. ope
i
at 10.255MHz
+
The re-
sulting 455KHz
+
is filtered by a ceramic filter.
to a logarithmic
amplifier, demodul
and used to provide
a
spectrum analyzer presentation
on an
x-v
oscillos
4-2
DETAIL CIRCUIT DESCRIPTION
1.
Al (Chassis) Assembly
The
sis assembly contains the power supplies
fil
jIst L.O., meters, indi
and controls.
regula
by three terminal
2.
A2 (PLL
and Total
Oriver) Assembly
• -12V. +5V),
supplies are
The output of a hi
lity crystal
oscillator {VI, Q
is
divided down to
A sample of the
L.O. is fed
to
Q9
to a programmmable divider.
The CARRIER FREQUENCY thumbwheel p
di
so that when the
L.O.
is lO.7MHz
the thumbwheel sett
the divider output is 5KHz.
The reference signal and the programmable sig-
nal are compared
and the resultant error voltage is fil
by
active filter U
U16 and then fed to
AfC port of the
t
L.a.
U12
. is used to light
L.O. UNLOCK led
if
lock or reference is 1
Ull ;s
used to genera
KHz pulse for the calibrate function.
Advertisement
