Wavetek 3000 Instruction Manual page 177

OPTION lA
After
removing the RF signal causing
the overlaod, the
switch can be reset
by momentarily turning the front-panel
AC POWER switch off .
Normal operation of the UNLEVELED
lamp is a steady glow if instru-
ment is
unleveled . If the
RF
circuit breaker is
tripped while
the instrument is
unleveled, the
UNLEVELED lamp will vary in
in-
tensity instead of flashing on and
off .
CIRCUIT DESCRIPTION
Figure 8/lA-1 is a block diagram of RF
Level Programming Option 1A .
Refer to
this diagram and the appropriate sche-
matics in section 6 to follow the circuit
information contained in this Section .
Reverse Power Protection Circuit
The Reverse Power Protection circuitry
is located in the Programmable Attenautor
and in Attenuator Drive module M41A-1 . With
the instrument's AC POWER switch turned
off, the Protection
Circuit internal
relay, in series with the RF output, is
in its normally open position . This
prevents damage to
the Attenuator or
Output
Amplifier if reverse
power is
inadvertently applied to the
RF OUT
connector while instrument is not in use .
As soon as AC power is applied to instru-
ment, the comparator, IC1, will compare
the RF monitor voltage to a fixed
reference
voltage of approximately 5
volts . As long as the monitor voltage
is less than the 5 volt reference, the
comparator output will be approximately
+17 volts . This positive output turns
on relay driver Q1, which energizes the
Protection Circuit relay, thus completing
the RF output circuit .
The positive output from TC1 also turns
on Q2 ; thus effectively grounding pin 7
o£ timer IC5, which operates as an astable
oscillator . With pin 7 grounded, the
8/IA- 4
NOTE
timer is inoperative
and its output at
pin 3 is "high" .
The "high" output from
IC5 turns off Q3 and thus
prevents the
current flow to the front-panel UNLEVELED
lamp .
If an external RF signal exceeding 6 VRMS
is applied
to the instrument's
RF OUT
connector, the RF monitor voltage will go
above 5 volts .
This will produce a
negative output from IC 1, and the feedback
provided by the negative latch diode and
resistor will latch IC1 in
this state .
The negative output from
ICI will turn
off relay driver Q1 and
cause the
Protection Circuit relay to return
to
its normally open position, thereby
removing the external RF signal from the
attenuator circuitry .
The negative output from comparator IC1
also turns
off Q2, thus removing the
ground from pin 7 of
timer IC5, which
allows the timer to oscillate . The output
at pin 3 of IC5 them varies between 0 and
+7 volts . As the output swings down toward
0, Q3 turns on and supplies current to
the front-panel UNLEVELED lamp ;
then,
as the output rises toward +7 volts, Q3
turns off and prevents current flow.
This on/off cycle of Q3 causes the
UNLEVELED lamp to flash .
The comparator cam be
unlatched _by
momentarily removing AC
power to the
instrument after the external RF signal
is removed .
Front-Panel RF Level Control
When Option lA is installed, the RF output
level is adjustable over a range of 110
dB (+13 dBm to -97 dBm) as follows : The
output can
be reduced 10 dB by level
program relay K2 on Meter
Board B005 .
The Programmable Attenuator
provides
another 90 dB of attenuation in 10 dB
steps, and the OUTPUT
VERNIER provides
continuous adjustment of the output level
over a 10 dB
range . These combine to
provide a total attenuation of 110 dB .