Section IV
Example 3:
FREQUENCY SPAN
.............
SK/DIV
START/CENTER
.................
START
GAIN (Point A)
.............
;
......
- 1.5
RAMP VOLTAGE (Point A)
.....
0 V
to
- 6 V
·
RAMP
CONTRIBUTION
TO OUTPUT
...............
0 V
to
+
9V
FREQUENCY DIAL
................
5 kHz
DIAL CONTRIBUTION
TO OUTPUT
..................
+ 0.45 V
OUTPUT RAMP
......
+ 0.45 V
to+
9.45 V*
FREQUENCY
SWEEP
......
5
kHz
> 50 kHz
*Out
of
Range
4-97. Out
of
Range Detector.
As
illustrated in Example 3,
certain combinations
of
FREQUENCY and FREQ SPAN
settings cause the voltage at the output
of
the Dial Mixing
Amplifier
to
go
belOw
the
0 V (0 Hz) lower limit or above
the + 9 V (50 kHz) upper limit.
When
either limit
is
exceeded, the VTO frequency is driven
out
of
range. This
could cause erroneous responses to appear on the display.
The Frequency Out-of-Range Detector senses when the Dial
Mixing Amplifier
output
is more negative than 0 V
or
more
positive than + 9 V and, in
tum,
generates an (L)Data flag
which is applied
to
the Digital Storage section. In the
Digital Storage section, the (L)Data flag clears the memory
locations where the frequency is out
of
range.
As
a result, a
clean baseline appears on
the
display (Figure 4-14).
Model 3580A
Figure 4-14. Frequency Out Of Range.
4-98. VTO and Tracking Oscillator. Refer
to
Figure 4-15
for the following discussion.
4-99. The 0 V
to
+ 9 V ramp from the Dial Mixing
Amplifier
is
applied
to
the
VTO and
to
the inverting
port
of
the VTO Error Amplifier. At
the
inverting port
of
the Error
Amplifier,
the
ramp voltage
is
summed with a de voltage
from the front panel ZERO CAL potentiometer. The sum
of
the two voltages serves
as
a reference for the frequency
control loop.
4-100. The VTO. The VTO
is
a conventional oscillator
circuit
that
is
tuned by changing the de bias on
two
varactor diodes which are the capacitive elements in the
LC
tank circuit. The 0 V
to
+ 9V ramp coarse tunes the VTO
av
TO
+9V
{FROM
DIAL MIXING
AMPLIFIER)
COARSE
TUNING
r-~o.
-oUfPUfl
i.JLM.!:!.z...=
~~M~;_lJ
SUMMING
+V
JUNCTION
-v,'
1--====------------IOOkHz
TO
150kHz
{TO
INPUT MIXER)
~=====FREQUENCY
DISCRIMINATOR=======.
0
'ZERO'
L~AJ._J
4-14
r====TRACKING
OSCILLATOR====\
50
kHz
LOW-PASS
FILTER
r-----,
L~~T-~Ef
J
CAL
POWER
___
~
{FROM
A9).
Figure 4-15. VTO And Tracking Oscillator (AS).
IOkHz
CAL SIGNAL
{TO
A9)
3580A·C-3567
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