Tektronix DAS9100 User Manual

Service Administrative Support
1450 RF LEVEL INDICATION (CONT.)
mode, the following information
was provided as part of an IOC to
the field office involved.
The AGC circuit in the 1450
samples the output video and
corrects the gain of the IF
section in order to offer a 1 volt
video signal at the output
connectors. The sample points are
controlled by the SYNC TIP and
BACK p0RCH controls.
When used in the BACK P0RCH mode,
the signal (sync tip to peak
white) is corrected for the
desired output. BACK P0RCH is a
nominal voltage of O volts, and
the Sample-and-Hold circuitry uses
this point for reference.
In the SYNC TIP mode, the signal
is sampled at sync tip. The
voltage difference between sync
tip and back porch must then be
compensated for prior to the
conversion function
(Sample-and-Hold) to achieve the
desired O volt point. If this
compensation was not done, the
circuit would see a signal that
might represent sync tip to peak
white plus sync tip to back porch,
and decrease the IF gain
accordingly in order to put a one
volt video signal out.
Since sync is a normal component
of video modulation, it therefore,
follows that absence of modulation
means absence of sync, and since
the correction for sync is still
being applied to the AGC circuit,
a corresponding offset will be
reflected in the RF Level
indications. Calculations for the
14.50-1 indicate the difference to
be approximately 2.SdB between
modulated and unmodulated signals,
as observed on the front panel RF
Level indicator.
October, 1883 laau• 31
If the customer desires to measure
RF Levels with and without
modulation, BACK P0RCH is the
suggested 100de. This also assumes
that any modulating signals
applied to the RF carrier have
been properly DC restored for the
application involved. Improper DC
restoration in Amplitude Modulated
systems will increase carrier
levels, and in Frequency
Modulation systems, will increase
bandwidth consumption. Both of
these will present the broadcaster
with technical problems in his
transmitter's capability and legal
problems with his regulating
agencies.
w2 Issue 13-16
1502/1503 DEFECTIVE CHART RECORDER
PAPER
The possibility exists that some
defective rolls of chart recorder
paper (006-1658-01) may be in stock in
the Service Centers. The defect
involves the cardboard center of the
roll being slightly longer than the
width of the paper. This causes
binding of the roll in the recorder
and inability of the motor to properly
advance the paper.
Stock should be checked and purged if
necessary.
w2 Issue 13-18
1800 SERIES TEST STATION, SIGNAL SENSE
CARD, MANUAL CORRECTION
Reference:
Signal Sense Card Manual 070-3213-00
1804 Test Station Manual 070-3331-02
Schematic Diamond 57A
1805 Test Station Manual 070-3338-00
Schematic Diamond 57
(ARTICLE CONTINUED ON THE NEXT PAGE)
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Tektronix DAS9100 Specifications

General

Brand	Tektronix
Model	DAS9100
Category	Measuring Instruments
Language	English