TFT 923 User manual
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CONTENTS
Section I - General
Inf
onnation
Paragraph Title Page
1.1 Equipment Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1
1.2 Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-2
1.3 Warranty Information
..
........
..
..........................
14
1.4 Claims for Damage
in
Shipment . .
..
.....
.
..
.........
..
.
........
1
-5
Section
II
-Imtallation
2.1 Unpacking and Inspection
..............
...
. .
.............
...
.
2-1
2.2
Power Requirements
...
.
..............
........
. . . . . .
.......
2-1
2.3 Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-1
Section m-Operation
3.1 General Information .
....
.....
. . .
..
.
........
.
..
.
...........
3-1
3.2
Tum
-
on
and Warm-up
...............
.
.........
. . .
..........
3-1
3.3 Controls, Indicators and Connectors .
.....
...........
......
......
3-1
3.4
Modulation Measurement Using MODULATION Meter
..........
..
.....
3-5
3.5 Modulation Measurement Using Peak Indicators . .
............
. . . . .
...
3-5
3
.6
Residual Noise Measurement
....
. .
......
........
.
......
.......
3-5
3.7 Distortion Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-6
3-8 Carrier Power Alarm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-6
3-9 Modulation Alarm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-6
Section IV -Theory
or
Operation
4.1 Block Diagram Description . . . . . .
...
. .
....
.
......
...
..........
4-1
4.2 AM Detector, Lowpass Filter, and Summing Amplifier ....
......
.
.......
4-2
4.3 Audio Attenuator
..
. . . .
............
.
.....
..
......
.
........
4-
3
4.4 Carrier Level LED Drivers
............
.
.........
. . . .
.......
. .
4-
3
4.5 Audio Amplifiers
.......................
....
.............
.
4-3
4.6 Modulation Meter Amplifiers
.......
..
.......
.....
..
......
.....
4-3
4.7 Peak Flashers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
44
4.8 Calibration Signal Generator
.......
. . . .
..
............
.
.......
.
4-5
4.9 Pow
er
Supplies
..........
.
......
..
..................
. . .
..
4-5
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CONTENTS
(Continued)
Section V -Maintenance
Paragraph
Title
Page
5.1 General Information
..............
....
.....
....
.......
. . . . .
5-1
5.2
Periodic Maintenance
........
. . .
....
. .
...
..
.
..........
.....
5-
1
5.3 Meter Lamp Replacement
....
.
...
. . .
.....
.....
...
.
.....
...
. . .
5-1
5.4 Calibration Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5-1
5.4.1 Audio Attenuator Circuit
..
..
......
.....
..
.
......
. . . . .
...
5-1
5.4.2
Carrier Level Indicators
...
.......
.
...
.......
. .
.........
5-2
5.4.3
Peak Flashers
...........
.....
.................
. . . . • .
5-
2
5.4.4
Modulation Meter
.........................
...
...
.
....
5-2
5.4.5
Carrier Power Alarm . .
...........
• .
.....
.
.............
5-2
5.4.6
Modulation Alarm
.................
...
. .
.....
• . .
......
5-2
Appendix A -
RF
PreseJector Option
(See separate table
of
contents provided with Appendix
A)
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Illustrations
Paragraph Title Page
1.1-1
TFf
Model 923 AM Modulation Monitor - Basic Block Diagram . . . . . . . . . . .
1-1
3.3-1 Front Panel Controls and Indicators
.....
...
.
........
....
.......
3-1
3.3-2 Rear Panel Controls, Indicators and Connectors .
..
................
..
3-3
Tables
Paragraph Title
3.3-1 FrontPanel Controls and Indicators Page
3-2
3.3-2 Rear Panel Controls, Indicators and Connectors . . . . . . . . . . . . . . . . . . . . 3-4
JV
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6-2
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6-4
6-5
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CONTENTS (Continued)
Section
VI
-Engineering Drawings
Title Dwg
No
. Rev.
AM Monitor Model Assy
..................
5102-3879 . . . . . . . . . . B
AM Monitor Front Panel Assy . . . . . . . . . . . . . . .
AM
Monitor Chassis Assy . . . . . . . . . . . . . . . . . .
AM
Monitor System Block Diagram . . . . . . . . . . . .
AM Monitor Main Board Schematic Diagram . . . . . .
AM Monitor Main Board PCB Assy . . . . . . . . . . . .
5102-3880
5102-3881
6600-2764
6601-3946
6608-3946
. . . . . . . . . . A
. . . . .. . . . . B
....... ... A
. . . .. . . ... A
..... . . ... A
V
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Secti
on
I
GENERAL
INFORMATION
1.1 EQUIPMENT DESCRIYI'ION
The
TFT
Model 923 AM Modulation Monitor is intended for continuous modulation
monitoring
of
an
AM
transmitter operating in the 500kHz to
40
MHz
frequency range
to
enable
the station tocomply with therequirements
of
Section 73.
56
of
FCC
Rulesand Regulations.
The
Monitor requires a minimum I-volt
nns
RF
input from the monitored transmitter.
No
tuning is
required
at
the Monitor.
Features include:
• Flasher to indicate 125
% positive modulation
peaks.
• Adjustable flashers to indicate positive modulation peaks from
50%
to 150% .
• Adjustable attenuator for reading residual noise and low modulation percentages on
modulation meters.
•
Linear
phase filter design, eliminating overshoot
on
complex audio waveforms.
• Wide audio bandwidth, allowing distortion measurements on high frequency
modulation.
• Internal calibration circuit which digitally synthesizes a modulated carrier for both
+125% and -100% modulation to check the modulation meter and peak_flashers.
An optional, tunable
RF
Preselector can be installed
in
the Monitor for off-the-air
monitoring
of
signals in the 500
to
1990kHz range. Figure 1.1-1 is a basic block diagram
of
the equipment.
"'
[7
ANTENNA INPUT
500
kHz to 1990kHz -
HIGH LEVEL
RF INPUT
500
kH
zto
40
MH
z -
-
110/220V
AC
INPUT -
-
PRES
EL
EC
T
OR
BOARD
(OPTIONAL)
j
DC
POWER
MAIN BOARD
POWER
SUPPLY -
-
455
kHz
I F
Figure 1.1-1.
TFT
Model 923 AM Modulation Monitor -Basic Block Diagram
1-1
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1.2
SPECIFICATIONS
RF INPUT,
WIDE
BAND
DEMODULATOR
Frequency Range . . . . . . . . . . . . . . . . . . . . .
High-Level RF Input
......
. . . . . . . . .
...
.
Impedance
........
.
.......
.
.......
.
MODULATION
METER/TEST/RIGHT
Meter Range . . . . . . . . . . . . . . . . . . . . . . . .
Attenuator
Range
. . . . . . . . . . . . • . . . . . . . .
Accuracy
..............
.
...
. .
..
...
.
Frequency Response . . . . . . . . . . . . . . . . . . .
MODULATION
METER/LEFf
Meter Range . . . . . . . . . . . . . . . . . . . . . . . .
PEAK
MODULATION
INDICATORS
1
25
% Peak Indicator . . . . . . . . . . . . . . . . . . .
Variable Peak Indicator
...........
..
. . . .
Accuracy (400 Hz Tone)
..
..............
.
Frequency Response
(high
le
ve
l
RF
i
nput)
Response Time Accuracy
500
kHz
to
40
MHz
1 V
to
5 Vrms
50 ohms
0 to
133%
with
"dB"
scale for
measurement
of
positivemodulation
or the right channel.
0 to- 50
dB
in
10
dB
steps
+2.0% at
100%
modulation,
sinusoidal test tone 400
Hz
50
Hz to
15
Hz
±2
.0%, 30 Hz
to
25
kHz,
±5.0%
0
to
133%
and
"dB"
scale for
monitoring negative
modulation
or
the left channel
Internally set to
flash
when
modulation exceeds +
125
%
Set
by
the front panel 3-digit
switch
in 1% steps from
50%
to
150%
on
both
positive and negative peaks
2% at
98
% modulation
±3%,
50
Hz to
15
kHz at
98%
modulation
Approximately
10
cycles
of
10
kHz
1
-2
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MODULATION
CALIBRATOR
Type: . . . . . . . . . . . . . . . . . . . . . . . . . . . . Built-in modulation calibrator
generates a "-" 100
%,
"+
"
125
%
AM modulated carrier which is
switched into the detector input by
the front panel "CAL" button
Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . ±1.0%
Audio Outputs, Rear Panel
Impedance . . . . . . . . . . . . . . . . . . . . . . . . . 600 ohms. balanced, 5k ohms
unbalanced
Balanced Output Level . . . . . . . . . . . . . . . . . . 0 dBm (.775 Vrms)
Unbalanced Output Level . . . . . . . . . . . . . . . . 5 Vrms
THD
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.15% typical, 0.25~
max
at
99%
modulation, 400 Hz tone
Signal-to-noise Ratio . . . . . . . . . . . . . . . . . . . > 75 dB Below 100%
(high level RF input)
Transient Response . . . . . . . . . . . . . . . . . . . . <
1.
0%overshoot
(high level RF input)
Baseband
Frequency
Response
. . . . . . . . . . . . .
±0
.5
dB
,
20
Hz
to
25
Hz
ALARM
OUTPUT
Relay contactclosures for activation
of
an external alarm upon loss
of
modulation
or
carrier.
PHYSICAL
AND
ENVIRONMENTAL
Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19" W x 5 1/4" H x 14" D
Weight . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25
lbs. shipping weight
Power
: . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115/230 VAC,
50
/60
Hz
, 30 watts
maximum
Operating Temperature . . . . . . . . . . . . . . . . . . 0° to
50
°C
1-
3
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1.3 WARRANTY
TFf
inc.warrants each
of
the instruments
of
its manufacture to
be
produced to meet the
specificationsdelivered to
the
BUYER; and
to
be
free from defectsin material and workmanship
and will repair
or
replace,
at
its expense,
for
a period
of
one
year
from the date
of
delivery
of
equipment, any parts which
are
defective from faulty material
or
poor
workmanship.
Instruments found
to
be defective during the warranty period shall be returned
to
the factory
with transportation charges prepaid
by
BUYER.
It
is
expressly agreed that replacement and
repair
shall
be
the
sole remedy
of
BUYER with respect to
any
non-conforming equipment and
parts thereof and shall
be
in
lieu
of
any other remedy available
by
applicable law. All returns
to
the factory
mu
st
be
authorized
by
the
SELLER, prior
to
such returns. Upon examination
by
the
factory,
if
the instrument is found to
be
defective,
the
unit will
be
repaired and returned
to
the
BUYER, with transportation charges prepaid
by
SELLER.
Transportation charges
for
instruments found
to
be
defective within
the
first (30) days
of
the
warranty period will
be
paid both ways
by
the SELLER.
Transportation charges
for
warranty return
s,
wherein failure is found
not
to
be
the fault
of
the
SELLER
, shall
be
paid both ways
by
the BUYER.
This
warranty does
not
apply
to
instruments which, in theopinion
of
theSELLER,
ha
ve
been
altered
or
misused.
NO
OTHER WARRANTY IS EXPRESSED
OR
IMPLIED.
TFf
IS
NOT
LIABLE
FOR
CONSEQUENTIAL DAMAGES.
1-4
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1.4
CLAIMS FOR DAMAGE
IN
SHIPMENT
Your instrument should be inspected and tested
as
soon as
it
is received.
The
instrument is
insured for
safe
delivery.
If
the instrument isdamaged in any way
or
fails to operate properly,
file a claim with the carrier,
or
if
insured separately, with the insurance company.
WE
SINCERELY
PLEDGE
OUR IMMEDIATE AND FULLF.ST COOPERATION
TO
ALL
USERS
OF
OUR
PRECISION ELECTRONIC INSTRUMENTS.
PLEASE ADVISE US
IF
WE
CAN
ASSIST YOU
IN
ANY
MANNER
TFr,
Inc.
3090
Oak.mead
Village Drive
Santa Clara,
CA
95051
(408) 727-7272
1-5
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Section
II
INSTALLATION
2.1 UNPACKING AND INSPECTION
Upon receiving the instrument, inspect the packing box and instrument for signs
of
possible shipping damage. Operate the instrument inaccordance with theprocedures
of
Section
3
of
this manual.
If
the instrument is damaged
or
fails to operate properly, file a claim with the
transportation company
or
with the insurance company,
if
insured separately.
2.2
POWER REQUIREMENTS
The Model 923 is factory wired
to
operate from either a 117-volt
or
a
230
-volt AC
source.
The
line frequency must be 50/60 Hz. Maximum power required is
30
watts.
2.3 INSTALLATION
a. Mount the instrument in the equipment rack.
b. Plug the line cord into an appropriate power source
(see
Section 2.2 above)
c. Turn the front panel level INPUT LEVEL control (Figure 3.3-1, Item 3) fully
counterclockwise.
CAUTION
Do
not connect the cable to rear panel HIGH LEVEL
RF
INPUT
connector until the INPUT LEVEL control is turned fully
counterclockwise. Also, make sure that the RF input from the
transmitter is no more than
5V
rms before connecting the cable to
the
HIGH
LEVEL RF INPUT connector.
d. Connect the cable from the high-level modulated
sampling
point
on
the
transmitter,
or
from the antenna common point sampling loop, to the rear-panel
HIGH
LEVEL
RF
INPUT connector.
e. Connect all other rear-panel terminals that
are
used. Refer to Section
3.3.2
for
descriptions
of
the use
of
the various outputs.
f.
Tum
theINPUT LEVELcontrol clockwise until
the
INPUTLEVEL "SET" LED
is on.
2-1
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3.1 GENERAL INFORMATION
Section
III
OPERATION
The Model 923
AM
Modulation Monitor displays the modulation percentage
of
the
carrier being monitored.
Both
positive and negative modulations
can
be selected for monitoring.
A front panel LED flashes
when
the modulation percentage exceeds
125
percent on
po
sitive
peaks. A pair
of
front-panel LEDs
flashes
when
the
modulation percentage exceeds a preset
limit. Both positiv~
and
negative peaks
can
be monitored at the same time.
The Monitor can also be
used
to measure residual noise on the carrier and modulation
percentage
of
low-level modulation.
3.2 TURN-ON AND WARM-UP
The monitor contains
no
on-off switch,
and
is on whenever it is plugged into
an
appropriate power source. Since the instrument is completely solid-state, a warm-up
of
no
more
than 5 minutes is sufficient
for
it
to
achieve its rated accuracy.
3.3 CONTROLS, INDICATORS
AND
CONNECTORS
Front
and
rear panel controls, indicators
and
connectors are shown
in
Figures 3.3-1
and
-2. The functions
of
these items are
li
sted
in
Tables 3.
3-1
and
-2.
0
0
VIFT
IHC
•
IIOOUlATOw'LEFT
•
0
NUT
uve.
• 0
+IIOOUlATnnESTIRIGKT
0
MOOa
923
AM
MONITOR
Figure 3.3-1. Front Panel Controls and Indicators
0
0
3-1
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ITEM
1
2
3
4
Table 3.3-1. Front Panel Controls and Indicators
NAME
-MODULATION/LEFT
% PEAK MODULATION
INPUT LEVEL
+
MODULATION
/TEST/
RIGHT
FUNCTION
Analog meter. Indicates negative modulation percentage
of
carrier being monitored as described
in
Section
3.4
.
3-digit pushwheel
switch
and 3 LEDs. Used
to
indicate
that
a modulation peak has exceeded a reference level
selected via
the
MODULATION
METER
switches
(Item 6).
The LEDs can respond
to
fast
transients and peaks
that
the MODULATION meters cannot.
The pushwheel
switches
should be set
to
the
desired
modulation reference modulation percentage. The ± %
LED
pair
will
then flash when modulation
peaks
exceed
the selected reference.
The +
125%
LED
will
flash when modulation exceeds
125%
regardless
of
the setting
of
the pushwheel
switches.
Three LED indicators and one potentiometer control.
Used
to
adjust
the
carrier
input
level
from
the
transmitter
as described in Section
2.3
. The indicator can also be
used
to
observe changes in carrier
power
during
modulation.
SET
LED lights when
input
signal level is
correct. LOW LED lights
when
input
level is
too
low
.
HIGH LED lights when
input
level is
too
high.
Analog meter. Indicates positive modulation percentage
of
carrier being monitored as described
in
Section 3.4.
The scale
to
be read depends
on
which
MODULATION
METER
attenuation
switch
(Item
6)
is depressed, as
follows:
Modulation
Meter
Switch
100
%
(0
dB)
30%
(-10)
10
% (-20)
3%
(-30)
1% (-40)
0.
3%
(-50)
Applicable
Meter
Scale
Top scale
Middle scale
Top scale +
10
Middle scale +
10
Top scale +
100
Middle scale +
100
The + MODULATION meter is also use
to
indicate
residual noise, as described in Sec.3.6. Residual noise in
dB
referenced
to
100
% modulation is
the
sum
of
the
reading
on
the
lower
meter scale and the
dB
label on the
depressed MODULATION
METER
switch.
3-2
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0
0
6
7
8
Table 3.3-2. Front Panel Controls and Indicators (Continued)
MODULATION
METER
FUNCTION
LEFT
METER
CAL
Just potentiometer. rating
+ MODULATION meter as described in Section
3.4
.
Set
of
six interlocked pushbutton switches.
Each
switch
selects
an
attenuator
to
set the scale
of
the
+MODULA
Tl
ON/TEST/RIGHT meter (Item
4)
Two
interlocked pushbutton switches (MONO and
STEREO
INPUT) and one alternate-action pushbutton
switch (CAL) used
as
follows:
MONO Used
for
monitoring
AM
Monaural
broadcast programs only.
S
TERE
O INPUT
CAL
Reserved
for
future use.
On
-off
control
of
modulation
calibrator.
Use
of
the calibrator is
explained
in
Section 3.5.
Screwdriver-adju
st
potentiometer. Used
for
calibrating
-MODULATION meter
as
described in Section 3.4.
0
-
--
□
0
4 3
Figure 3.3-2. Rear Panel Controls, Indicators and Connectors
3-3
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ITEM
1
2
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Table 3.3-2. Rear Panel Controls, Indicators and Connectors
NAME
(none)
STEREO
LEFT
CHAN IN
RIGHT CHAN IN
ANT
INPUT
(PRESELECTOR)
HI/LOW
LEVEL SELECTOR
(PRES
ELECTOR)
OVERLOAD
455
kHz OUTPUT
HI-LEVEL
RF
INPUT
500
kHz -
40
MHz
1-5 VRMS
TB2
TBl
FUNCTION
Houses the
power
fuse,
power
line noise filters and
input
voltage selector
for
115
or
230VAC
.
Two
BNC
input
connectors reserved
for
future
usage
.
BNe
Connector. Used
to
connect an antenna
to
the
RF
Preselector
when
this option is installed.
2-position slide
sw
i
tch
. Effective only
with
RF
Preselector
is installed. In
the
HI
position, a
20
dB
attenuator is
inserted in
the
input
line. LOW position is
for
RF
input
levels
of
0.1
to
10
mVrms
. HI position is
for
RF
input
levels
of
10
to
50
mVrms
.
LED
.
Effect
ive
only
when
RF
Preselector is installed.
Lights when
RF
input
is
too
high
for
proper operation.
The
RF
input
level can be reduced
by
20
dB
by
changing
position
of
the
HI/LOW Selector (Item
4)
to
HIGH
BNC connector. Used
to
connect
RF
Preselector
output
to
Model
923
RF
input.
BNC Connec
tor
: Provides a means
for
introducing
transmitter carrier through a
50
-
ohm
cable. Maximum
voltage is 5 Vrms.
Terminal Board. This board carries the following output
terminals:
Loss
of
Carrier Alarm. Terminals (Pin 4 and Pin
5)
. When
RF
input
drops
below
a preset level, this pair
of
terminals
provides
contact
closures
that
can handle up
to
2 amperes
at
up
to
28
voe.
Modulati
on
Alarm. Terminals (Pin 4 and Pin
5)
. When the
modulation on
the
monitored carrier drops
below
10
%
for
a preset
time
peri
od
, this pair
of
terminals provides
contact
closures
that
can handle up
to
2 amperes
at
up
to
28
voe
.
Balanced Audio Output. Terminals (Pin 6 through 8).
These terminals
prov
ide 2.2 Vp-p modified
75
µsec
AM
Standard de-emphasized, balanced audio
output
across a
600
-ohm load.
Unbalanced Audio
Output
. Terminals (Pin
10
and Pin 11).
Th
is pair of terminals
prov
ides
14
Vp-p, modified
75
µsec
AM Standard de-emphasized unbalanced audio
output
across a 5 k-ohm load.
Terminal Board. Provides flashers and metering data
for
future Remote Meter Panel.
3-4
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3.4
MODULATION MEASUREMENT USING MODULATION METER
The + and -MODULATION Meters givea quasi-peak indication
of
positive and negative
peak modulations, respectively. For maximum accuracy, the Modulation meters calibration
should be checked regularly and adjusted
if
necessary. The meters are calibrated
by
depressing
the
CAL
FUNCTION and the MODULATION METER
100%
pushbuttons.
If
the MODULATION meter
reading is not exactly 100%, adjust the LEFT METER CAL (Figure3.3-1, Item
8)
until it
is
.
If
the
MODULATION meter reading is not exactly 125%, adjust the RIGHT METER CAL.
3.5
MEASUREMENT
OF
MODULATION USING PEAK lNDICATORS
The peak flasher are intended to catch fast transients and peaks that the meter cannot
respond to. The +
125%
flasher operates regardless
of
the settings
of
other front-panel switches,
and indicates when the positive modulation peaks exceed
125
%.
To use variable
PE
AK
flashers, set the pushwheel switches
to
the desired
modulation
percentage. The variable Peak flasher
will
then indicate when the modulation exceeds
to
the
preset value in the selected direction.
Accuracy
of
the peak flashers should be checked regularly, as follows. Depress the
FUNCTION
CAL
button; the +
125
% flasher sho
uld
light regardless
of
the pushwheel settings.
With the FUNCTION CAL switch depressed and the pushwheel switches reading 100, the PEAK
flasher should be on,
at
101
% it should be off.
3.6
MEASUREMENT
OF
RESIDUAL NOISE
Residual noise
on
the transmitter carrier
of
up
to 40 kHz can be read to
an
accuracy
of
1
dB
, and noise
of
up
to 70
kHz
can
be
read to 2 dB. Thus residual sideband measurements can
be made on transmitters using newer modulated techniques that might produce
unwanted
high-frequency spurs.
To
read residual noise, tum off the transmitter modulation. Starting with 0
dB
MODULATION METER switch. depress each switch
in
tum until a reading is obtained on the
+MODULA
TI
ON
meter. The residual noise in
dB
below
100%
modulation is the algebraic
sum
of
thelower-scale meter reading on the
+MODULA
TION meterand the value
of
the MODULATION
METER switch depressed. Residual noise measurements should be made
with
monitor reading
positive peaks.
3-5
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An
alternate
method
of
using
measuring residual
noise
that does not require turning off
transmitter modulation is to depre
ss
the MODULATION METER
switch
that
has
been
found
to
give
a suitable meter reading for the expected value
of
residual noise.
Although
the meter will
read
off-scale during normal modulation, it will
not
be
damaged
;
and
residual nojse can
be
read
on
the meter during
luUs
in
the program.
3.7
DISTORTION
1\fEASUREMENTS
Connect the
di
stortion analyzer input
to
the rear-panel UNBALANCED AUDIO
OUT
at
TB2,
and
tune
theanalyzer to the
fundamental
frequency ofthe audio
used
to modulate the transmitter.
Residual distortion
in
the Monitor is typically 0.15 percent at
99
percent modulation.
3.8
CARRIER
POWER
ALARM
The MODULATION/LOSSOF CARRIER
ALARM
terminals (pins 4
and
5ofTBl) short circuit
when
the carrier leveldrops
below
a preset level. This level is internally adjustable
between
0%
and
20%
below
normal level, and
is
factory set at
10%
below
normal. There is no
front
panel
control for this feature.
Itisemphasized that the CARR
IER
ALARM
terminals are DPDT gold alloy relay contacts.
The voltage at the terminals must
not
exceed +30 volts
and
the current
through
the terminals
must
not
exceed 2 amperes.
3.9
MODULATION
ALARM
The rear panel MODULATION/LOSS OF CARRIER
ALARM
terminals (pins 4
and
5 of
TBl)
short circuit
when
the modulation on the monitored carrier drops below
10%
for a preset
period
oftime. This period is internally adjustable
between
2
and
30 seconds. There is
no
front-panel
control for this feature.
3-6
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Section
IV
THEORY OF OPERATION
4.1 Block Diagram Description
The modulated carrier to be monitored is brought in through the
RF
INPUT
connector
and
is
adjusted by means
of
the
CARRIER
LEVEL
potentiometer
to
1V rms at the input to the AM detector.
This corresponds to the level at which the
CARRIER
LEVEL
"SETR
LED
is
lit. The
AM
detector
is
a full-wave rectifier which cancels out the carrier frequency. This provides a wider bandwidth
through the lowpass filter to the metering circuit without allowing the carrier
to
be passed.
Theoutput
of
the
AM
detector is a DC voltage, which is proportional
to
the input carrier
level and the audio modulation riding
on
the DC. This composite signal is fed through the linear
phase lowpass filter which eliminates the carrier and its harmonics and does not produce overshoot
on square-wave audio
or
complex music waveforms. TheDC out
of
the filter is amplified
to
drive
the
CARRIER
LEVEL
LEDs. This circuit is factory adjusted so that a 1-volt rms input to the
AM
detector turns
on
the SET LED. Theamplified DC
is
also used to activate
an
external carrier power
alarm by providing a relay contact closure
when
the DC level out
of
the lowpass filter drops
10%
below normal level.
The audio out
of
the lowpass filter
is
applied
to
a voltage-controlled attenuator to
keep
the
modulation meters
and
flashers calibrated as the RF level varies within ±30%
of
the normal level.
The attenuator is in a loop with a buffer amplifier and comparator. The buffered DC voltage out
of
the attenuator is compared with a reference DC voltage, and any difference is applied
to
the
attenuator to correct its DC output. Holding the DC level constant maintains the calibration
of
the
meter and flasher circuits.
The audio output from the buffer amplifier
is
fed
to switch SWl-C to measure positive
modulation peaks,
and
it is also inverted
and
applied to switch SWl-B to measure negative
modulation peaks. Either
of
these can be selected
by
the switches for driving the
sMODULATION
meters through the meter amplifiers, attenuator circuits, and meter detectors. The adjustable
attenuator makes it possible to read audio voltages down to 70
dB
below
100%
modulation, for
measurement
of
residual noise and
hum
without
an
external voltmeter.
4-1
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The positive and negative modulations are also applied to the Pulsewidth Differential
Detector (PMDD). When the front-panel switches are set to from 50% to
150%
, they control a DC
reference voltage to the detector. This voltage
is
compared with the + and -modulation audio
peaks in the PMDD peak comparator. When the audio peaks reach the value
of
the reference DC
voltage, pulses are generated in the comparator and sent
to
the PMDD
peak.
flasher (which
is
a
programmed microprocessor), against a pulsewidth duration, Tx, set by a thumbwheel switch.
If
the incoming pulse is greater than Tx, a 1.5-second high output is generated by the microprocessor
to
activate a corresponding LED
on
the front panel. The push-wheel switches can be set in 1%
steps
from
50%
to
150%
on both positive and negative peaks.
The +
125%
peak modulation detector is factory set for
125%
on positive peaks only, but
can be internally adjustable from
115%
to
125%
.
Theaudio from the buffer amplifier is amplified
and
fed
to two standard audio outputs; one
(UNBALANCED AUDIO
OU
T)
is
5 volts rms into 5000 ohms,
and
the other (BALANCED AUDIO OUT)
is
0
dBm
into 600 ohms.
The calibration circuit consists
of
a 500 kHz oscillator which simulates a carrier.
Modulations
of
-
100
%and +
125
%are digitally generated, and the modulated carrier is fed into the
AM detector when the front-panel
CAL
function switch
is
depressed. Since the modulation
percentage
of
the calibration signal is precisely determined by thedigitalcircuitry and is independent
of
the carrier level and carrier frequency, the
MODULAT
I
ON
meters and the
peak
flashers can be
calibrated regardless
of
the reading on the INPUT LEVEL LEDs.
4.2
AM Detector, Lowpass Filter, and Summing Amplifier (Figure 6-S)
The detector circuit consists
of
inputtransformer T2 and diodes
CR3
and CR5 in a full-wave
rectifier circuit. The diodes are biased to reduce distortion which may occur when modulation
approaches 100%. This bias
is
factory adjusted by means
of
potentiometerR77, and should not be
readjusted in the field unless
an
AM Generator
with
distortion less than 0.1%
at
99%
modulation
is
available.
U8
, U9 and associated components constitute
an
active, linear phase, Gaussian,
Jowpass
filter. The 3-dB cutoffis at 80 kHz, making it possible to measure all sidebands out to 70
or
80
kHz. The bias on the AM detector diodes adds a DC component to the audio out
of
the lowpass
filter that is not due to the carrier level. To compensate for this, the same bias that is applied to
4-2
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