AM Modulation Monitor
923A
AM Modulation Monitor
Features & Benefits
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•
•
•
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125 % positive modulation meter
100% negative modulation or
carrier level deviation meter
No overshoot linear phase
filtering for high accuracy
500 kHz to 40 MHz bandwidth to
cover standard AM Broadcast
and MW
Built-in P.M.D.D. circuit for
digital peak indicator modulation
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Built-in voltmeter amplifies proof-ofperformance measurements
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NRSC compliant
Built-in modulation calibrator
eliminates need for external
calibration
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Optional frequency agile AM band
preselector for off-air monitoring
General Description
The TFT Model 923 AM Modulation
Monitor is a precision broadband
demodulator which permits extremely
accurate proof-of-performance mea-
surements for an AM transmitter and for
monitoring the modulation level of
an off-air broadcast signal when it is
used in conjunction with the optional AM
broadcast band RF preselector.
Multiple Meters
Two large front panel meters are
standard for simultaneous display of
positive and negative modulation. The
left meter can be switched to monitor
modulation of carrier shift level. The
right meter can be switched to measure
SNR or frequency response.
Digital Peak Modulation
Indicators
Model 923 contains three peak flashers
for quick visual monitoring. These
indicators "catch" modulation peaks
which cannot be observed on the
Modulation Meter.
The "+" 125%
indicator is factory set, the "+" and "-"
indicators are digitally programmable
from the front panel for peaks from 50 to
150% in 1.0% increments.
TFT's exclusive microprocessor based
Peak Modulation Duration Differentiation
(P.M.D.D.)
circuit
measures
true
modulation peaks and separates
peak modulation from transients for
accurate measurement and display.
Internal Self-Calibration
The 923 also contains a built-inmodulation calibration circuit which
provides both the "±" 100% and "+"
125% peak modulation reference
levels. The signal used for calibration
is a true amplitude modulated RF
carrier generated internally. The AM
calibrator provides an accurate
validation of the 923's modulation
measurement.
Built-in AC Voltmeter
The 923 includes a built-in 50 dB
attenuator calibrated in 10 dB steps
for making measurements of
frequency response as well as
signal to noise ratio to -70 dB below
100% modulation.
RF Preselector Expansion
Although the basic 923 is designed for
high level RF feed directly from a transmitter, it can be used
for off-air monitoring by adding a preselector module. The
preselector is frequency synthesized from 500 kHz to 1,990
kHz and tunable digitally in 1 kHz steps from the front
panel. The only requirement for off-air modulation
monitoring is an antenna; no external RF amplifier is
needed. 50 dB of AGC insures that adequate input signal
is available for AM transmitters at different powers and
patterns.
Audio Outputs
The rear panel audio outputs conform to NRSC response
for studio monitoring system
SPECIFICATIONS
RF INPUT, WIDEBAND DEMODULATOR
AUDIO OUTPUTS, REAR PANEL
Frequency Range ..... ...... 500 kHz to 40 MHz
Impedance……. ...... ...... 600 ohms, balanced,
5K ohms, unbalanced
High-Level RF Input.. ...... 1 V to 5 Vrms
Balanced Output Level ...... 0 dBm (.775Vrms)
Impedance…… ....... ...... 50 ohms
Unbalanced Output Level ... 5 Vrms
MODULATION AND TEST METER
THD .... ..... ....... ...... ...... 0.15% typical, 0.25% max. at 99%
modulation, 400 Hz tone
Meter Range..... ....... ...... 0 to 133% with "dB" scale for
measurements of positive
modulation or the right channel
Signal-to-Noise Ratio
(high level RF input) .... ...... >75 dB Below 100%
Attenuator Range ..... ...... 0 to -50 dB in 10 dB steps
Transient Response. ...... (high level RF input)<1.0%
overshoot
Accuracy........... ....... ...... ± 2.0% at 100% modulation,
sinusoidal test tone 400 Hz
Frequency Response…... 50 Hz to 15 kHz, ± 2.0%
30 Hz to 25 kHz, ± 5.0%
MODULATION METER
Meter Range..... ....... ...... 0 to 133% and "dB" scale for
monitoring negative modulation or
the left channel
PEAK MODULATION INDICATORS
"+" 125% Peak
Indicator............ ....... ...... Internally set to flash when
modulation exceeds +125%
Variable Peak Indicator . ...... Set by the front panel 3-digit switch
in 1% steps from 50% to 150% on
both positive and negative peaks
Accuracy (400Hz Tone) ...... ± 2% at 98% modulation
Frequency Response
(high level RF input) .... ...... ± 3%, 50 Hz to 15 kHz at 98%
modulation
Response Time Accuracy ... Approximately 10 cycles of 10 kHz
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
Baseband Frequency
Response ... ....... ...... ...... ± 0.5 dB from NRSC response
ALARM OUTPUT
Relay contact closures for activation of an external
alarm upon loss of modulation or carrier.
RF PRESELECTOR OPTION
Frequency Range .... ...... 500 kHz to 1,990 kHz,
in 1kHz tuning steps
RF Sensitivity.... ...... ...... 100µ V for 40 dB SNR, 1.0 mV for
55 dB SNR below 100% Mod.
Input Impedance ...... ...... 50 ohms nominal
IF Bandwidth..... ...... ...... ±40 kHz, 40 dB
Image Rejection ...... ...... > 60 dB
AGC Range ...... ...... ...... 50 dB (approximately 100 µV to 50
mV)
Output. ..... ....... ...... ...... 455 kHz, 1.0 Vrms into 50 ohms
PHYSICAL AND ENVIRONMENTAL
Size & Weight ... ...... ...... 19" W (48.3 cm) x 5 1/4" (13.3 cm) H
x 14" (35.6 cm) D;
17 lbs. (7.7 kg ) shipping weight
Power . ..... ....... ...... ...... 115/230 VAC, 50/60 Hz,
30 watts maximum
Operating Temperature ...... 0º to +50ºC
Accuracy............. ....... ...... ± 1.0%
1953 Concourse Drive, San Jose, California 95131-1708 USA
Tel: (+1) 408-943-9323 • Fax: (+1) 408-432-9218
http://www.TFTInc.com
Printed in U.S.A.© 2002 TFT Inc., Contents Subject to Change
PN 9500-0138 Rev. F 99032M
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