Belar FMHD-1, Руководство оператора

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FMHD-1 Operator’s Manual
Page 6 FMHD-1
The FMHD-1 does not have a power switch. When the AC-line power cord is connected to the power
supply module and the module is connected to the DIN power jack on the FMHD-1 chassis, the unit is
operating.
3.2 Monitoring Location & RF Signal Source
The FMHD-1 is designed to operate either at the station’s transmitter site or at a remote receiving
location using an outdoor antenna. Operation using a direct feed from the transmitter provides the most
accurate readings and should always be used for proof-of-performance or system performance
measurements.
Monitoring hybrid IBOC HD™ FM broadcasts at the transmitter presents multiple connection
possibilities for the
FMHD-1 . It is important that the RF samples used for monitoring (as well as for
determining compliance with spurious emissions limits) be taken as close to final antenna feed point as
possible. This way, all of the effects of any band-limiting or other imperfections in the RF system are
included in the performance and spurious emissions measurements.
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Common-Line Facilities:
For the low-level combining case, a single high-level RF sample to feed the FMHD-1 is taken from the
transmission line to the antenna . Similarly, in the case where the analog and digital signals are combined
at high-level in a coaxial combiner, a single high-level RF sample is taken from the transmission line to
the antenna. It is important that the directional coupler used to obtain this RF sample have sufficient
bandwidth to provide an accurate representation of spurious emissions. Additionally, it is recommended
that the directivity of the coupler be at least 30dB to prevent reflected power in the line from overloading
or corrupting the readings of the
FMHD-1 or other instrumentation used to evaluate the RF system .
Separate-Line Facilities:
In the situation where two transmitters independently feed either a dual-feed antenna or two separate
antennas via two transmission lines, the two high-level inputs on the
FMHD- 1 are fed RF samples
obtained from each transmission line. A precision RF combiner in the monitor sums the analog and
digital RF components to form the complete IBOC RF signal, while maintaining isolation between the
two RF feeds. Two rear-panel set screws allow adjustment of the relative amplitudes of the analog and
digital portions of the HD signal so that the
FMHD-1 spectrum display and relative power calculations
accurately represent the off-the-air signal. Just as for common-line HD facilities, it is important that the
directional couplers used to obtain the RF samples have sufficient bandwidth and directivity to both 1)
provide an accurate spectral representation of the transmitted signal and 2) to prevent overloading or
corruption of the readings of the
FMHD-1 or other test instruments.
NOTE: Because the analog and digital RF signal levels in separate-line facilities are adjusted
independently at the monitor, it is necessary to have completed the NRSC-recommended mask
compliance measurement immediately prior to setting the input levels on the
FMHD-1 . Otherwise the
monitor’s indication of the analog-to-digital sideband power ratio will be inaccurate.
Monitoring at the Studio:
Placing the FMHD-1 at a remote site, typically the main studio, requires an outdoor receiving antenna
capable of providing an un-impaired off-air signal of the station(s) to be monitored. For remote
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Recommendations for measurements of hybrid FM IBOC transmission systems are presented in National Radio
Systems Committee document NRSC-G201-A, NRSC-5 RF Mask Compliance: Measurement Methods and Practice,
April 2010. This document is available on-line at www.nrscstandards.org.