Belar FMHD-1, Руководство оператора

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FMHD-1 Operator’s Manual
BELAR ELECTRONICS LABORATORY, INC. Page 29
7
FMHD-1 RF Spectrum Screen
7.1 General
The FMHD-1 incorporates a sensitive and accurate DSP-based FFT (Fast Fourier Transform) spectrum
analyzer which displays the frequency spectrum of the hybrid FM IBOC HD™ signal. This signal
contains both the analog FM RF signal and the digital IBOC OFDM subcarrier sets (or sidebands). The
output is an X-Y graph showing the relative amplitude of signal energy, displayed along the vertical (Y)
axis, as a function of frequency, which is displayed along the horizontal (X) axis. This capability can be a
great aid in troubleshooting problems in the transmission system and in verifying system performance.
The frequency axis of the display indicates “0” at the carrier frequency as set on the Main Tuning Screen.
RF frequencies less than the carrier appear on the negative-axis portion of the display; frequencies greater
than the carrier appear on the positive portion of the axis. The maximum frequency span that may be
displayed at one time is 750 kHz, but the RF center frequency may be shifted off-center to enable views
as far as 825 kHz to either side of the carrier. Horizontal frequency resolution of the RF Spectrum Screen
is from 50 Hz-per-pixel to 1,500 Hz-per-pixel. A RF spectrum mask may be superimposed on the display
to quickly evaluate compliance with FM emission limitations.
The spectrum display is controlled by the rotary encoder acting upon both on-screen parameters and
parameters appearing in a dedicated sub-menu. The six on-screen measurement parameters available are
described below.
7.2 On-Screen Spectrum Parameters
The rotary encoder controls the spectrum display via six on-screen parameters and several additional
parameters that appear in a sub-menu. The six measurement parameters appear across the top of the
display and in its lower-right corner. The parameters are:
dB:
The “ dB ” parameter, for “decibel”, effectively varies the measurement scale by moving that portion
of the signal spectrum visible on the display up and down vertically. The decibel labels along the vertical
axis change accordingly. Note that the “decibel-vertical-span”, or the difference between the dB reading
at the top of the display and that appearing at the bottom, is not changed by the dB parameter. That span
is controlled by the vertical resolution step size, which varies the decibel resolution per pixel and the
number of pixels that fall vertically in the display area. The vertical span is adjusted by the “
VR
”
parameter discussed below.
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The largest vertical-axis label is +100dB; the minimum vertical-axis label that appears is –150dB.
Depending on the “
dB
” and “
VR
” parameter settings, one or neither of these two range limits may fall
within the range of a given spectrum display.
VR:
The “
VR
”, for vertical resolution, is equivalently amplitude resolution of the displayed spectrum. It is
the number of decibels assigned to each vertical pixel in the range of the display. It can be set to between
0.1 decibel (dB) and 0.8 decibel-per-pixel, in 0.1-decibel steps.
C1:
This parameter sets the decibel amplitude (vertical position) of an adjustable yellow horizontal line
(“Cursor 1”) running across the display. The C1 cursor is used to help measure the amplitude of signals
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The decibel span of the display is set by the product of “VR,” or the voltage ratio parameter, and the 181 horizontal rows of pixels which make
up the vertical portion of the X-Y spectrum display.