Belar FMHD-1, Operator'S Manual

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FMHD-1 Operator’s Manual
Page 32 FMHD-1
(This simplifies measurements for stations operating with asymmetric digital sideband power levels.)
Adjusting the “
LSB Limit ” and “ USB Limit ” parameters allows the mask to measure conformance
with both the waveform noise and spurious emission limit and the hybrid IBOC digital sideband power
level limitation.
The mask is set so that with the RF Spectrum Screen set for 1-kHz-per-pixel horizontal resolution, the
decibel amplitudes conform to the standards for measuring sideband power levels, noise, and spurious
emissions. For a discussion of the measurement techniques, refer to
NRSC-G201-A .
5
Typically, when
making these measurements, the top of the vertical axis would be set to “
0dB
” using the “
dB
” on-screen
parameter setting. (An un-modulated analog FM carrier would just reach this 0-dB reference line, but the
FMHD-1 internal computational capability determines the analog FM carrier power with a modulated
carrier.) Table 6 of NRSC-G202 shows the appropriate power spectral density “limit-lines” for an RF
mask to guarantee conformance with the prescribed total digital-sideband-power-to-carrier-power-ratio
6
.
(This ratio is also called the digital sideband power ratio relative to analog, or “dBc”.) For a –20dBc
power ratio, Figure 6 from
NRSC-G202 recommends using –40.0dBc/kHz as a mask limit-line for
emissions. Likewise, for a –14dBc digital-to-analog power ratio, the recommended mask power level
limit is –34dBc/kHz. The limit-line amplitudes are all numerically 20dB below the allocated digital-to-
analog ERP ratio, but measured in dBc/kHz. This discussion is applicable only for the case of symmetric
sideband powers. Tables in
NRSC-G202 cover cases for asymmetric IBOC digital sidebands powers.
7
With
Mask
set to "
OFF
”, no spectral-limit overlay appears.
LSB Limit:
This parameter adjusts the power level of the lower sideband mask step relative to 0 dBc, the reference
analog carrier level of 0dB. It is varied using the rotary encoder from “
–40.0 dB ” to “ –20.0 dB ” in
0.1-dB increments.
USB Limit:
This parameter adjusts the power level of the upper sideband mask step relative to 0 dBc, the reference
analog carrier level of 0dB. It is varied using the rotary encoder from “
–40.0 dB ” to “ –20.0 dB ” in
0.1-dB increments.
Measurements:
When "
ON
”, this feature activates a standardized computation of the power of the analog FM carrier and
the individual hybrid IBOC sideband power levels. The horizontal resolution is automatically set to
1-kHz resolution bandwidth and the averaging count set to 50. The following results are displayed along
the right edge of the screen:
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This spectral mask is defined in the NRSC-5-B hybrid FM IBOC waveform noise and emission limit graph, Figure 1,
in document
NRSC-G201-A
, April, 2010, available from the http://www.nrscstandards.org web-site.
6
Power limits for the hybrid IBOC digital sidebands were set in FCC Order DA 10-208, adopted January 27, 2010
and released January 29, 2010. The Order is included as Annex 1 in NRSC document NRSC-G202, of September,
2011. It is available at http://www.nrscstandards.org .
7
The 191 digital carriers falling in the 69-kHz-wide IBOC bands each side of the analog carrier frequency for the
MP1 service mode result in a PSD numerically 21.4 dB below the nominal digital sideband power ratio relative to
analog, but expressed in units “dBc/kHz”. Example, for a prescribed ratio FM Digital ERP to FM Analog ERP of –
20dBc, the power spectral density of the IBOC digital sidebands is –41.4dBc/kHz. This is the sideband level we
expect to see in a 1-kHz resolution bandwidth reading on a spectrum analyzer. So the NRSC-recommended limit-
lines at –40dBc/kHz provide some margin for error.