**DA 05-1321**

**May 10, 2005**

**OET Clarifies Emission Mask Measurements for DTV Transmitters**

The out-of-channel emission masks for full service DTV transmitters are
specified in Section 73.622(h) of the Commission's rules and, for low
power stations, Section 74.794 (47 C.F.R.§§ 73.622(h) and 74.794) . This
notice clarifies those rules and provides guidance for conducting
measurements of the out-of-channel emissions of DTV transmitters.

The out-of-channel emissions requirements for full service and low power
DTV transmitters are specified for a measurement bandwidth of 500 kHz
and are defined as attenuations relative to the total in-channel power.
The rules also state that measurement bandwidths narrower than 500 kHz
are permitted if appropriate correction factors are applied.

Direct measurements of such emissions with a 500 kHz resolution
bandwidth (RBW) may not be possible because 500 kHz RBW may not be
available on available measurement equipment. In addition, because
resolution filters on spectrum analyzers do not have "brick wall"
responses, leakage of the main DTV signal into the skirts of the
resolution filter can corrupt out-of-channel emission measurements
sufficiently to cause an apparent failure to satisfy the mask;
consequently, a sufficiently low RBW must be selected to prevent main
signal leakage into the filter skirts from causing such an apparent
failure. Depending on shape factor of the analyzer's resolution filter,
proper measurements in the 500 kHz window adjacent to each channel edge
may require an RBW of 30 kHz, 10 kHz, or even less.

Considering the above, the Commission will allow compliance with the DTV
full service or low power masks to be demonstrated by either of two
methods.

Method 1: Scale the measured power to a 500 kHz bandwidth based on
10 log (500 kHz/equivalent noise bandwidth of the resolution filter) and
compare each point on the spectrum to the emission mask; or,
equivalently, scale the measured attenuation by 10 log (equivalent noise
bandwidth of the resolution filter/500 kHz) and compare each point.

Method 2: Sum the power across a 500 kHz window in order to implement an
effective measurement bandwidth of 500 kHz; this power level is then
compared to the mask value computed at the center frequency of the 500
kHz band.

Method 2 may be performed either by manually summing the measurements or
by using the band power function on a spectrum analyzer or vector signal
analyzer. In implementing Method 2, it is sufficient to measure a
contiguous sequence of 500 kHz windows.

A single method may be used for all measurements, or, if desired, one of
the above methods may be employed in the 500 kHz window adjacent to each
channel edge and the other for all measurements further from the band
edge. Similarly, a single RBW may be used for all out-of-channel
measurements, or, if desired, a larger RBW may be used in the regions
beyond 500 kHz from the channel edge than that used in the
500[ ]{.smallcaps}kHz adjacent to the each channel edge in order to
speed sweep times or to limit the number of resolution windows that must
summed when performing manual integration across a 500 kHz span. In any
case resolution bandwidth must be no greater than 500 kHz.

In selecting sweep span and resolution bandwidth on spectrum analyzers
with digital displays, care must be taken to ensure that the frequency
bin width on the analyzer \[span/(# of points in the spectrum - 1)\] is
less than or equal to the RBW. This caution does not apply to analyzers
that have fully analog displays (typically built before the mid 1970's).

It should be noted that, depending on the type of spectrum analyzer
used, additional correction factors may be necessary to accurately
represent the average power of the noise-like DTV transmission.

For additional information regarding this notice, contact Steve Martin,
Technical Research Branch, Office of Engineering and Technology, at
(301)362-3052. A detailed procedure for measuring DTV emission spectra
is under development by the IEEE. Information regarding that standard
can be obtained at the following URL:
<http://www.ieee.org/organizations/society/bt/stnds.html>

Chief, Office of Engineering and Technology

**FCC**