This column has
used the term
“D/U ratio” (desired/
describing how the
FCC might repack the
UHF TV spectrum. In
fact, the FCC must use
this parameter because
FCC/OET Bulletin No. 69
in dealing with interference gives the interference
protection in terms of D/U ratios
(see the bulletin’s Table 5A).
Except for co-channel interference,
(CCI) all D/U ratios are negative numbers
expressed in decibels. This gets awkward
when a large negative number such as –60
dB means that a receiver is extremely robust
while less robust receivers have a D/U
such as –30 dB. I think that the use of D/U
to characterize the robustness of receivers
is a new application.
The D/U ratio of a receiver is the largest
difference between desired and undesired
signal powers at the RF input of that receiver
at which it is able to operate. This
is the way I think of it because this makes
clear that D/U is the limiting parameter for
reception of DTV signals. The critical D/U
ratio is constant over a wide range of U signal
|Fig. 1: ATSC signal spectrum at the input to a transmitter
Historically, we lived in a noise-limited
world just as Marconi did when he transmitted
the letter S across the ocean circa
1901. His was the only station on-the-air.
Many engineers believe the increased
number of stations on the remaining UHF
channels will result in increased DTV-DTV
interference when repacking is fully implemented.
The FCC recognizes co-channel interference
and adjacent channel interference
(ACI). My concern is over the extensive
use of what we old-timers called taboo
UHF channel interference (TCI). The FCC
does not recognize TCI between ATSC signals.
I believe we will soon move into an
interference-limited world, like it or not.
ACI is caused by third-order nonlinear
distortion products generated in the high-power
amplifiers of DTV transmitters.
These third-order distortion products are
largely attenuated by the RF mask filter at
the transmitter so that only a small fraction
are radiated. But that small fraction is what
Fig. 1 shows an ATSC signal in a UHF
channel at the input of the power amplifiers
of a TV transmitter. This signal spectrum
is confined to the channel limits 6.0
|Fig. 2: ATSC signal spectrum at the output of the transmitter (magenta trace) and after the RF
mask filter (blue trace)
|Fig. 3: ATSC signal spectrum at the input to the transmitter (green trace) and after the RF mask
Fig. 2 shows the spectrum of the ATSC
signal at the output of the final amplifier.
This spectrum is spread out over three
contiguous channels. The side channels
do not carry any useful information. What
is there is sideband splatter (noise). This
noise is the cause of ACI. If we have two
stations allocated adjacent channels, half of
the sideband splatter (noise) of each transmitter
falls in the other station’s channel.
This was well-known to the FCC, so they
set a limit on the radiated noise power of
each ATSC signal. They did this by defining
an RF mask, which every full-power ATSC
signal must comply with.
The “IEEE Recommended Practice for
Measurement of the 8-VSB Digital Television
Transmission Mask Compliance for
the USA” was largely written by my colleague
Linley Gumm. It can be obtained
from the IEEE where it is known as IEEE
The effect of the RF mask is shown in
Fig. 3. The noise power under the mask in
each adjacent channel is 46.5 dB below the
power of the radiated signal in the allocated
channel. This is about 22 watts average power,
enough to cause a lot of interference to
other stations on adjacent channels.
Recently Gumm and I conducted a test
of seven modern DTV receivers for their
robustness to ACI. Both our D signal on
channel N and the U signal on channel
N+1 were generated by Rohde-Schwarz
DTV signal generators.
The U signal was deliberately distorted
by carefully overloading an amplifier
to simulate the transmitter’s output
signal. As Fig. 1 shows, the spectrum of
the D ATSC signal is strictly confined to
the channel N. As Fig. 2 shows, the spectrum
of the U signal on channel N+1
spills from the U channel into both adjacent
channels. We filtered the distorted
U signal (with sideband splatter) with
a RF mask filter, courtesy of Larcan. The
filtered signal spectrum (blue trace) is
shown in Fig. 3 along with the transmitter
input signal (green trace).
We measured the threshold D signal
power at which our bank of seven
modern ATSC receivers could acquire
the D signal and display artifact-free pictures.
Table 1 reports our results with
the simulated broadcast signal shown
in Fig. 3 (blue trace). Table 1 also shows
our results when the U signal was taken
directly from the signal generator and
was therefore free of sideband splatter
(noise) in the D channel.
|Sideband Splatter in D Channel 49.5 dB Below U Power in Adjacent Channel
Note: Sideband Splatter 49.5 dB below U power is 3 dB below FCC Limit (U -46.5 dB). This is typical of ATSC Transmitters
Table 1: Laboratory test results for ACI with seven modern ATSC receivers
Without sideband splatter, the D/U
ratio was significantly changed for the
better (about 6 dB) compared to the D/U
when we simulated a real-world transmitter.
Thus, we have demonstrated that
ACI is due to the transmitter-generated
nonlinear distortion products (sideband
splatter). Those differences are given at
the bottom of Table 1. This difference,
typically 6 dB, means that the interference
in the D channel was four times
greater with sideband splatter 49.5 dB
lower than the U power, than if the U signal
were to be radiated free of sideband
The FCC limit on sideband splatter is
46.5 dB below the U power so our results
are not “worst case,” but we believe
they are representative of what transmitters
do in practice.
Next month, more test results with
these modern ATSC receivers will be revealed
and analyzed. Stay tuned,
Charles Rhodes is a consultant in the
field of television broadcast technologies
and planning. He can be reached
via e-mail at firstname.lastname@example.org.