|Charles W. Rhodes
By now, almost everyone
of Broadcasters is suing the Federal Communications
Commission over rules for the TV spectrum auction and channel repack in 2016. This suit concerns
the procedures by which the FCC will calculate
the number of persons affected by
co-channel (CC) and/or adjacent channel
There is another class of interference
by undesired (U) signals on other channels.
Some of these are known from the
days of analog TV in the UHF band as taboo
channel interference (TCI). Recently
we covered interband interference (IBI),
which always involves two or more U signals.
About four years ago this column discussed
a specific kind of IBI, frequency
(FMI). The second harmonics
of FM radio signals (88–108
MHz) fall in the high VHF
band (174–216 MHz). When
the tuner of a DTV receiver is
overloaded by one strong FM
radio signal, only the second
harmonic of that FM signal
can cause jamming of a VHF
signal on Chs. 7–13.
When there are two strong
FM signals, the second harmonics
of each may be generated
in the tuner of DTV
receivers. In addition to these
harmonics new frequencies
will also be generated in the
These are of the form
2*Fa–Fb and 2*Fb–Fa; and
two other third-order inter-modulation
which are also generated
(2*Fa+Fb and 2*Fb+Fa). The
latter do not fall in any of our TV bands so they are usually ignored
in the literature, but these
can cause IBI. As you see, FMI is
one form of IBI. This column recently
showed that two strong
high VHF band (174–216 MHz)
signals may generate IM3 of the
form 2*Fa+Fb and 2*Fb+Fa that
fall in the portion of the UHF
band that will remain broadcast
spectrum after repacking.
What if there are three high
VHF band signals (Fa, Fb, and Fc)?
These generate IM3, which fall in
the UHF band:
2*Fa+Fb, 2*Fb+Fc, 2*Fc+Fa
2*Fb+Fa, 2*Fc+Fb, 2*Fa+Fc
They also generate another
class of intermodulation products:
triple beats (TB). These are
of the form Fa+Fb +/–Fc. If all
three signal frequencies are additive,
the TB frequency is about
three times higher in frequency—and in
the case of three high VHF signals—the
TBs fall in the UHF band.
|Fig. 1: Number of triple beats per channel for various allocations
Now let us look at TBs generated by
three strong U signals in the UHF band.
These are of the form: Fa+Fb–Fc, Fb+Fc–Fa
and Fc+Fa–Fb. Note that any one of these
can be subtracted from the sum of the other
two. Fig. 1 shows the number of TBs for a
range of U signals involved.
Four signals generate 10 TBs. For example,
consider U signals on Chs. 33,
34, 35 and 36. The spectrum of these IM3
extends from Chs. 30–39 (581 + 587 –
599 = 569 MHz, Ch. 30). 608 plus 602
– 584 = 626 MHz (Ch. 39) as shown
in Fig. 1:
• Five U on Chs. 32–36 produce
13 TB (Ch. 28–40);
• Seven U on Chss 31–36 produce
19 TBs (Ch. 24–41).
Each TB has about 6 dB more noise power
than an IM3 being generated by these
My colleague Stanley Knight ran these
calculations and also plotted the total
noise power in each channel by the IM3
and TBs for each set of U channels as
shown in Fig. 2. This needs some further
Starting with four U signals on Chs.
33–36 there are 10 TBs and a smaller number
of IM3. Both TBs and IM3 occupy three
consecutive channels. The upper side channel
and the lower side channel are about 6
dB weaker than the center channel of an
IM3. We found that the noise in Ch. 30
is 3 dB greater than IM3, but Chs. 32
and 34 suffer 9 dB of noise. Ouch!
|Fig. 2: Noise power spectrum of triple beats and their sides, and IM3 beats and their sides relative to the noise of
a single IM3 interferer per channel for various allocations
Now look at the curve for seven U channels
(30–36). The noisiest channel has a
noise floor 16 dB above what a signal IM3
would have produced.
The reason why there is so much noise
dumped into channels by a large number of
U signals is that the number of TBs grows almost exponentially with the number of U
signals. This is shown in Fig. 2.
That is the larger half of the misery. The
smaller half is due to desensitization of
DTV receivers when there are strong (overloading)
U signals present. Desensitization
results when the wide band AGC System
(widely if not universally used in all DTV receivers,
not just ATSC receivers), senses the
mixer is being overloaded by the total power
of multiple strong U signals, and thus,
lowering the gain of the RF amplifier. But
of course this also attenuates the desired
(D) signal power at the mixer. That is automatically
compensated for by increased
IF amplifier gain. Alas, the increased IF gain
also means that mixer noise at the second
detector is increased and there goes the
signal-to-noise ratio. If it goes below 15.2,
dB reception fails.
While the FCC certainly intends to take
into account the effects of CCI and ACI in
its planning for repacking, it will not take
into account any of the other forms of interference.
This column has shown that there
are many forms of interference other than
CCI and ACI for DTV receivers. You might
say that DTV receivers should not use this
wideband RF AGC System. However, it must
be used to meet the ACI limits, which the
FCC has rightly imposed.
Since about 2012 most DTV receivers
have a tuner built on a silicon chip and IC.
This greatly restricts the RF selectivity of
modern DTV receivers. You just cannot put
high Q factor inductors on a chip.
Someday a new technology for fabricating
filters may come into use in consumer
products such as DTV receivers, but
there is an existing population of nearly
100 million modern ATSC receivers out
there. Perhaps 25 million of those receive
signals from an antenna at the residence.
How many of them will lose one or more
channels due to interference that the FCC
had to ignore to proceed with its mandate
from Congress to auction TV spectrum
for the public good? It is possible that the
NAB may choose to amend its suit to add
some of the other sources of interference
in its lawsuit or others may also sue. Stay
Charles Rhodes is a consultant in the
field of television broadcast technologies
and planning. He can be reached via email
at [email protected].