Readers of this column
while the FCC
still does not recognize
interference to DTV reception
caused by pairs
of strong undesired (U)
DTV signals, such interference
is real. DTV signals
on Channels 30 and
33 generate third-order intermodulation
products (IM3) centered in Channels 27 (2*
30–33) and 36 (2* 33–30).
These IM products fall close to the frequencies
of the U signals so that they are
“Inter-Band Interferences.” Alas, another set
of IM products is also generated. These are
of the form 2Fa+Fb and 2Fb + Fa. Using the
previous example, Channel 30 center frequency
is 569 MHz, and Channel 33 is 587
2* 569 MHz + 587 MHz = 1725 MHz and
2* 587 + 569 MHz = 1743 MHz. Both are far
above the UHF band so they do not present
any interference to DTV reception. This is
why most technical articles discuss the 2Fa-
Fb and 2 Fb-Fa equations and may not take
up the other pair.
|Fig. 1: Two carriers at the center frequencies of Channel 9 and 11 are shown: Mkr. #1 188.95 MHz
(9) and Mkr. #2 200.90 MHz (11).
Two strong DTV signals on high VHF
band channels present quite a different situation
for DTV reception on modern ATSC
receivers as these have little RF selectivity.
Consider two U signals on Channel 9 (189
MHz center frequency) and Channel 11
(201 MHz center frequency). These generate
IM3 centered at 2*189 + 201 = 543 MHz
and 2*201 + 198 = 555 MHz in Channels
32 and 34.
All third-order intermodulation products,
(IM3 and Triple Beats), occupy three
contiguous channels, so there can be interference
to reception on Channels 30, 31,
32, 33, 34, and 35.
|Fig. 2: The IM3 generated by these carriers (Fig. 1) are shown: Mkr. #3 578.85 MHz and Mkr. #4
590.80 MHz. These are of the form 2Fa + Fb and 2Fb + Fa. These fall in the UHF band, between 537
and 633 MHz (Chs. 25 and 41).
I have provided spectrum plots that illustrate
this situation. I’ve used unmodulated
carriers at the center of Channel 9
(189 MHz) and Channel 11 (201 MHz) in
Fig. 1. These signals were passed through a
slightly nonlinear amplifier to generate IM3.
Fig. 2 shows the IM3 (2Fa + Fb and 2Fb+ Fa)
in the UHF band.
We have measured the effect of a pair
of high VHF band ATSC signals on Channels
9 and 11 upon the sensitivity of seven
modern ATSC receivers, five of which have
a “Silicon Tuner on a Chip.”
Fig. 3 shows the minimum usable desired
(D) signal power (Channel 28) of
each receiver when there are two U signals
(Channels 9 and 11) present at the stated
power per channel.
Also shown is the noise-limited minimum
usable D signal power for each receiver
Note that when U equals –38 dBm/
channel, D minimum has reached the receiver’s
noise-limited sensitivity for most
receivers. Therefore, any U power below
–38 dBm would have no effect upon reception.
Above –38 dBm/channel, the U signals
on these VHF channels reduce the performance
of some of these receivers. Above
–26 dBm/channel, all receivers are affected.
They are all being desensitized by the IM3
from these VHF band signals.
|Fig. 3: D minimum Ch. 28 with and without U signals on Channels 9 and 11.
The D/U ratio for receiver #53 is –45.7
dB per channel. The FCC assumed that
modern DTV receivers would have a D/U
of –60 dB. The best receiver (#52) closely
approached this with D/U equaling –57.7
dB. None met the FCC D/U ratio.
DIFFICULT TO OVERCOME
You have reason to be concerned with
inter-band interference because it is difficult
to overcome in the field. A 75 ohm
high-pass filter rejecting signals in the
VHF band would do this, but some viewers
would have to switch this filter into a
bypass mode whenever a VHF channel is
D. So even if such filters were free, VHF
broadcasters may be at a competitive disadvantage
where inter-band interference
is encountered. That is, where the U signal
power per channel exceeds –38 dBm.
Another example of inter-band interference,
FMI, was described in this column
(“A New Kind of FMI on the Horizon,” Jan.
20, 2010). Multiple strong FM radio signals
can and do generate second-order distortion
products when the front-end of ATSC
receivers is overloaded. This problem can
be readily and inexpensively fixed with an
FM Trap available at many retail stores such
as Radio Shack. The FM Trap does not have
to be bypassed so the family may remain
unaware of its existence assuming that
somehow they were told that this simple
remedy could restore reception of high
VHF-band DTV signals. Alas, any filter to fix
inter-band interference to UHF reception
may have to be bypassed to receive high
VHF band stations.
What else could be done about this inter-band interference? Co-siting of the DTV
transmitters lowers the difference in received
signal power between stations. With
lower D/U ratios, FMI could be avoided.
Charles Rhodes is a consultant in the
field of television broadcast technologies
and planning. He can be reached via email