|Charles W. Rhodes
Last month, we discussed third-order
seldom covered in the technical
literature: 2Fa + Fb, and 2Fb + Fa.
We started with spectrum plots for two
unmodulated carriers at the center of
Ch. 9 (189 MHz) and Ch. 11
(201 MHz). These IM3
products 2*189 +
201 = 579 (MHz) and 2* 201 + 189 =
591 (MHz) fall in the UHF band, but the
signals generating them are in the high VHF band. Therefore
I call this inter-band interference (IBI).
With a pair of Rohde & Schwarz DTV signal generators,
I set one to generate an ATSC signal on Ch. 9, and the
other on Ch. 11. Fig. 1 shows the spectrum, which
was not expected to look like this at all. Marker #2 (579.0
MHz) and Marker #3 (591.0 MHz) are at the IM3 center
frequencies. But what were the frequencies of Markers
#1 and #4?
|Fig. 1: Spectrum plot of third-order distortion products generated by two ATSC signals on Chs. 9 and 11—Mkr #1: center frequency third-harmonic center frequency
Ch. 9; Mkr #2: IM3 Chs. 9 and 11; Mkr #3: the other IM3 of Chs. 9 and 11
center frequencies; Mkr #4: center frequency of the third harmonic of Ch. 11.
I soon found that Marker #1 (567.0 MHz) is the third
harmonic of Fa (189 MHz) and Marker #4 (603 MHz) is the
third harmonic of Fb (201 MHz). These third harmonics are
about 6 dB lower in power than the IM3
products and they
The third harmonic of Fa falls within UHF Channel 30
while the third harmonic of Fb falls in Channel 36. These
being third-order distortion products of signals nominally 6
MHz wide are nominally 18 MHz (three channels wide). So
the noise power spectrum of Fig. 1 extends from Channel
29 up to Channel 37.
Amazing, but the third-order distortion products of two
high VHF band undesired (U) ATSC signals can cause interference
to Channels 29–37.
But, what about the other third-order distortion products
2Fa – Fb and 2Fb – Fa? Those intermodulation products
fall in Channels 7 and 13 respectively. 2*189 – 201 =
177 MHz. (Channel 7) and 2*201 – 189 = 213 MHz. (Channel
While the IM3 products are somewhat larger than the
third harmonics as shown in Fig. 1, these harmonics should
not be ignored. We must also keep in mind that IM3 products
cover three contiguous channels.
Higher than third-order
distortion products exist,
but in general, their power
is much smaller than the
products (second and third
order). Therefore they are
not usually significant. The
IM3 products generated by
DTV signals on Chs. 9
and 11 are 2*189 + 201 MHz
= 579 MHz and 2*210 + 189
MHz = 591 MHz. These distortion
products fall in the
UHF band, but they are not
centered in UHF channels.
For example, the center
frequency of channel 30 =
569 MHz, while the new
IM3 product is centered at
567 MHz. This is true for
all of these third-order distortion
products of two or
more high VHF band signals;
their center frequencies
are 2 MHz below the
center of the UHF channel.
In addition to IM3, we also have triple beats
(TB) of the form Fa + Fb +Fc. Where Fa, Fb and
Fc are in the high VHF band, these TBs fall in
the UHF Band. The center frequency of the lowest
TB of the high-band VHF signals (Channels
7 + 8 + 9) is equal to 549 MHz. The highest TB
(channel 11,+12+13) falls at 621 MHz.
These center frequencies are in Chss
26 and 39 respectively, but their spectrum
starts a 549 – 9 = 540 MHz signal that corrupts
Ch. 25. The spectrum ends at
621 + 9 = 640 MHz, corrupting Ch.
40. A TB is a more potent source of interference
than is an IM3 which is also generated
by the same pair of these signals.
DTV RECEPTION AFTER REPACKING
|Table 1: VHF triple beats per UHF channel
|Table 2: Third-harmonics of center frequencies of high VHF channels
|Table 3: UHF channels subject to interference by high VHF band signals
I know of no site where all seven
high VHF signals can, at the present, be
received at about the same power. But
consider that one community has Chss
8, 10 and 12 and another community
may have the rest, 7, 9, 11 and 13.
Between these communities, reception
of all seven DTV signals may be possible.
After repacking, I expect that some
communities may be allocated five or six
or even all seven high-band VHF channels.
Why not? The crowding of what
will remain of the UHF band will spill over into an already-crowded
high VHF band. When—and not if—this happens,
I expect IBI on a significant scale. Some of the TBs from
the high VHF band will fall in former UHF channels so
cellphones using the new 600 MHz band will have to be
designed with enough RF selectivity and/or wide enough
linear dynamic range to avoid IBI.
My colleague Stanley Knight, came up with Table 1,
which shows the distribution of TBs in the UHF band. The
center frequencies of the third harmonics of high-band
VHF signals are given in Table 2, which also identifies the
UHF channel within which these center frequencies fall.
Those UHF channels subject to IBI from two high VHF
band signals is shown in Table 3.
There is more to IBI that will be covered in my next
column. Stay tuned.
Charles Rhodes is a consultant in the field of television
broadcast technologies and planning. He can be reached
via e-mail at email@example.com.