This time each year I usually present a technical paper dealing
with some aspect of DTV reception at the IEEE International
Conference on Consumer Electronics and this year is
no exception. Very few broadcasters attend this conference. The
attendees are the engineers who design consumer electronics
products. This conference is where east meets west, so I can connect
directly with the engineers who will design most of the DTV
receivers for the North American market.
My paper addresses the study that colleagues Linley Gumm and
Stanley Knight and I have worked on in 2012. We tested 26 NTIAapproved
converter boxes vintage 2008 for their rejection of interference. We assume
that they represent, more or less, the population of some 30 million ATSC converter
boxes in use in American homes today. With, say, 120 million ATSC receiving devices
in the United States, these are a significant part of the population.
The analog front-end of converter boxes and those of DTV receivers are similar, if
not identical. Finding out how well these behave in an interference-limited environment,
such as we expect after the FCC carries out it announced plan to re-pack the
UHF TV spectrum, is a timely idea.
TABOO CHANNEL INTERFERENCE
The FCC regulates co-channel and adjacent channel interference by its channel
allotment plan. It does not regulate what I call “taboo channel interference,” a term
left over from analog television, meaning one undesired signal two to eight channels
above or below the desired signal, or 14 to 15 channels above the desired signals.
Taboo channel interference exists because of receiver performance limitations, such
as RF selectivity and the effects of nonlinearity, especially of the mixer. So we tested
the UHF taboos.
The metric the FCC uses is called the “Desired/Undesired” (D/U) signal power
ratio. In analog systems, the D/U of a given receiver for a specified offset between
desired and undesired channels is a constant;
it does not change with the power of the undesired
signal. It is also called the “protection
ratio.” If a receiver is nonlinear (mixer overload),
the D/U changes with undesired power.
Our first finding was that, with these 26 ATSC
converter boxes, one undesired signal did not
drive the mixer into overload as the D/U ratio
we measured was constant for a undesired power
of from –11 dBm to –41 dBm. This was great
news.But, we noted there were two groups of
data. We determined that four of these ATSC
converters have a double-conversion tuner
(DCT) and 22 have a single-conversion tuner
(SCT). They behave differently to undesired
signals. An SCT has one mixer to convert the
desired signal to the intermediate frequency, 44
MHz. The local oscillator frequency is set to be
44 MHz above the desired channel.
|Fig. 1: Double-conversion tuners outperform single-conversion tuners in rejecting interference on N+14 and N+15, but SCTs outperform in rejecting other interference.
A second signal 44 MHz above the local oscillator frequency will also be converted
to the intermediate frequency of 44 MHz.
This is called the “image response.” The image
frequency is 88 MHz above the desired
channel. Removing the undesired signal 88
MHz above the desired signal must be done
by a filter before the mixer. That means the
filter must track the desired frequency. This
is called a “tracking filter.” These are expensive
to manufacture, so an alternative to a
tracking filter was invented. It is the doubleconversion
tuner. It does not need a tracking
filter. The desired signal is up-converted to a
first intermediate frequency, which is above
the highest frequency to be received.
Whether the viewer wants to watch
Ch. 2 or 51, that signal is up-converted
to 1222 MHz in the four units under test
with a DCT. Every desired signal frequency
is now 1222 MHz. A bandpass filter, which
passes all frequencies from 1212 MHz to
1232 MHz, and which attenuates all frequencies
below 1200 MHz and above 1240
MHz, rejects nearly all undesired signals,
especially the image frequencies. A second
mixer converts the desired signal of 1222
MHz down to 44 MHz, the second intermediate
We found that the median value of the
undesired signal power for SCTs was quite
different from that of the units with DCTs.
If we had published the median for all 26
units, the report would not reflect the performance
limitations of the four DCT tuners
because that would be masked by the dominant
22 units with an SCT.
Anyone planning the repacking of the
TV spectrum by using this single median
would be misled by such data. Remember
that, at the median desired power, or median
D/U power 50 percent of receivers fail.
That didn’t seem to be reasonable criteria
So we used a trick every statistician
knows; we also calculated the variance of
standard deviation. I picked a variance of
1.0 at which 84 percent of receivers have
not failed. If the median was –36 dBm, and
the variance is 5 db, then if the desired power
is increased from –36 dBm to –31 dBm,
84 percent of these units would be working.
While the DCT tuners did better at rejecting
image frequency interference, they
did worse than the SCT for the other interference
tests. This is shown in Fig. 1.
A single, strong undesired signal desensitizes
the receiving device because it has a
wideband RF AGC system. This controls the
gain of the RF amplifier, by sensing the total
signal power at the RF input to the mixer.
(Remember, it is the mixer that gets overloaded.)
When the RF amplifier gain goes down,
the IF amplifier gain goes up and the
noise power generated by the mixer goes
up too, thus the SNR drops. At an SNR
of 15.2 dB, the ATSC signal cannot be decoded,
hence reception fails. For example,
if a given unit has a noise-limited threshold
of –84 dBm, an undesired signal at, say,
–23 dBm causes Dmin to increase from
–84 dBm to –65 dBm; the loss on sensitivity
is 21 dB. This is the desensitization for that
unit at that undesired power.
Wideband RF AGC is widely, if not universally,
used in DTV tuners to allow them
to receive the desired signal when there is
a strong undesired signal on an adjacent
channel. Our tests show how effective this
Manufacturers of consumer electronics
do not reveal what is inside the product.
We determined which units have a DCT by
measuring the frequency of the very small
local oscillator signal power present at the
“F” connector. Spectrum analyzers require
a low-noise preamplifier ahead of the analyzer
to detect such small signals.
We discovered an easier method. Double conversion
tuners in DTV receivers probably
have a first intermediate frequency of
1222 MHz. This is the second harmonic of
Channel 37 (611 MHz). Units with a first intermediate
frequency of 1222 MHz are sensitive
to an undesired signal on Channel 37.
A signal generator with output at 611 MHz
will quickly separate DCT and SCT units.
Charles Rhodes is a consultant in the
field of television broadcast technologies
and planning. He can be reached via email