[opendtv] Re: 20050727 Wolfsson's Wednesday Words (Mark's Monday Memo)


Mark Schubin wrote:

 >
 > - Non-masked Charlie Rhodes has an even more interesting piece in TV 
Technology.  Here's an excerpt:
 >      "This brings us to double conversion tuners like the type Zenith 
designed for testing its 8-VSB DTV modulation technology.  It was the 
outstanding interference rejection of that double-conversion tuner that 
led the FCC to conclude that the UHF taboos did not apply to DTV":
 > <http://www.tvtechnology.com/features/digital_tv/f_charles_Rhodes.shtml>
 >

I've included that article here in hopes someone can explain it better. 
  Does it help or have implications in explaining the Zenith mystery 
boxes tested at Mark's apartment?  And will the lack of double 
conversion tuners mean we have to set aside extra channels to avoid 
interference in some areas?

- Tom

----------------------------------------------------
Digital TV: Charles W. Rhodes

The Superheterodyne Concept and Reception

The first television receivers were designed to receive BBC 
transmissions from the Crystal Palace transmitter in London. This was, 
incidentally 405-line all-electronic television, then regarded as HDTV 
by those who saw it. As there was only one transmitter in all of the 
United Kingdom, extremely simple receivers were practical. This simple 
receiver topology encouraged home constructors to do it themselves, and 
many did.

All later television receivers were of the superheterodyne type. In 
fact, all radios, television receivers, terrestrial and satellite, and 
all radar sets employ this superheterodyne principle, invented in 1916 
by Edwin Howard Armstrong when he was serving in the U.S. Army in 
France. The concept came as a solution to the problem of trying to 
amplify the weak "wireless" signals with the primitive triode vacuum 
tubes then available. He hoped to detect approaching aircraft by hearing 
the impulse noise of the ignition systems; that would require a lot of 
sensitivity before the detector tube. Tubes in those days were ill 
suited to amplify high-frequency signals, so Armstrong reasoned, "Why 
not heterodyne the high-frequency signal to a much lower frequency where 
it could be efficiently amplified as much as one might wish?"

This is the fundamental concept of all superheterodyne receivers.

The concept might have worked out given more time, but World War I ended 
too soon. Major Armstrong went home and invented the superhet radio 
receiver and went on to also invent the super-regenerative detector (the 
circuitry of the proximity fuse used in World War II) and in 1933, 
wideband frequency modulation radiobroadcasting.

His FM transmitting tower is still in use at Alpine, N.J. atop the 
Jersey Palisades. After 9/11, TV stations operated from that historic 
tower to serve the New York area.

Today we don't use vacuum tubes in receivers, but all radio and TV 
receivers use Armstrong's superheterodyne receiver principle. The 
strengths and weaknesses of this invention are important to the future 
of terrestrial TV broadcasting, so please read on ; you can quickly 
become an expert on superheterodyne receivers and amaze your boss.

INTERFERENCE REVIEW

This column has said a lot about signal distortion, especially IM3 
(third order inter-modulation) distortion. This is because third-order 
distortion is responsible for most interference between signals; it is 
both inherent and always bad in amplifying devices. But there is a very 
useful order--second-order distortion--that is also inherent in active 
devices, and without it, we would not have radio or television.

Second-order distortion produces sum and difference frequencies of pairs 
of frequencies present at the input. Let us call the signal frequency Fs 
and for the frequency of a local oscillator (LO), we'll use Fo. 
Second-order distortion produces two new frequencies: Fo + Fs and LO - Fs.

The difference frequency is usually the useful component in the output 
of the frequency mixer. You might wonder how the feeble radio signal can 
produce any kind of distortion? Alone, it cannot. The much stronger 
power from the LO drives the mixer into nonlinearity and causes it to 
generate the sum and difference frequencies. The signal carrier and its 
sidebands are shifted in frequency, but not otherwise altered by the 
mixer. The modulation of the signal is not affected in this frequency 
conversion process.

Armstrong's superheterodyne receivers were revolutionary in 1923. They 
were so sensitive they could be used with a loop antenna instead of the 
usual long-wire antenna needed for TRF and regenerative receivers. The 
only tubes available were triodes and these could only amplify signals 
far below the broadcast radio frequencies. Early superhets used an 
intermediate frequency (IF) between 30 kHz and 90 kHz. And herein lies a 
problem.

Consider the receiver was tuned to 800 kHz by tuning its LO to 845 kHz. 
The useful mixer output was at 45 kHz and this was amplified in a 
cascade of tuned IF amplifiers.

But alas, what if there was a second station at 890 kHz? This would also 
be heterodyned to the IF and it would be heard too. The only way around 
this is to provide RF selectivity ahead of the mixer. The RF filter 
would be tuned to the desired signal, and thus the undesired signal 
would be attenuated.

The higher the IF, the greater the attenuation of the undesired signal. 
But until pentode amplifier tubes were introduced, an IF frequency above 
90 kHz was not practical. This form of interference was named "image 
response." This naturally causes a lot of confusion in the TV world, but 
this name is still being used.

ADDING PICTURES

Jumping from radio to TV, the first pre-World War II RCA television 
receivers were superheterodynes with a picture IF of 12.75 MHz. The 
sound IF was 8.25 MHz.

The frequencies used for television broadcasting were 44 to 90 MHz.

The picture carrier was 1.25 MHz above the lower channel edge, the sound 
carrier 0.25 MHz from the upper channel edge, and each channel was 6 MHz 
wide. The lowest picture carrier frequency was 45.25 MHz. The picture IF 
was 12.75 MHz so the LO was tuned to 58 MHz. Such receivers were 
vulnerable to image interference from a signal at 58 + 12.75 MHz = 71.00 
MHz (there were no TV channels between 72 and 78 MHz then). An aural 
carrier at 71.75 MHz would have produced a signal just outside the IF 
bandpass at 13.75 MHz, which would not have reached the second detector.

After World War II and up to 1952, television receivers used a higher IF 
near 26 MHz for the picture, and went 4.5 MHz lower for the aural IF, 
which was by then using the Armstrong's frequency modulation. This 
increase in the receiver IF was forced by the introduction of the high 
VHF band, from 174-216 MHz. Suppose the earlier IF had been retained in 
the post-World War II period. The Channel 7 picture carrier, 175.25 MHz 
plus the picture IF of 12.75 MHz would equal the LO frequency of 188 
MHz. The receiver also would have responded to an interfering signal at 
188 + 12.75 MHz = 200.75 MHz, but this is in Channel 11, so interference 
would have resulted from such a low IF. The receiver IF had to be moved 
up again. With a picture IF = 25.75 MHz, the LO to receive channel 7 = 
201.00 MHz and the image frequency is 201.00 + 25.75 = 226.75 MHz, which 
is why the high VHF band could not have been extended.

When the UHF band was opened in 1952, the receiver IF had to be 
increased again, this time to 45.75 MHz for the picture. It can be shown 
that Channel 29 will cause image interference to receivers with this new 
IF, so the UHF taboos, n+14 and n+15, were adopted by the FCC to prevent 
IF interference. The FCC did not mandate the new IF, but all 
manufacturers voluntarily adopted it to avoid marketplace disasters.

You might be asking, "Why didn't they go to a sufficiently high IF in 
1952 as they had done in 1946?" Let's do the numbers.

The UHF band in 1952 extended to Channel 83, (884-890 MHz). The IF would 
have to be above Channel 13, e.g., 230 MHz. The limitations of vacuum 
tube technology at that time prevented a high enough IF except for 
military radars, a few of which had 200 MHz IF late in World War II.

Even today, the cost of an IF amplifier flat over 6 MHz and centered 
around 230 MHz would be unacceptable. So the FCC did what it had to do, 
establish the UHF image taboos we still know as n+14 and n+15.

DUAL TUNER DESIGNS

This brings us to double conversion tuners like the type Zenith designed 
for testing its 8-VSB DTV modulation technology. It was the outstanding 
interference rejection of that double-conversion tuner that led the FCC 
to conclude that the UHF taboos did not apply to DTV. The performance of 
that double-conversion tuner became the basis for the DTV planning 
factors that enable DTV broadcasting in those vacant channels within 
existing broadcast spectrum.

A double-conversion tuner is a cascade of two single-conversion tuners. 
The first frequency conversion results in the desired signal being 
upconverted to the first IF. The first IF signal is filtered to reject 
image frequency interference. Zenith chose 915 MHz for its first IF, a 
frequency above the UHF broadcast band.

To tune Channel 2, (54-60 MHz), the first LO was tuned to 915 + 57 = 969 
MHz. The image frequency would be 969 + 57 = 1,884 MHz up there in the 
microwave region. This was easily filtered by a low-pass filter at the 
tuner input port because of the 2:1 difference in image frequency and 
the filter cutoff frequency (800 MHz).

After the first IF filter, the desired signal was heterodyned to the 
receiver's second IF of 44 MHz by a second LO operating at a fixed 
frequency of 915 + 44 MHz. Most of the signal amplification is done at 
the lower IF.

However, to my knowledge, no TV manufacturer uses a double conversion 
tuner, probably because of cost and the fact that need for such 
interference rejection has not yet become apparent in the marketplace.

To be fair, tuner designers know that the choice of the first IF 
frequency is always a compromise. It must be above the UHF band, but no 
matter what frequency is chosen, there lurks the danger that somewhere 
that frequency is being used, but not by broadcasters, and that the 
undesired signal will pass through the first IF filter to cause 
interference.

REDUCING TUNER COSTS

The only way to reduce tuner costs is by integrating the circuitry. In 
the case of a tuner, the active elements, transistors for amplification 
and diodes for mixers, are readily integrated along with 
interconnections. What cannot be integrated are the inductors for tuned 
circuits, so tuner designers seek inductorless circuit approaches.

The first IF filter in the Zenith double-conversion tuner was a bandpass 
filter at 915 MHz realized as a surface acoustic wave filter, which can 
be fabricated as an IC on a piezoelectric substrate, usually Lithium 
Niobate. This is a two-chip solution to the tuner problem, but the 
market demands a "tuner-on-a-chip" solution for cost reasons. Single 
chip solutions are known.

One of these is called the zero IF superheterodyne, also known as a 
direct-conversion receiver. DTV signals cannot be detected by a simple 
envelope detector; they must be detected synchronously. Locked-in LO 
frequency and phase to the received signal's pilot carrier is required.

Tektronix first employed synchronous detection of TV signals in its 
famous NTSC measurement grade demodulators in 1973. All DTV receivers 
have circuitry to produce the needed carrier locked to the pilot 
carrier, so why not feed the locally generated pilot carrier frequency 
into the mixer?

The output of this mixer would be a baseband (demodulated) DTV signal. 
Its spectrum would extend from DC up to 5.38 MHz.

What about the IF filter? There is no need! Its function is replaced by 
the low-pass (anti-aliasing) filter used to filter the baseband signal 
before it reaches the analog-to-digital converter. But without an IF 
filter, what about first adjacent channel signals--what rejects them? 
First, lets deal with the upper adjacent channel interference problem.

The pilot frequency of that undesired DTV signal will be at 6 MHz in the 
baseband signal. The low-pass filter should attenuate this undesired 
pilot at 6 MHz and all of its sidebands, which extend up to 11.38 MHz.

The lower adjacent channel presents a very different problem. Its pilot 
is 6 MHz below the local oscillator frequency, so it appears at 6 MHz in 
the baseband signal. The low-pass filter should remove both adjacent 
channel pilots, but the sidebands of the lower adjacent channel DTV 
signal will appear within the spectrum of the demodulated signal and 
cannot be removed. This is a critical drawback of the direct conversion 
topology for our 8-VSB DTV signal, which was brought to my attention by 
my friend, Dr. Oded Bendov.

But why worry? There is at least one direct conversion tuner-on-a-chip 
IC commercially available for the European DVB-T signal. Is it only a 
matter of time before someone introduces a direct conversion 
tuner-on-a-chip IC for use in North America? Would it work?

Yes, except where there is a lower adjacent channel also in use in the 
locality. Would such a DTV tuner meet the requirements set forth by the 
FCC? I have no idea; I'm not a lawyer. Perhaps this column will alert 
some of the people who might be working on such a tuner-on-a-chip for 
8-VSB based upon direct conversion.

Is there any other way to build a DTV tuner-on-a-chip that would 
eliminate image interference? I believe there is! Special mixers have 
been built in the form of a microwave monolithic IC device with image 
rejection mixer circuitry on the chip. In a conventional frequency 
converter, the LO frequency is above the desired signal frequency, so it 
is below the image frequency.

The difference frequency spectrum at IF is therefore inverted, i.e., the 
pilot is at the high end of the IF spectrum and the sidebands are now 
inverted too, so what you have at IF is a lower sideband signal, plus 
the pilot offset in frequency.

However the image frequency signal at the mixer output is not inverted, 
therefore it should be possible to separate the desired signal at IF 
from the image frequency interference. An IC designed for our 8-VSB 
system could offer a powerful solution at a very low cost, a cost which 
should follow Moore's Law downward 2:1 per 18 months.

Is anyone out there working on an image rejection tuner for 8-VSB? I 
hope so. Is anyone out there working on a zero IF or direct conversion 
tuner for 8-VSB? I hope not.

Charlie Rhodes is a consultant in the field of television broadcast 
technologies and planning. He can be reached via e-mail at 
charleswrhodes@xxxxxxxxxxxxxxxx
 
 
----------------------------------------------------------------------
You can UNSUBSCRIBE from the OpenDTV list in two ways:

- Using the UNSUBSCRIBE command in your user configuration settings at 
FreeLists.org 

- By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word 
unsubscribe in the subject line.

Other related posts: