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■ SAW (Surface Acoustic Wave)
Filter no Longer Required due to
the Adoption of a Low IF System
To select only the desired channel, TV tuners
required a highly selectivity SAW filter in the
tuner output block. Although SAW filters have
superlative selectivity, their large package has
been an impediment to tuner miniaturization.
Furthermore, since the signal loss in the SAW
filter is large, the power consumption in the
driver amplifier required to drive the SAW
filter is proportionately large.
A low IF technique is adopted in the Sony CMOS
silicon tuner to make it possible to implement
the channel selection filter on the chip itself.
Although it was difficult to integrate this filter
on the chip with the intermediate frequencies
used previously (57 MHz in Japan), if a low IF
is used, a high selectivity low loss active filter
can be implemented relatively easily. Also, in
the demodulator IC block in the following stage
as well, this approach has the advantage that it
is easier to get good performance from the A/D
converter if the frequency is lower.
The low IF technique is said to have just one
aspect, its image rejection characteristics, in
which it is inferior to other methods. While it is
common to reject images with a filter located in
the previous mixer stage in earlier IF systems,
it is extremely difficult to reject images using
an actual filter, since the image interference
corresponds to the channel next to the desired
channel. In Sony's CMOS silicon tuner, an image
canceling circuit is used for image rejection.
The image interference rejection performance
is determined by the relative precision of the
IC internal signal. (See figure 4.) The signal
relative precision requirement, calculated from
the image rejection performance, required in
a TV tuner is 0.03%.
This is a value that is
about two orders of magnitude higher than the
precision that can be achieved with normal IC
processes. To achieve these rigorous specifications,
Sony optimized the circuit structure and
design specifications, and strove for thorough
symmetry down to the finest parts of the layout,
and achieved fully adequate image rejection
characteristics.
■ Built-in Tracking Filter to Improve Interference Rejection Performance
In existing TV tuner modules, it was necessary
to have tracking filters that use air core inductors
and varactor diodes at the previous and following
stages of the input amplifier to exclude interfering
signals from other broadcast waveforms and
other sources. There are, however, sample-to-sample
variations in the characteristics of air core
inductors and to achieve optimal characteristics
at all times, it was necessary to adjust the winding
spacing of those coils each time by hand.
Therefore high skill levels were required of the
staff performing those adjustments in the mass
production process. In this CMOS silicon tuner,
however, surface mounting chip inductors with
comparatively low sample-to-sample variations
and IC internal variable capacitors are used in
the tracking filter. Although it is still necessary
to correct for the sample-to-sample variations
in the filter characteristics as it was with air
coil tuners, this adjustment is performed by
switching the IC internal variable capacitors
using control data. Therefore it is possible to
automate this process with software and other
tools, and stable tuner characteristics can be
acquired without depending on the skill and
experience of the employees.
■ Low Noise/Low Distortion RF
Amplifier Circuit
In the CMOS silicon tuner, an extremely high
ability to withstand interference is required in
the input block RF amplifier to make it possible
to eliminate the air core filter, which has
superlative selectivity characteristics. Sony took
full advantage of the limited supply voltage and
incorporated a wealth of distortion characteristics
improving ideas in this CMOS silicon tuner.
As a result, this device achieves the same ability
to withstand interference as existing coil tuners,
despite using a 2.5 V supply voltage. This tuner
can produce clear video even in a poor signal
environment in which the tuner is surrounded
by multiple broadcast towers.
■ Low Phase Noise PLL Circuit
Achieves High Reception Sensitivity
Extremely high signal purity (low phase noise)
is required in the local oscillator used in TV
tuners to achieve high quality video. In existing
TV tuners, such characteristics are achieved by
providing a resonant circuit with an extremely
high Q that, like the tracking filter, uses an
air core inductor and a varactor diode. In this
CMOS silicon tuner, Sony adopted an ultrahigh
frequency oscillator and a high-speed divider circuit to implement a local oscillator with
superlative phase noise characteristics while
eliminate in special components such as the
air core inductor. By combining a PLL circuit
capable of dividing by fractional parts, called a
fractional PLL, with this local oscillator, Sony
achieved both the high resolution of the 31.25
kHz step size required in TV tuners and low
phase noise. Although conventional fractional
PLL circuits emit spurious radiation characteristic
of fractional circuit operation, Sony
diffused this spurious radiation by combining
a ΔΣ modulator in this CMOS silicon tuner so
that the signal quality is not degraded.
■ Reduction of Spurious Radiation that
could Degrade Picture Quality During
Analog Broadcast Reception
One characteristics that is critical for analog
broadcast reception and is particularly difficult
to design for is spurious radiation. The
local oscillator and crystal oscillator that are
included in the tuner circuit always incorporate
a high-frequency component, and if that
component leaks into the tuner input or output,
it can degrade the picture quality of a specific
broadcast channel.
Sony positioned this issue as one of the most
critical for the CMOS silicon tuner from the start
of this development effort, and proceeded with
development while focusing on the spurious
radiation problem. To determine the amount
of spurious radiation appearing in a signal,
in addition to the amount of spurious signal
generated by the source and the degree of sensitivity
of the circuits that are affected by that
signal, it is also necessary to know the amount
of propagation over the transmission paths that
connects these components. Estimating this
propagation level is, however, one of the most
difficult points when designing for low spurious
levels ("spurious design"). Sony constructed a
spurious design environment that uses a noise
analysis tool for CMOS silicon tuner design,
and designs chips with an even higher level
of precision. (See figure 5.) Sony achieved a
spurious level that is more than satisfactory as
a TV tuner specification by, based on the results
acquired from that tool, inserting effective filters
in the power supply, adding guard bands, and
other measures.
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Features of this Tuner
Vol.56