Televisions, video recorders, set-top boxes and broadband cable receivers all have a common element: the tuner. Although all other electronic components in these devices shrink as semiconductor technology shrinks, consumer applications often use huge "tuner tanks" to achieve this critical function. Challenging restrictions on tuner design are the reason this technology persists, but market forces are pushing silicon tuners to the forefront.
The tuner designer must overcome many challenges. The input signal in broadcast television and cable applications lies in the frequency band of 48 MHz to 861 MHz, and the signal strength may have a wide dynamic range. For example, in broadcast television applications, the signal to be selected may have adjacent undesired channels whose signal strength exceeds 100 times.
A typical tuner design uses a single conversion receiver architecture, although other architectures are also possible. The structure of a single conversion tuner includes a preselection filter, a low noise amplifier (LNA), a down converter and an intermediate frequency (IF) amplifier.
1. Constraints of silicon tuner design
1) Pre-selected filter tracking
The preselection filter takes the full frequency signal frequency band and reduces it to a smaller frequency band containing the channel of interest. In view of the wide frequency range of the channel, this means that the preselection filter must be a tracking bandpass filter whose center frequency can vary across the signal spectrum. LNAs with RF automatic gain control functions usually follow a preselected filter.
The downconverter stage is traditionally a heterodyne system. The downconverter is designed with channel selection, which involves adjusting the local oscillator (LO) so that the difference between its frequency and the signal of interest falls within the bandpass of the IF filter. This stage uses high-performance, narrow-band, fixed-frequency filters—usually surface acoustic wave (SAW) devices—by selecting and excluding all other options. This is followed by an IF amplifier with variable gain control, allowing the system to match the strength of the selected signal to the needs of the demodulation and detection circuit the tuner is driving.
Considering the wide frequency and signal strength range of the input signal, using this architecture to generate a good-performance tuner will bring many challenges. One is the pre-selection filter. In order to cover the full signal bandwidth, typical broadcast TV tuner implementations require filters to operate in three different frequency bands: VHF (very high frequency), 48 to 88 MHz; medium VHF, 174 to 216 MHz; and UHF (super High frequency) at 470 to 861 MHz. A common implementation is to use separate filters, one for each filter.
2) Multi-band operation
The preselection filter selects the operating frequency band, but it may still be necessary to implement a tracking filter to provide the required selectivity. The tracking filter must maintain a relatively fixed bandwidth, although the center frequency can change over many octaves. The realization of such a filter usually requires a large number of passive components, such as inductors, which must be manually tuned in the factory to obtain proper performance. This demand for passive components and manual tuning greatly increases the size and cost of the tuner. A typical tuner can measure 2.5 x 2 x 0.75 inches.
However, the preselection filter is not the only component with design challenges. The LO in the downconverter must also handle a wide frequency range. The preselection filter only reduces the bandwidth of the input signal. The signal of interest may still fall anywhere in the 48 to 861 MHz range, and the LO must basically cover this range. In addition, the LO must exhibit low close-range phase noise or DTV channel reception will be compromised. The integrated circuit oscillator achieves such a wide frequency range that cannot be tuned, and at the same time exhibits low phase noise using the typical 3-volt power supply voltage of today's electronic systems. A power supply of up to 30 V may be required.
To meet all these performance requirements, most suppliers choose to retain the traditional TV and VCR tuner designs, despite their cost and size. But market pressures are beginning to force change. One of the elements is the authorization of the Federal Communications Commission, that is, all TVs sold in the United States have begun to use tuners capable of receiving digital TV broadcasts. This task forces suppliers to change the basic structure of their products, creating opportunities for innovation in tuner design.
The growth in demand for the portable entertainment market has also promoted changes in tuner design. Portable means battery-powered or handheld devices and prohibits the use of high voltages in LO implementations. In addition, portable devices require much smaller implementations than typical tuners. In the growing flat panel display/TV market, small size is also important. In a flat panel design, the size of the tuner may be the limiting factor for product thinning.
Another trend that affects tuner requirements is that consumers want to receive multiple channels at the same time. This means that more than one tuner is required, which takes up more space, which affects the system size and increases the cost of the tuner for the final product. Market pressure to reduce size and other trends have promoted the use of silicon tuner designs.
3) Eliminate manual tuning
There are many goals for silicon tuner design. One of the main goals is to eliminate the need to manually adjust external components in the tracking filter. There are two effects in silicon. One is that eliminating most external components also eliminates their ability to absorb and dissipate unwanted RF energy from the excluded frequency band. Silicon tuners must use innovative circuit designs in LNAs and mixers to manage unwanted energy without damaging the transistors.
The second impact is the need for a new RF architecture. Early silicon tuner designs tried to adopt a double conversion method, which provided selectivity without manually tuning external components. The first conversion shifts the frequency of the input signal upward. The RF SAW filter reduces the bandwidth before converting to IF for the second time. The filter device represents the main cost of this design.
Recently, self-calibration technology is being used to overcome the changes in semiconductor process manufacturing. Some also eliminate the need for high-voltage power supplies for the LO and the need for RF SAW devices. Instead, they only use SAW filters in the IF stage, which have a much lower frequency and are lower cost devices than RF SAW filters.
Implementing these designs in silicon requires advanced semiconductor process technology. Chip suppliers usually only characterize the process of their digital VLSI implementation. To implement a silicon tuner, the process must be characterized based on RF performance. In addition, the process must have a way to create an inductor of the correct value and have a sufficiently high Q for low phase noise LO implementation or RF filter design. Such a process can now be used.
In addition to semiconductor processes, silicon tuners require careful chip design. RF has many opportunities for radiated and conducted interference. In a single-chip silicon tuner design, the proximity of on-chip signal lines and the sharing of circuit substrates exacerbate this. Controlling this interference requires a layout that separates critical circuits and includes shielding patterns. The design also requires careful creation and management of on-chip power and ground distribution networks. In addition, the design must include on-chip and off-chip filtering components to break the interference signal path.
All these problems have been solved, and with the advent of silicon tuner devices, product designers have begun to make ways to get rid of the old tuner-in-a-can. Satellite and cable receivers were the first to adopt this method. They process signals with roughly the same power in each channel. This channel uniformity slightly simplifies the tuner design, enabling early silicon tuner equipment to meet the requirements.
However, terrestrial broadcast reception must use a tuner that can provide selectivity over a wide range of channel power levels. The possibility of combining strong signals in adjacent channels with weak channels of interest imposes strict restrictions on the selectivity of tuner design. Until recently, innovative RF architectures and improved RF semiconductor processing have allowed silicon tuners to achieve the required performance at low cost.
By eliminating the need for manual adjustments, these silicon tuners can increase manufacturing yields and provide more reliable performance than older designs. They meet the needs of portable devices by eliminating the need for high-voltage power supplies and allowing compact implementations. Given the market's influence on these attributes, silicon tuners are expected to align TV receiver designs with other parts of the electronics industry.
Ravi Shenoy ([email protected]) is the analog director and RF technology of LSI Logic (Milpitas, California).
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