Traditional RF tuners may be used to receive terrestrial or cable broadcast signals in the range of about 50 megahertz (MHz) to 860 MHz. Such tuners may employ single conversion intermediate frequency (IF) technology. A single conversion tuner may mix a received RF signal with a local oscillator (LO) signal in a single mixing stage to produce an IF signal. A common IF frequency may be about 36 MHz, for example.
The tuner may include a number of tracking filters used to protect a desired channel from interfering signals. Certain interfering signals, including an image channel and intermodulation products, may be generated in a subsequent stage without the tracking filters. The image channel may lie above the desired channel at twice the IF output frequency, for a tuner utilizing high side mixing. The image channel may be particularly problematic, because it may lie directly on a desired channel after an IF conversion. A tracking filter may be centered on the desired channel to suppress one or more undesired channels prior to conversion.
Some tuner front-ends may include RF circuitry for a number of sub-bands, for coverage of a full broadcast spectrum. An RF circuit associated with each sub-band may include a tracking filter of substantially identical design to tracking filters associated with the other sub-bands.
Some tuners may include a first tracking filter comprising a single inductor-capacitor resonant network located prior to an automatic gain control (AGC) stage. A center frequency associated with the first tracking filter may track with a center frequency of the desired channel. The first tracking filter may select the desired channel from the full received spectrum and may provide a first attenuation to the undesired channels, including the image channel. This may provide protection from intermodulation that might otherwise be generated in a subsequent stage.
A second tracking filter may comprise two resonant networks arranged as a double-tuned, loosely-coupled structure. The second tracking filter may be located prior to a conversion stage. A center frequency associated with the second tracking filter may track the desired channel frequency. The second tracking filter may provide a higher resonant quality factor (Q) attenuation to the undesired channels, including the image channel.
In a typical implementation, the tracking filters may be replicated for each segmented band. The tracking filters may be formed from numerous discrete components, including varactor diodes and air coils. An inductance value associated with the air coils may vary from a few hundred nanohenries (nHs) in the low band to a few nHs in the high band. The air coil inductors may be characterised by a high Q factor, typically in excess of 50, and may require manual adjustment. A tracking filter thus composed may not be suitable for semiconductor integration.