Television tuners receive carriers over a predetermined range, e.g. from 50 MHz to over 860 MHz for some standards. This is a relatively wide range of frequencies compared to other electronic devices, such as cellular telephones, which operate over a frequency range of 100 MHz or so. The incoming signal power to a tuner can vary by many orders of magnitude depending on a variety of factors. For example, the distance of the tuner from the signal source or the environmental conditions may cause significant changes in the signal strength for mobile tuners. In cable systems, as many as 138 channels can be present, and each channel can vary by more than 30 dB.
The change in signal strength is problematic. As the signal is introduced to the tuner, the signal is amplified by a low noise amplifier (LNA). However, if the amplitude of the signal is too large, the LNA will distort the peaks of the signal and thus degrade the output of the tuner. Accordingly, it is desirable to provide continuous control of the gain of the tuner over a wide range of input analog voltages.
However, providing control adds complexity, as well as distortion and non-linearity problems when the gain control range is extended. Altering the gain of the tuner changes the noise figure and intermodulation distortion of the tuner; noise figure generally increases as the gain decreases while intermodulation distortion increases as the gain increases. The noise figure is the ratio of the total output noise power to the output noise due to a standard input source resistance, e.g. a 50 ohm resistance or 75 ohm resistance (for cable). Intermodulation distortion of the tuner is nonlinear distortion in the output of the tuner containing frequencies that are linear combinations of the fundamental frequencies and all harmonics present in the input signals. Thus, it is also desirable to provide good linearity and a good noise figure over the entire gain control range.