Telecommunications – Receiver or analog modulated signal frequency converter – Signal selection based on frequency
Patent
1997-05-29
1999-12-07
Eisenzopf, Reinhard J.
Telecommunications
Receiver or analog modulated signal frequency converter
Signal selection based on frequency
4552261, 455 674, 4552422, 455324, H04B 126
Patent
active
059998024
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention concerns a so called "direct conversion tuner" which is especially useful in a television receiver.
BACKGROUND OF THE INVENTION
An early type of tuner known as a "tuned radio frequency tuner" (TRF) included several radio frequency (RF) amplifiers which were all tuned to the frequency of the RF signal of desired transmission channel followed directly by a detection section, without an intervening mixer employed in later tuners. Such a tuner could provide relatively distortion free performance due to the absence of a mixer. However, TRF tuners tended to be large in size and subject to stability and gain control problems due to the number RF amplifiers which were needed. Moreover, TRF tuners did not provide a consistent or adequate degree of signal selectivity.
The type of tuner which is primarily used today is known as a "heterodyne" or "superheterodyne" tuner. In its simplest form, known as a "single-conversion" tuner, it comprises a tunable RF amplifier followed by a frequency conversion stage, including a mixer and a local oscillator. The frequency conversion stage produces an intermediate frequency (IF) signal which corresponds to the received RF signal but has a lower frequency. The IF signal is filtered by an IF filter section and the resultant signal is coupled to a detection section. The combination of conversion stage and the following IF filter section provides a significantly better selectivity characteristic than a TRF tuner. The frequency of the local oscillator signal is offset (usually higher) from the frequency of the desired RF signal by the desired frequency of the IF signal. In a television receiver, the local oscillator signal is controlled so that it places the frequency of the IF picture carrier corresponding to the RF picture carrier at a nominal frequency, for example, at 45.75 MHz in the United States and 38.9 MHz in Europe.
A single conversion tuner can be made quite small and relatively inexpensive. However, it produces unwanted intermodulation and cross-modulation products due to third and higher order components of the signal transfer characteristics of the mixer included in the frequency conversion stage. Various unwanted conversion products, known in the tuner fields as "image", "one-half IF" and "IF beats", continue to be a problem. The IF filter is designed to minimize unwanted conversion products and also to provide rejection of responses due to adjacent channels (selectivity). Thus, the selection of the IF frequency is a compromise. As a result the rejection of unwanted conversion products and selectivity of the tuner may not be adequate.
The deficiencies of a single conversion tuner have become especially troublesome due to the increasing number of "contiguous" channels now available in large cable television systems. With the advent of digital television transmission systems, such as for high definition television (HDTV), the problem becomes still more difficult because these systems make full use of the available channel spectrum and only a small guard band of a few hundred kilohertz (KHz) exists between channels. In addition, the overall frequency response of a tuner for tuning digital television signals must be flat to the edges of the channel, but nevertheless, have a very steep "roll-off" (attenuation) at the edges for adequate adjacent channel rejection. This makes the design of an appropriate IF filter more complicated since no Nyquist slopes and sound traps, which tend to ease IF filter design, can be used in digital systems. In addition, it is contemplated that both analog and digital television signals will be transmitted during a transition period. In that case, even more adjacent channel selectivity will be required for good reception of the digital signals because digital television signals will be transmitted with much less power than analog television signals.
The "double-conversion" variation of the superheterodyne tuner was developed to overcome the shortcomings of the single-conversion tuner. In this type of tuner
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Proceedings of the IRE, "A Third Method of Generation and Detection of Single-Sideband Signals", D.K. Weaver, Jr. pp. 1703-1705.
1993 IEEE, "Digital TV Receiver with a Low IF", Dietmar Ehrhardt et al, pp. 331-339.
Akiyama Kuniyuki
Eisenzopf Reinhard J.
Emanuel Peter M.
Kincaid Lester G.
RCA Thomson Licensing Corporation
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