Telecommunications – Receiver or analog modulated signal frequency converter – Signal selection based on frequency
Reexamination Certificate
1999-02-12
2001-07-17
Hunter, Daniel (Department: 2684)
Telecommunications
Receiver or analog modulated signal frequency converter
Signal selection based on frequency
C455S339000, C375S347000, C375S350000
Reexamination Certificate
active
06263195
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to tuner circuits in data communications systems. In particular, the present invention relates to a wide-band digital tuner in a communications satellite.
2. Description of Related Art
A tuner, in general, is capable of down converting from any frequency within a specified frequency range. The tuners in satellite data transmission systems are usually analog or digital with a single serial input received from the output of an analog-to-digital converter (ADC).
The basic structure of a prior art tuner used in a communications satellite is illustrated in the block diagram of FIG.
1
. An output signal IF
in
which has been shifted down in frequency to an intermediate frequency, is applied to analog to digital converter (ADC)
12
which digitizes the signal IF
in
into an output containing a large number of samples at a high rate.
The tuner
14
receives the digitized signal IF
in
outputted by the analog to digital converter
12
and processes it into quadrature signal processing paths
16
and
18
which each contain a frequency converter
20
which downwardly shifts the input signal IF
in
to a lower frequency. The frequency converters
20
of the I signal processing path
16
and the Q signal processing path
18
respectively receive input carriers COS(&ohgr;T) and SIN(&ohgr;T) from the quadrature digital sinewave generator which cause the frequency converters to produce the quadrature I and Q signals which are downshifted in frequency to a lower carrier frequency. The input to the quadrature digital sinewave generator
22
is a frequency command F
in
which commands the quadrature digital sinewave generator
22
to output the quadrature carriers COS(&ohgr;T) and SIN(&ohgr;T) of the appropriate frequency to cause the frequency converters to shift the input signal IF
in
to the lower carrier frequency for further signal processing. The envelopes of the lower frequency quadrature carriers produced by connection of COS(&ohgr;T) and SIN(&ohgr;T) to the frequency converters
20
are modulated with the quadrature components of data present in the intermediate frequency input signal IF
in
. The outputs from the frequency converters
20
are applied to suitable low pass filters
24
which attenuate frequency components outside the desired lower carrier frequency band to which the I and Q data components are shifted. (Alternatively, tuner
14
could utilize bandpass filters instead of lowpass filters
24
.) The output I and Q signals are applied to downstream demodulator processing
26
of a conventional nature including channelization, discrete Fourier transformation (DFT) and other known signal processing techniques.
The bandwidth of tuner
14
is limited and switching between multiple IF frequencies is typically accomplished before the payload is converted in ADC
12
to the single serial digital signal provided to tuner
14
. These multiple IF frequencies may, but need not, be located at uniformly-spaced positions in frequency. The process of switching between different IF frequencies is sometimes referred to as “hopping”. For security purposes, hopping is often performed in a quick random pattern known only to the transmitting and receiving stations. Analog components impart the disadvantages of signal attenuation, heightened noise and lower reliability. These problems are exacerbated when the data transmission system utilizes frequency hopping among different channels.
Most tuners do not operate well at the high rates associated with wideband signals in the gigabit per second range. Channelizers, in particular, are commonly used to separate an input wideband signal of a specific spectrum received from an antennae into a plurality of narrowband channels. The wideband signal may carry different channels using different frequency bands, different time slots, different spread spectrum coding, or a combination of any two or more of these techniques. The channelizers may be wideband channelizers and/or narrowband channelizers used to separate a wideband signal into smaller sections of constituent channels.
The channel distribution in a wideband satellite communications application may be a plurality of 2 GHz channel groups, each comprised of twenty (20) 100 MHz sub-band channels, or 320 MHz channel groups, each comprised of four (4) 80 MHz sub-band channels. Each sub-band channel in these implementations may be channelized into smaller sections of narrower band channels, such as, for example, four (4) narrowband channels of 20 MHz each. Of course, neither the wideband nor the narrowband signals are limited to any particular spectral range and may separately determined for different applications.
BRIEF SUMMARY OF THE PREFERRED EMBODIMENTS
A preferred application of the present invention is in a wideband satellite communications system transmitting payload data over a number of different frequency channels at extremely high data rates. However, it also has application to any frequency-hopped communications system in which the transmitted waveform requires dehopping. Furthermore, it may be used to prepare channel information from a waveform containing channels at non-uniformly spaced frequencies.
In an exemplary implementation of the present invention, a communications satellite has an analog front-end, an ADC located functionally “closer” to the antennae, and a serial-to-parallel converter, also known as a demultiplexer or demux, providing several digital output signals in parallel. The analog front-end is simplified to perform a coarse tuning or “de-hop”, thus avoiding many of the disadvantages associated with the conventional analog front-end. The ADC is a modern commercially available unit or a specially designed unit able to sample at extremely high speed data rates. The single input from the ADC is quickly switched by the demux to a number of output branches. Each output switch outputs at a slower rate than the ADC even though the aggregate rate remains the same. This reduction in data sampling rate permits the processing of wide bandwidth signals resulting from coarse tuning.
The fine tuning is performed in an advanced digital tuner receiving the plurality of parallel digital signals inputs from the demultiplexer. Parallel architecture and processing in the advanced digital tuner allows the high sample rate data to be processed at a much slower clock rate. The digital tuner filters out half of the spectrum with a complex bandpass filter, such as a Hilbert Transform filter, and down converts the other half to complex baseband. An alternative architecture would allow downconversion first and then filtering out half of the spectrum with a half-band filter. Because the “half-spectrum” is less than one-half of the Nyquist rate, the tuner can down sample by 2 and no aliasing occurs. This prepares the output signal from the tuner for downstream demodulator processing such as channelization and data detection
REFERENCES:
patent: 5577031 (1996-11-01), Smith
patent: 5590156 (1996-12-01), Carney
patent: 5592480 (1997-01-01), Carney et al.
patent: 6011785 (2000-01-01), Carney et al.
patent: 6134229 (2000-10-01), Schwaller et al.
patent: 6147713 (2000-11-01), Robbins et al.
Caso Gregory S.
Liu Sam H.
Niu Edward L.
Hunter Daniel
Keller Robert W.
Nguyen Thuan T.
TRW Inc.
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