Pulse or digital communications – Repeaters – Testing
Patent
1985-06-19
1987-01-27
Griffin, Robert L.
Pulse or digital communications
Repeaters
Testing
375 89, 370120, H04L 2728
Patent
active
046399356
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an apparatus in a transmitter of digital signals, each sent with its own frequency, for correcting frequency-dependent attenuation in the transmitting medium, and more particularly to such apparatus including a filter means where at least one of the frequencies can be attenuated for retaining a desired relationship between the amplitudes of the signals associated with different frequencies.
BACKGROUND ART
Frequency-dependent attenuation of a transmitting medium can be corrected by a filter with an adjustable frequency characteristic. In adjusting such a filter there is required knowledge of the attenuation properties of the channel in question and a new adjustment is required for alterations in the channel, e.g. alterations due to re-disposition of the cable etc. Should there be an alteration in the channel, there is an obvious risk that the filter adjustment is forgotten or that an adjustment is incorrect. To avoid these difficulties, filters are used with a variable frequency characteristic, which can be controlled in cable transmission by a DC current applied to the line, the strength of the current being dependent on the length of the line. Since the channel signal attenuation is not unambigously related to the strength of the DC current, compensation will be incomplete, resulting in that a transmitted signal can be wrongly detected.
DISCLOSURE OF INVENTION
The above problems are solved in accordance with the invention by the received signal voltage being detected and the amplitude differences between signals associated with different frequencies being used to control the filter attenuation. The invention is characterized by the disclosures of the accompanying claims.
BRIEF DESCRIPTION OF DRAWINGS
The invention will now be described in detail in connection with the appended drawing where FIG. 1a is a diagram of the frequency-dependent attenuation in two different transmission cables,
FIG. 1b is a diagram of the frequency characteristic of a compensation filter,
FIG. 2 is a block diagram of a demodulator provided with an apparatus in accordance with the invention for correcting frequency-dependent attenuation in transmitting four different frequencies,
FIG. 3 is a diagram showing the time variation of the signals at different instants in this demodulator,
FIG. 4 is a block diagram of a further demodulator for correcting attenuation in transmitting two frequencies and
FIG. 5 is a diagram showing the signals in this demodulator.
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1a illustrates how signals are attenuated in transmission in two separate cables with different lengths corresponding to graphs a and b in the diagram, in which A is the signal amplitude level and f its frequency. For low frequencies, the cables attenutate the amplitude to the respective given levels A1 and A2, and for increasing frequency the attenuation in the longer cable increases at a frequency fa according to the graph a and the attenuation of the shorter cable increases at a frequency fb according to the graph b. FIG. 1b illustrates two characteristics 1 and 2 for a variable filter, which can compensate for the attenuation of the long and the short cable. The transmitted signals have the frequencies f0, f1, f2 and f3 and the filter is conventionally selected so that its attenuation is fixed at the highest of these frequencies, f0, but can be varied for the remaining frequencies. This may be carried out with the aid of control signals which change the filter parameters, e.g. by changing the resistance of a field effect transistor. With full compensation, all the signals beyond the filter up to the highest desired frequency f0 will be attenuated to the same level. Since the characteristic of the filter can be varied for the transmitted frequencies, the filter can completely compensate the received signal in spite of the filter characteristic for intermediate frequencies not completely corresponding to the cable attenuation.
FIG. 2 is a block diagram of a demodulator pro
REFERENCES:
patent: 2855506 (1958-10-01), Schabauer
patent: 3205442 (1965-09-01), Stamboulis
patent: 3810019 (1974-05-01), Miller
patent: 3820042 (1974-06-01), Mueller
patent: 4298983 (1981-11-01), Kawai et al.
patent: 4409593 (1983-11-01), Bose
patent: 4528674 (1985-07-01), Sweeney et al.
Nippon Denk K. K., "Patent Abstract of Japan", vol. 7, NR. 13E153, abstract of Japan-57-170656, Oct. 20, 1982.
Powell, K. E., "FSK Demodulator Circuit", IBM Technical Disclosure Bulletin, vol. 21, Jun. 1978, No. 1, pp. 339-340.
Jarnestedt Goran
von Campenhausen Rolf
Chin Stephen
Griffin Robert L.
Telefonaktiebolaget LM Ericsson
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