Wave transmission lines and networks – Automatically controlled systems
Patent
1995-11-13
1998-05-26
Pascal, Robert
Wave transmission lines and networks
Automatically controlled systems
455123, 455125, 333174, H03H 701
Patent
active
057572470
DESCRIPTION:
BRIEF SUMMARY
This Application is a 371 of International Application No. PCT/FI93/00566, filed Dec. 29, 1993.
BACKGROUND OF THE INVENTION
The invention relates to a method for adjusting the frequency of an RF bandpass filter, especially a combiner filter, comprising deriving an RF sample signal proportional to the power of a modulated RF signal passing forward to the bandpass filter, reflected from the input of the bandpass filter or passed through the bandpass filter; down-converting the sample signal by another RF signal; detecting the signal strength of the mixing result; adjusting the centre frequency of the passband of the bandpass filter on the basis of the detected signal strength.
One known way to connect several radio transmitters to tho same antenna or antenna line is to connect each radio transmitter through a separate bandpass filter having its centre frequency tuned to the transmitting frequency of the radio transmitter. Such bandpass filters are called combiner filters. The function of the combiner filter is to feed the transmitting signal of the respective radio transmitter to the antenna with the smallest possible losses and prevent the leakage of transmitting signals of different frequencies from other radio transmitters, from the direction of the antenna, to this particular radio transmitter as efficiently as possible. Conventionally, combiner bandpass filters are tuned fixedly to the transmitting frequencies of the radio transmitters. So it has not been possible to change the transmitting frequency of the radio transmitter without simultaneously changing the combiner filter or its tuning.
In certain cases, however, it is desirable that the frequencies of radio transmitters can be changed simply and rapidly. One such case is the base station of a cellular mobile radio system to which pre-determined transmitting and receiving channels have been assigned. In cases where the channel allocation of the system can be altered, if desired, by varying the transmitting and receiving frequencies of the base stations, the channel capacity of the system can be utilized in a flexible and efficient way in varying conditions. Therefore combiner filters have been developed in which the centre frequency changes automatically with the transmitting frequency.
The adjustment of known automatically tunable combiner filters is based on measuring RF power reflected from the input of the filter or RF power passing through the combiner filter, and on locking to a minimum/maximum value of the measured power. Two problem with this adjustment method are, however, the low adjusting accuracy and the narrow dynamic range. As the frequency selectivity of the entire adjusting system relies on the combiner filter, power components of other radio transmitters leaking through the combiner filter to its input cause the minimum reflection suppression value of the reflection suppression measurement at the filter input to be about 7 dB. which results in a narrow dynamic range for the measurement. In the adjustment method based on the measurement of the power passing through the combiner filter, the measuring dynamics of the maximum power value also remain low for the same reasons as have been given above. In addition, the automatically tunable combiner filter based on this prior art adjustment method does not allow variation in the relative power levels between the radio transmitters, that is, the "mutual dynamics" value is almost 0 dB, as a change in the power level of one transmitter affects immediately the power measurement in the adjustment circuits of the combiner filters of the other radio transmitters, thus causing adjustment error.
Finnish Patent Application 912 255 (which corresponds to Sarkka U.S. Pat. No. 5,420,552, issued May 30, 1995) discloses an adjusting arrangement for a combiner bandpass filter where the adjusting accuracy and dynamic range have been improved by frequency-selective power measurement. Referring to FIG. 7, the prior art measurement is based on mixing a transmission P.sub.f passing forward to a combiner filt
REFERENCES:
patent: 5235294 (1993-08-01), Ishikawa et al.
patent: 5420552 (1995-05-01), Sarkka 333 17.1
Heikkila Pekka
Koukkari Eero
Nokia Telecommunications Oy
Pascal Robert
Summons Barbara
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