Telecommunications – Transmitter and receiver at same station – With a common signal processing stage
Patent
1994-06-01
1996-02-20
Eisenzopf, Reinhard J.
Telecommunications
Transmitter and receiver at same station
With a common signal processing stage
455 76, 370 24, H04B 140
Patent
active
054937019
DESCRIPTION:
BRIEF SUMMARY
THE FIELD OF THE INVENTION
The invention relates to a frequency-modulated transceiver wherein the transmitter comprises a frequency-modulated high frequency oscillator, and the receiver comprises, coupled to a cascade, a high frequency mixer whose local oscillator signal is a modulated output signal from the high frequency oscillator, and an intermediate frequency mixer having an intermediate frequency local oscillator signal containing transmission modulation.
BACKGROUND OF THE INVENTION
In radio devices operating in the microwave regions (>1 GHz), such as radio links, it is advantageous to minimise the number of high frequency and microwave components, since they are the most expensive and often the most critical components in the system. For instance, in a full-duplex radio operating on a high frequency (microwaves, millimetric waves) it is very costly to manufacture separate local oscillators for the transmitter and the receiver. On the other hand, the use of the same high frequency oscillator both for the transmitter and the down-conversion section of the receiver causes certain problems. The most preferred way to generate the transmission signal is frequency modulation of the common high frequency oscillator, but in such a case the transmission modulation will be summed to the received signal as well, and it has to be somehow cancelled.
One known method for cancelling the transmission modulation from a received signal is to sum a baseband signal corresponding to the transmission modulation in an appropriate delay and phase relation to a baseband signal of the receiver, whereby the transmission modulation in the receipt signal is cancelled.
In Finnish Patent 80,549, the residual modulation is eliminated by using in the receiver a second intermediate frequency mixing stage in which local oscillator is also modulated with a transmission signal so that the transmission modulation in the received signal is cancelled in the second mixing.
There are certain drawbacks in the above-mentioned methods. In the first solution the detector of the receiver has to handle the modulations in both the receiving and transmitting directions, wherefore the required bandwidth increases and thus the selectivity of the receiver diminishes.
Furthermore, both methods have the problem of matching of the linearity of modulation and countermodulation, which makes it difficult to completely eliminate the transmission modulation from the received signal. The matter is further complicated by the fact that the properties of oscillators vary according to the ambient conditions. This problem can be mitigated to some degree by an automatic control of the compensation during operation, but this increases the complexity of the apparatus.
SUMMARY OF THE INVENTION
The above problems are avoided with a frequency-modulated transceiver as disclosed in the introductory paragraph. The transceiver is characterised according to the invention in that it comprises a third mixer for generating the intermediate frequency local oscillator signal from a modulated output signal of the high frequency oscillator.
In the transceiver according to the invention, the receiving portion comprises at least two mixing stages. In the first mixing stage, the local oscillator signal is a transmission signal, and in the second intermediate frequency mixing stage eliminating the transmission modulation from the received signal, the local oscillator signal is produced by mixing from the transmission signal.
With the solution according to the invention, the compensation of the transmission modulation from the received signal is more efficient than in the prior art methods, because, in the apparatus of the invention, the local oscillator signal in the second mixing stage, i.e. the compensation signal, is derived from the signal to be compensated, and therefore the properties of the signals correspond well one to another. For instance, the nonlinearity of the modulation will not affect the resulting compensation.
BRIEF DESCRIPTION OF THE DRAWINGS
In the follow
REFERENCES:
patent: 4491969 (1985-01-01), Datta
patent: 4520474 (1985-05-01), Vilmur
patent: 4520475 (1985-05-01), Berg
patent: 4633511 (1986-12-01), Koga et al.
Eisenzopf Reinhard J.
Nokia Telecommunications Oy
Vo Nguyen
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