Abstract: A Cartesian loop transmitter for transmitting baseband signals is disclosed. The disclosed Cartesian loop includes a forward path having a first input for receiving the baseband signals and a second input and a feedback path having an input from the forward path. The forward path including an upconverter to upconvert the input signals to a signal to be transmitted, an attenuator having selectable attenuation and a plurality of power amplifier stages. The feedback path including a pin diode attenuator having selectable attenuation and a downconverter to downconvert the input from the forward path. The feedback path provides an error signal indicative of the non-linearity of the forward path to the second input of the forward path.

Abstract: The invention provides methods and apparatus for substituting a reference spread spectrum signal for a pilot tone or providing a reference spread spectrum signal where no pilot tone has been used. The invention has particular relevance when a large number of input channels are required for a broad band linear amplifier and when an omni-directional antenna is required for a repeater which may be prone to instability.

Abstract: Broadband amplifiers which are intended to be linear suffer from distortion and although many techniques have been devised to overcome this problem significant improvements are required especially at frequencies above 100 MHz. An error signal is formed from the output of the amplifier 4 by comparison in a subtractor 11 with its input and the error signal is combined in a coupler 17 with the amplifier output to reduce distortion. The present invention provides automatic amplitude and phase control of the signals forming the error signal by means of components 24 and 25 in a multi-channel input and in the error signal by means of components 13 and 14. These components receive control signals from feedback networks 18' and 20' which each process two of three input signals from: couplers 34, 36 before the subtractor 11, a coupler 22 which passes the error signal and a coupler 21 after the coupler 17.

Abstract: Broadband amplifiers which are intended to be linear suffer from distortion and although many techniques have been devised to overcome this problem significant improvements are required especially at frequencies above 100 MHz. An error signal is formed from the output of an amplifier by comparison in a subtractor with its input and the error signal is combined in a coupler with the amplifier output to reduce distortion. The present invention provides automatic amplitude and phase control of the signals forming the error signal by two groups of adjustment means, one group in a multi-channel input path and one group in the error signal path. These groups receive control signals from respective feedback networks which each process two of three input signals from: couplers before the subtractor, a coupler which passes the error signal and a coupler situated after the coupler which combines the amplifier output signal and the error signal.

Abstract: In transparent tone-in-band communication systems a notch in the frequency band is usually formed and then the resulting spectrum is translated in frequency to an intermediate frequency range as part of the transmission process. In a transmitter of the present invention a more simple arrangement is used in which the notch is formed directly in an intermediate frequency range by using mirror filters to divide an input signal into two portions and supply respective mixers, each having one output sideband in the intermediate frequency range. The mixers receive different reference frequencies and the mixer outputs are supplied to a summing circuit whose output is passed to a band pass filter to remove the unwanted sidebands. The original frequency spectrum is restored at the receiver by mixer processes of shifting the two selected sidebands similar to, but the inverse of, those used in the transmitter.

Abstract: In previous transparent tone-in band systems the notch has been removed from the frequency spectrum before a carrier signal carrying data has been demodulated. The present specification describes an arrangement in which data is recovered by first and second mixers from a signal having a spectrum containing a notch. Phase and frequency locking is obtained: either by using a third mixer fed from the inputs to the first and second mixers to provide, indirectly, one signal for a fourth mixer which also receives the carrier signals and provides demodulating signals for the first and second mixers; or by a correction system coupled to the outputs of the first and second mixers for correcting demodulating signals supplied to these mixers.

Abstract: In those transparent tone-in-band systems which employ a single oscillator in the transmitter to generate a frequency notch between upper and lower portions of a band which is to be transmitted, there is not complete freedom to choose the width of the notch. In the present invention which also uses a single transmitter oscillator, one portion of the band transmitted is selected using a filter. A mixer is used to change the frequency of the input band in a way which does not allow the lower sideband to "fold" back at zero frequency and interfere with the lower transmitted portion. A further filter selects the other portion for transmission from the output of the mixer.

Abstract: A communication system which uses a transmitter and a receiver. The transmitter divides a band of interest in the frequency spectrum into upper and lower portions, and frequency translates one of these portions in order to provide a frequency notch between the portions. The receiver of the system includes a receiver processor which receives the upper and lower portions, and which restores the original frequency spectrum. The receiver processor at least partially determines the final position of the restored portion.

Abstract: When modems are used in SSB systems a problem arises in controlling variable data rates. If a transparent tone-in band system is used, the notch in the band transmitted will be required for a control signal for the SSB receiver. The present invention allows a further possibility for transmitting control signals, for instance one representative of data rate, by varying the width of the notch in the transmitter and determining notch width in the receiver. The notch width is controlled by frequencies applied to two mixers and reference signals for these mixers are derived by respective multipliers from a clock signal. In a receiver the process of restoring the original frequency spectrum also employs two mixers receiving signals from two oscillators. The frequency of one of the oscillators is controlled to ensure that lower and upper portions of the restored spectrum have the correct frequency and phase relationship.

Abstract: A communication system which uses a transmitter and a receiver. The transmitter divides a band of interest in the frequency spectrum into upper and lower portions, and frequency translates one of these portions in order to provide a frequency notch between the portions. The receiver of the system includes a receiver processor which receives the upper and lower portions, and which restores the original frequency spectrum. The receiver processor at least partially determines the final position of the restored portion.