Gain control and frequency stability in digitally-channelised RF bi-directional amplifiers

A method, for use in a repeater, for maintaining frequency stability of a reference oscillator subject to temperature variation, the method comprising:

• providing a stable external frequency reference source having at least intermittent availability;
• continuously monitoring a current temperature of the reference oscillator;
• while the stable external frequency reference source is available, applying a primary frequency control signal such that an output of the reference oscillator is synchronised to the stable external frequency reference source, and recording a corresponding control signal value in association with the current temperature of the reference oscillator;
• and
• in the event that the stable external frequency reference source is unavailable, applying a secondary frequency control signal to the reference oscillator corresponding with the control signal value previously recorded in association with the current temperature of the reference oscillator.

A radio repeater for amplification of a plurality of distinct frequency channels, and configured to control channel gain, comprising:

• a front-end subsystem including a mixer arranged to translate the distinct frequency channels from an input radio frequency (RF) band to a first intermediate (IF) band;
• a digital signal processing sub-section including at least one analogue-to-digital converter (ADC) for converting the IF band into a corresponding digital signal comprising all of the distinct frequency channels, the digital signal processing sub-section being configured to process the digital signal to select a set of wanted channels from the distinct frequency channels, and further including a digital-to-analogue converter (DAC) to convert said set of wanted channels into corresponding analogue signals within a second IF band;
• and
• a back-end subsystem including a plurality of mixers arranged to translate the analogue signals in the second IF band to an output RF band,
• wherein the back-end subsystem further comprises, for each channel in the set of wanted channels:
• a coupler arranged to provide a monitor signal corresponding with a power level of an output channel;
• a feedback loop arranged to receive said monitor signal, and including a power control circuit for comparing the power level with a predetermined maximum output signal level, and to generate a corresponding control signal;
• and
• a variable attenuator arranged to receive the control signal, and to apply attenuation to the output channel in order to maintain the output channel power level at or below the maximum output signal level.

A frequency reference generator comprising:

• a receiver configured to receive an external frequency reference source signal having at least intermittent availability;
• a voltage-controlled oscillator (VCO) having a nominal frequency corresponding with a desired reference frequency, and a control voltage input;
• a correlator, configured to compare an external frequency reference derived from the external frequency reference source signal with corresponding samples of the VCO output frequency, and to generate an error-correction signal corresponding with a different frequency;
• a temperature sensor, environmentally coupled to the VCO in order to measure VCO temperature;
• a memory device, configured to record error-correction signal values in association with corresponding temperature values detected by the temperature sensor;
• and
• a controller having a control signal output coupled to the VCO control voltage input, the controller being configured to:
• while the external frequency reference source signal is available, generating a control signal determined such that an output of the VCO is synchronised to the external frequency reference source signal, such that the error-correction signal is substantially zero;
• and
• in the event that the external frequency reference source signal is unavailable, retrieving from the memory device a previously recorded error-correction signal value corresponding with a current temperature detected by the temperature sensor.