GB2298750A - Feedforward and feedback gain control in radio receiver - Google Patents

Description

RADIO RECEIVER Fleld of the Invention This invention relates to a radio receiver that processes received signals digitally. The invention is applicable to, but not limited to mobile radio equipment having automatic gain control circuitry.

Background of the Invention Many modern day communications systems use digital technology with linear modulation techniques to provide efficient usage of the available spectrum. One such communications system using linear modulation technology is single side band (SSB). The use of linear modulation techniques requires the receiver to operate in the linear region of all its subcircuits. Hence, linear SSB receivers do not include non-linear stages, such as envelope detectors or limiters and, as a result, are more susceptible to weak interfering signals.

An SSB receiver design is typically based on a heterodyne concept, wherein two or three frequency conversions are generally used to downconvert a high frequency SSB modulated signal to the audio frequency band.

Each frequency conversion facilitates improved bandwidth reduction of the received signal by using, for example, crystal filters with narrowing bandwidths as the frequency is decreased, in an attempt to reject interfering signals.

In addition to the linearity requirement, modern day communications receivers are required to operate across a large dynamic range of received signal levels. In the case of a voice communications system it is desirable to have a large dynamic range receiver in order to maximise the coverage area of the communications system for a given audio quality. To facilitate this linear operation across a wide dynamic range of received signal levels, linear receivers, as well as more traditional receiver designs, typically employ automatic gain control (AGC) circuits. The requirements of the AGC design for an SSB radio include amplitude control of the received signal over 100 dB of dynamic range and the AGC operation to respond within 10 msec's of activation.

A traditional AGC circuit is composed of two elements: an attenuator and a detector. The detector measures the level of the received signal and the AGC attenuator provides the means to control the level of the received signal. Attenuators at radio frequencies are generally used as they have a superior linear performance over baseband continuous attenuators and they do not require high cost PIN diodes. It is also possible to implement an AGC attenuator arrangement by switching amplification stages "on" or "off' within the receiver path, thereby avoiding the need for additional circuitry.

The range of attenuation that the AGC attenuator can provide typically limits the dynamic range of the AGC.

To achieve a dynamic range in excess of 100 dBs at least some of the attenuation arrangement must be positioned prior to any digital signal processing (DSP) element due to the practical restriction of a DSP's dynamic range. The requirement of an SSB AGC circuit to operate within 10 msec's places a severe limitation on the use of feedback techniques for AGC circuits due to both the signal processing delays incurred and the potential instability of feedback networks.

The design of linear, heterodyne receivers with AGC circuitry, to set accurately the level of the signal in the linear region of the receiver's subcircuits, using digital signal processors and analog-to-digital converters has proved to be difficult. An AGC circuit uses the largest received signal, measured by the detector, to control the AGC step attenuator levels. If a large interfering signal is received at the measurement detector, and it is within the crystal filter bandwidths but outside the desired signal bandwidth, the interfering signal may be used to set the AGC levels. In a typical linear receiver this creates a problem by providing too much signal to the analog-to-digital converter (ADC) i.e. operating the ADC outside its linear operating range.This operation in the non-linear region of an ADC results in "clipping" of the received signal as known to those skilled in the art. The consequence of clipping to the communications system is a poor reception of the received audio signal.

Thus it is desirable to have accurate AGC operation, operating across a wide dynamic range (e.g. 100 dBs) and responding very quickly (e.g. in less than 10 msec's). It is also desirable to remove the potential problem of an interfering signal, occurring within the pass-band of the crystal filter but outside the desired channel bandwidth, being used to control the AGC operation. This effect would result in clipping of the ADC and result in a poor reception of the audio signal to the communications system user.

Summarv of the Invention According to the present invention, a radio receiver is provided comprising a receiver circuit having an automatic gain control (AGC) attenuator, radio frequency conversion circuitry for providing an analog baseband signal and an analog-to-digital converter (ADC) of finite dynamic range for converting the analog baseband signal to a digitized baseband signal. The radio receiver further comprises a digital signal processor (DSP) - for processing the digitized baseband signal and providing a feedback AGC output to the AGC attenuator to maintain the analog baseband signal within the dynamic range of the ADC. The DSP comprises a digital attenuation arrangement and provides a continuous feedforward AGC output to the digital attenuation arrangement to maintain a desired level of the digitized baseband signal.

In this manner the DSP maintains the analog baseband signal within the dynamic range of the ADC and provides a continuous feedforward AGC output to the digital attenuation arrangement to control accurately the level of the digitized baseband signal.

Preferably the DSP further comprises a digital band-pass filter for filtering the digitized baseband signal and a first automatic gain control detector for providing a signal to the automatic gain control attenuator dependent upon the digitized baseband signal input to the digital band-pass filter. The DSP further comprises a second automatic gain control detector coupled to the output of the digital band-pass filter for providing a signal to the digital attenuation arrangement dependent on the signal from the digital band-pass filter.

In this manner, the DSP ensures that only the desired received signal is used in the continuous feedforward AGC circuits to control the level of the received signal.

The automatic gain control attenuator may be a step attenuator that provides attenuation in sufficiently large steps to attenuate the analog baseband signal with each step from the upper range of the dynamic range of the analog-to-digital converter to the lower end of its range and viceversa.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the drawings.

Brief Description of the Drawings FIG. 1 is a block diagram of a radio receiver architecture employing an AGC circuit according to a preferred embodiment of the invention.

FIG. 2 is a block diagram of an improved radio receiver architecture that eliminates clipping of the analog-to-digital converter according to the preferred embodiment of the invention.

FIG. 3 shows input power versus attenuation graphs emphasising the AGC operation of the radio receivers of FIG. 2 or FIG. 3.

Detailed Description of the Drawings Referring first to FIG. 1, a block diagram of a radio receiver 10, according to a preferred embodiment of the invention is shown, wherein the radio receiver 10 processes baseband signals digitally. The radio receiver 10 comprises a number of receiver sub-circuits. The receiver sub-circuits are an automatic gain control (AGC) step attenuator 12, a radio frequency (RF) conversion network 14, a crystal filter 16, an analog-to-digital converter (ADC) 18 and a digital signal processor (DSP) 20. The DSP 20 is comprised of a digital band-pass filter 22, a detector 28 and a digital attenuation arrangement 24.The detector 28 is operably coupled via a feedback path 32 to the AGC step attenuator 12 and the digital attenuation arrangement 24 In operation, a received input signal 11 is fed into the AGC step attenuator 12 to attenuate the received input signal 11, if required. The attenuated signal is down converted via the RF conversion network 14, to a suitable processing frequency and the down-converted signal is input to the crystal filter 16 to remove out-of-band signals that were either received by the receiver or were generated internally in the RF conversion network 14.

The crystal filter output is then fed into the ADC 18 to provide a digitized representation of the down-converted received signal. The ADC output signal is fed into the DSP 20 where the signal is filtered by the digital bandpass filter 22 and fed to both the detector 28 and the digital attenuation arrangement 24. The detector 28 measures the level of the received signal to determine the optimum setting for the AGC step attenuator 12. The attenuation level of the AGC step attenuator 12 is controlled by the DSP 20 and provides a coarse attenuation of the received input signal 11. The DSP 20 sets the level of the AGC step attenuator 12, via the feedback path 32, according to the measured value of the received signal.

When the detector operation is performed within the DSP 20, the DSP 20 can be used to perform an additional feedforward AGC operation using the digital attenuation arrangement 24. The digital attenuation arrangement 24 divides the received digitized baseband signal with its approximated RMS signal, as known to those skilled in the art. The digital attenuation arrangement 24 divides the received digitally-filtered input signal by the root mean square (RMS) value of the continuous, digitallyfiltered, feedforward input signal 30, fed by the detector 28, to provide the audio frequency output 26. The continuous feedforward AGC operation is limited, due to quantization effects, to a range of 30 dB. When the continuous feedforward AGC operation is coupled with the feedback AGC step attenuator operation the AGC dynamic range can be extended to approximately 100 dB.The AGC step attenuator is controlled by the DSP and provides 0 dB, 25 dB, 50 dB or 75 dB of attenuation.

The use of a continuous feedforward AGC circuit in the DSP 20, coupled with an AGC step attenuator at a radio frequency, advantageously ensures that the automatic gain control of the received signal level is not affected by the delays inherent in continuous feedback AGC approaches.

FIG. 2 shows a schematic block diagram of a radio receiver 10 in accordance with a second aspect of the invention. The radio receiver 10 receives an input signal 11 which is fed into the AGC step attenuator 12.

The AGC step attenuator 12 is connected to a RF conversion network 14, which in turn is connected to a crystal filter 16. The crystal filter output is then fed into an ADC 18. The ADC output is input to a DSP 40. The DSP 40 is comprised of a digital band-pass filter 22, an over-under range detector 54, a detector 48 and a digital attenuation arrangement 44. The over-under range detector 54 is operably coupled via a feedback path to the AGC step attenuator 12. The detector 48 is operably coupled to the digital attenuation arrangement 44 In operation, the AGC step attenuator 12 attenuates the received input signal 11. The attenuated signal is down converted to a suitable processing frequency and input to the crystal filter 16 to remove out of band signals.The filtered signal is then fed into an ADC 18 to provide a digitized representation of the received analog signal and fed into the DSP 40 where the signal is input to the digital band-pass filter 42 and the over-under range detector 54. The over-under range detector 54 of the DSP 40 measures the ADC output and the DSP 40 controls the attenuation level of the AGC step attenuator 12 using the feedback signal 56.

Advantageously the output from the ADC 18 is measured prior to the digital band-pass filter 42 and provides an accurate measurement of the signal input to the ADC 18. Hence, interference signals occurring inside the crystal filter bandwidth but outside the bandwidth of the digital band-pass filter 42 are detected and used to set the level of the AGC step attenuator 12. This removes any possibility of the received signal level exceeding the dynamic range of the ADC 18 thereby causing clipping of the ADC 18.

The ADC output is also input to a digital band-pass filter 42 arranged to filter out any signals which do not occur within the desired channel bandwidth. The digitally filtered output is then input to both the detector 48 and the digital attenuation arrangement 44. The detector 48 measures the level of the digitally-filtered received signal to provide a continuous feedforward, approximated root mean square (RMS) signal level to the digital attenuation arrangement 44 via the feedforward path 50. The digital attenuation arrangement 44 divides the digitally-filtered received signal by the RMS of the digitally-filtered continuous feedforward signal from the detector 48. The digital attenuation arrangement 44 then provides the audio output signal 46.

In this manner the DSP 40 accurately controls the AGC step attenuator 12 and optimises the level of signal attenuation throughout the receiver sub-circuits, in particular, ensuring that the received signal level is within the linear dynamic range of the ADC 18.

FIG. 3 shows three graphs that emphasise the relationship between the received input power, the attenuation operation and the audio level output from the AGC circuits of either FIGS. 2 or 3. Graph 60 shows the operation of the AGC step attenuator 12, controlled by the DSP 20 or 40.

The attenuation level of the attenuator is increased in 25 dB steps, up to an attenuation level of 75 dB, as the received input signal 11 is increased.

Graph 70 shows the operation of the continuous feedforward AGC of the DSP 20 or 40. The continuous feedforward AGC operates across a 30 dB range, 5 dB larger than the step attenuator value, to allow some cross-over between the attenuation functions.

Advantageously this cross-over of attenuation levels ensures that the final audio output signal remains constant for all input levels. The 5 dB margin between the 25 dB attenuation steps of the AGC step attenuator 12 and the 30 dB range of the feedforward attenuator imposes hysteresis on the AGC dynamic performance. The hysteresis is useful as it helps to stabilise the control loop.

Graph 80 shows the level of the audio output signal 46 for various input signal levels. As the received input signal increases, the combination of the receiver's step feedback and continuous feedforward AGC circuits cause the output signal to remain at the same level. This results in a flat response of the audio output across a wide dynamic range of input levels and provides clear audio quality to the radio user.

Thus, the invention provides a useful and effective method of controlling the received signal level throughout the receiver sub-circuits.

Claims (5)

Claims

1. A radio receiver comprising: a receiver circuit having an automatic gain control attenuator, radio frequency conversion circuitry for providing an analog baseband signal and an analog-to-digital converter of finite dynamic range for converting the analog baseband signal to a digitized baseband signal, and a digital signal processor for processing the digitized baseband signal, characterised in that the digital signal processor provides a feedback output to the automatic gain control attenuator to maintain the analog baseband signal within the dynamic range of the analog-to-digital converter and wherein the digital signal processor comprises a digital attenuation arrangement, characterised in that the digital signal processor provides a continuous feedforward automatic gain control output signal to the digital attenuation arrangement to maintain a desired level of the digitized baseband signal.

2. A radio receiver according to claim 1, further comprising, within the digital signal processor: a digital band-pass filter for filtering the digitized baseband signal to provide a digitized filtered baseband signal, and a first digital automatic gain control detector for providing a signal to the automatic gain control attenuator dependent upon the digitized baseband signal into the digital band-pass filter, and a second digital automatic gain control detector coupled to the digital band-pass filter for providing a signal to the digital attenuation arrangement dependent on the digitized filtered baseband signal from the digital band-pass filter, wherein the digital attenuation arrangement attenuates the digitized filtered baseband signal from the digital band-pass filter.

3. A radio receiver according to any of the preceding claims, wherein the automatic gain control attenuator is a step attenuator, providing attenuation in a plurality of sufficiently large steps to attenuate the analog baseband signal with each step from an upper end of the dynamic range of the analog-to-digital converter to a lower end of its range and vice-versa.

4. A radio receiver according to claim 3, wherein the digital attenuation arrangement operates linearly over a greater range than the step attenuation levels, thereby imposing hysteresis on the dynamic performance of the total automatic gain control operation.

5. A radio receiver according to any of the preceding claims, wherein the radio receiver is a single sideband (SSB) receiver.