AU662608B2 - Automatic gain control in superheterodyne radio signal receiver - Google Patents

Description

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i t 1 6 266 P/00/011 28/5/91 Regulation 3.2

AUSTRALIA

Patents Act 1990 i c i L I: IS r (lil r

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title: AUTOMATIC GAIN CONTROL IN SUPERHETERODYNE RADIO SIGNAL RECEIVER The following statement is a full description of this invention, including the best method of performing it known to us:c 2 This invention relates to an automatic gain control (AGC) process in a superheterodyne radio signal receiver and to the circuits used to implement this process. A worthwhile application of the process concerns telecommunications using the TDMA system (Time Division Multiple Access). According to the TDMA method, telecommunications signals are divided packets separated in time for transmission via radio links. The improved gain control in accordance I (IC, with the invention is particularly worthwhile for signals showing intensity variations with one or several of the following characteristics: variations are (i) Sfr fast compared to the duration of a packet; (ii) strong compared to the dynamic 10 range of the receiver amplification system; and/or (iii) unforeseeable, hence requiring an almost real-time reaction of the reception circuit.

The receiver concerning the present application is designed to receive RF signals (radiofrequency). In general, the intensity of the signals received is weak, and they must be amplified before they are used. They may be amplified either following direct demodulation in ultra high frequency, by using base band amplifiers for instance, or in irnermediate frequency (IF) which is much lower than the frequency of the signals received, generally in the order of 10 to 200 MHz. The process in accordance with the invention is applied only in the second case when using an intermediate frequency i.e. in a superheterodyne receiver.

Such a receiver, well known in the art, includes downstream of the reception antenna, a local oscillator which generates a signal having an adjustable frequency FLO equal to the reception frequency FR the ir4,rmediate frequency IF (IF FR FLO), and a mixer which converts the ultra high frequency signal received at the antenna (at frequency FR), into an intermediate frequency signal which will then be processed in the various circuits of the receiver according to the application envisaged. Prior to these various processing operations, aiming at extracting the useful information from the received signals, amplification of the IF signal is carried out, preferably with minimum signal deterioration.

Indeed, the choice of amplifier generally results from a compromise taking into account several parameters to be optimised, such as for instance performance: gain, linearity, passband, dynamic range, noise factor, distortions I- i- 3 caused by saturation or intermodulation, phase nise, amplitude/amplitude and amplitude/phase conversions, etc.; amplifier cost; aw in some applications: consumption, heat dissipation, size, weight, etc. In genenl, a more performing amplifier costs more. This is why many inventions in the last I-w years have aimed at lowering the cost of amplifier circuits for receivers while aintaining their performance.

The application envisaged for the receiver imposes its own constrai rs to the characteristics of the amplifier chain. For signals transmitted via a fixed ea radio link, random variations of the intensity received may be due to atmospheric propagation. The dynamic range of the amplifier chain must be sufficiently large to compensate the dynamic range of the signals during transmission, combined with the dynamic range of the random variations due to the conditions of propagation. For links between mobiles and a fixed station, high variations in the intensity of the signals received may result in differences in the length of the propagation path (the intensity received varies, according to a first Jr. ~approximation, inversely as the square of the distance covered).

It is common knowledge to use an automatic gain control circuit in a *receiver to overcome the variations in intensity of the signal received which, under some conditions, may be higher than the dynamic range of the receiver 20 amplifier chain.

In Australian Patent application No. 35486/93, there is disclosed an open loop automatic gain control system for heterodyne receiver. In the receiver in 96*4 accordance with this application, the intensity of the signal received is measured during a certain time, before the signal is applied to the amplifier chain input.

25 Thus, during all the time the intensity received is measured, it is necessary to delay the signal before it is applied at the amplifier chain input, to enable the AGC circuit to adjust the gain which will be applied for signal amplification. The problem arises on how to obtain the delay necessary to have the AGC open loop while leaving enough time to carry out the measurement and control the gain beforg the signal reaches the amplifier input.

In particular, in the case of transmission of signals in time packets, it is common knowledge to measure the intensity of a packet after it is applied to the amplifier chain input, to allow for automatic gain control of the amplifier in V,3 closed loop. The gain thus controlled is therefore applied to the next signals received. Conversely, within the context of the present invention, measurement of the signal intensity is used to control the gain which is used to amplify this very signal. This necessarily implies the use of some means to delay the signal for the entire duration of the measurement. In the state of the art, in a technical field closely related to field of the patent application, ie radars, it is common t, knowledge to use a SAW (Surface Acoustic Wave) delay line. In this case, the maximum measurement duration is limited by the maximum delay which can be obtained by a SAW delay line.

The use of surface acoustic wave delay lines is current in some receiver circuits operating at intermediate frequency, and it is even standard in respectable applications such as radars and sonars. Nevertheless, their use still present several drawbacks, which we could very vveil do without, especially in applications designed for large markets such as radiophone receivers. Indeed, these SAW components are expensive, bulky, fragile, and moreover, their performances are limited. The maximum delay obtained from SAW components is in the order of 25 ps, and their insertion loss is at least in the order of 25 dB.

-In cases where strong variations in the intensity received over a time scale s rter than the duration of a transmission packet are expected, the intensity f meas ment reliability becomes haphazard, the measurement can be made during the jration of the packet but outside a period of strong and quick variations. The'ih insertion losses of these components prevent them from being used in series i obtain a more significant delay in the main channel used by the signal before it is mplified and processed.

The process in accorda ce with the invention remedies these drawbacks.

According to the invention t lre is provided a method of providing automatic gain control in a superhetero ne radio signal receiver, comprising the steps of: Splitting the IF signal into two channels, on main channel and one secondary channel.

Delaying the IF signal by a time tau in the main ch nel.

Measuring signal power in the secondary channel during a 'mile shorter than or equal to tau 17 Qi Control the gain of the IF amplifier in relation to the measured power.

Delay amplification of the IF signal by T, wherein: The intermediate frequency (IF) is selected among frequencies commonly used for video signals in consumer-oriented equipment.

Indeed, selecting the intermediate frequency (IF) outside the frequency range usually used in the previous art for ultra high frequency signal receiver circuits (in radars for instance) makes it possible to use components which where developed and marketed for consumer-oriented applications, in chrominance signal circuits for colour televisions or video cassette recorders for instance.

The intermediate frequency (IF) offers the advantage of being selected Ii within a range of frequencies greater than 1 MHz and lower than 10 MHz, i.e.

Is 6, one order of magnitude below the IF frequencies usually used in radar ultra high v ttt ,Ot frequency signal receiver circuits.

Preferably, delay time tau is less than a millisecond, greater than a Smicrosecond (1 gs), equal to 30 Is, equal to 64 4s, or equal to 128 Aus. These delay times are greater than those usually obtained by SAW lines.

Indeed, currently found on the market are comb filters and glass delay 6 line components designed for consumer-oriented video applications, operating coo 20 for instance at nomrninal frequencies of 3.58 or 4.43 MHz, capable of supplying a nominal delay of 64 or 128 gs. Due to the large scale production of these S .4 components, they can be marketed at prices a few orders of magnitude lower than those of the SAW lines. Furthermore, the insertion loss of these video components is commonly lower than 10 dB, i.e. more than two orders of 04'W 25 magnitude in difference compared with SAW components.

0 According to a further aspect of the present invention, there is provided a receiver used to carry out the method of the invention comprising, a splitter, splitting the signal into two channels; a main channel; a delay line in the-main channel; an IF amplifier operating at intermediate frequency (IF) and having its

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output connected to the IF amplifier in order to control the gain, wherein said delay line is a video delay line.

Preferably, the video delay line is a component commonly used in consumer-oriented television equipment. Preferably, the video delay line is in accordance with those used in colour television chrominance circuits (for NTSC, PAL or PAL/SECAM systems). The video delay line may be in accordance with those used in video cassette recorders.

Other characteristics and advantages of the invention are highlighted in the following detailed description of the single attached figure, which diagrammatically shows an embodiment of a receiver circuit device used to implement the method in accordance with the invention. This example is not to be considered as limiting.

The single figure first shows the input of an intermediate frequency signal Then, a power splitter splits part of the IF signal power over a secondary 15 channel for power measurement and gain control. This secondary channel (4) :is a forward loop (also called open loop) for Automatic Gain Control (AGC).

On the main channel the output of splitter is connected to the input of a delay line where the IF signal is delayed by a time tau before it is applied to the input of the IF variable gain amplifier During signal aelay time tau over the main channel a power detector measures the power of that part of the IF signal forwarded over the secondary channel this allows one to know the signal power received. The result of this power measurement is then sent to some gain control means .Gain control means act on the IF variable gain amplifier before the delayed IF signal reaches, via main channel the input of amplifier The method in accordance with the invention is characterised by a particular selection of IF frequency, among the frequencies commonly used for video signals in consumer-oriented equipment. Such frequencies may be within a range of 1 MHz to 10 MHz, and a particular fall within the range of 3.5 MHz to 6.5 MHz such as 4,43 MHz 2 MHz or 3.58 MHz. This IF frequency selection makes it possible to use a video delay line to delay the signal in the main channel by a time tau One of thle advantages of the process in accordance with the invention is that the delay time thus obtained is .1 Lu 07d 6a sufficiently long to provide greater accuracy and reliability of the measurements made in the loop before AGC, hence greater gain control providing for better signal information at

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I U a e l 7 the output of the IF amplifier The size of these video components is smaller than those used in SAW lines, and these video components are also less fragile.

Their lower insertion loss makes it possible to use an amplifier chain with lower gain, thus saving on power consumption and reducing heat dissipation in the receiver circuit.

Further to the technical advantages resulting from the invention, there is a major advantage on an industrial point of view, that is the cost of video delay t ,#1

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line components is several orders of magnitude lower than the cost of SAW t delay lines.

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Claims (14)

1. A method of providing automatic gain control in a superheterodyne radio signal receiver, comprising the steps of: splitting the IF signal of said receiver into two channels, one main channel and one secondary channel; delaying the IF signal by a time tau in the main channel; measuring signal power in the secondary channel during a time shorter than or equal to tau controlling the gain control of an IF amplifier in relation to the measured power; delaying amplification of the IF signal by T, wherein tihe intermediate frequency (IF) is selected among frequencies commonly used for video signals in consumer-oriented equipment. S

2. A method as claimed in claim 1, wherein the intermediate frequency falls within a 1MHz to 10 MHz range.

3. A method as claimed in claim 2, wherein tf'a intermediate frequency falls within a 3.5 MHz to 6.5 MHz range.

4. A method as claimed in claim wherein the intermediate frequency is 4.43 MHz 2 MHz.

5. A method as claimed in claim 3, wherein the intermediate frequency is L 3.58 MHz 2 MHz. 614 4

6. A method as claimed in any one of claims 1 to 5, wherein delay time tau is shorter than one millisecond.

7. A method as claimed in any one of claims 1 to 6, wherein delay time tau is greater than 30 microseconds.

J8. A method as claimed in any one of claims 1 to 7, wherein delay time tau o, o° is 64 microseconds.

9. A method as claimed in any one of claims 1 to 7, wherein delay time tau is 128 microseconds.

10. A receiver for carrying out the method as claimed in any one of the above claims 1 to 9, comprising a splitter, splitting the signal into two channels; a main channel; a delay line in the rain channel; an IF variable gain amplifier operating at intermediate frequency and having its input connected to the delay as i ~-i r-l- 9 line output; a secondary channel; a signal power level detector connected to the secondary channel; a gain control device, located downstream of the power detector in the secondary channel and having its output connected to the IF variable gain amplifier in order to control the gain, wherein said delay line is a video delay line.

11. A receiver as claimed in claim 10, wherein said video delay line is a r, component commonly used in consumer-oriented television equipment.

12. A receiver as claimed in claim 10 or 11, wherein the said video delay line is in accordance with those used in colour television chrominance circuits.

13. A receiver as claimed in claim 10 or 11, wherein said video delay line is in accordance with those used in video cassette recorders.

14. A receiver substantially as herein described with reference to the figure of the accompanying drawing. DATED THIS SIXTEENTH DAY OF MARCH 1993 ALCATEL N.V. i lr. _r I *9 4b* (I t f.. C I ABSTRACT This invention relates to the use of an intermediate frequency in an ultra high frequency signal reception and demodulation circuit, prior to signal amplification. In accordance with the invention, astute selection of the intermediate frequency provides for easy implementation of this circuit. An intermediate frequency is selected for the reception circuit which corresponds to a frequency commonly used in television sets and video cassette recorders. This advantageous intermediate frequency selection makes is possible to use a very common and hence inexpensive component a video delay line to enable an open loop to act on the amplifier gain before application of the signal at its input. Therefore, a system in accordance with the invention includes a video delay line at a selected intermediate frequency which delays the signal before the input of an amplifier which is gain controlled by an open loop. Field of application: telecommunications and in particular those using TDMA, FIGURE 1. I