A RECEIVER FOR AMPLITUDE MODULATED SIGNAL

Abstract: A receiver for amplitude modulated signals, particularly a receiver in which the amplitude of such signals is measured and a digital output produced. A receiver is disclosed which is capable of measuring the amplitude of an input signal over an 80dBm range. The input signal is applied via a limiter (12) to a power splitter (14). First and second video detectors (16, 20) are coupled to the power splitter (14), in the case of the second video detector (20) an R.F. linear amplifier (18) is connected in the signal path to provide an overall 40dB's of gain relative to the input of the first video detector (16). Each video detector comprises a detector stage (22) whose output is connected to an amplifier chain (24, 26, 28) each amplifier in the chain having a gain of 10. The outputs of the detector stage (22) and amplifiers (24, 26, 28) are quantised and applied to a logarithmic analogue-­ to-digital converter (30). The outputs of the respective analogue-to-digital converters are applied to a combiner (32).

Description: The present invention relates to a receiver for amplitude modulated signals, more particularly to a receiver for measuring the amplitude of such signals. A wide bandwidth amplitude measuring receiver is known which is used to provide a measurement in digital form of a receiver signal in the microwave range of frequencies. This known receiver includes an analogue logarithmic amplifier whose output is supplied to an analogue-to-digital converter. In practice it is difficult to make a satisfactory analogue logarithmic amplifier because a wideband diode has to be used and this can cause stability problems. Additionally such a diode has difficulty in maintaining its square law characteristic over the full dynamic range which may be as high as 80dB. Further disadvantages of this known receiver are that it is bulky, expensive and difficult to apply temperature control. An object of the present invention is to provide a compact, less expensive amplitude measuring receiver which does not use an analogue logarithmic amplifier. According to the present invention there is provided a receiver for amplitude modulated signals, comprising an input terminal and a video detector coupled to the input terminal, the video detector including a detector stage, an amplifier chain connected to the detector stage and a logarithmic analogue-to-digital converter having a plurality of inputs respectively coupled to outputs of the detector stage and the amplifiers constituting said amplifier chain. The receiver made in accordance with the present invention can be made from integrated circuit elements and in consequence is compact, less expensive to make and lends itself to the application of temperature control to provide temperature stability. In implementing the receiver made in accordance with the present invention the amplifier chain comprises a plurality of series connected amplifiers. The quantisation of the input signal is carried out by dividing the dynamic range of the input signal into equal parts, for example 10dB, and arranging the gain of each amplifier such that signals in its part of the dynamic range can be quantised in an arrangment of comparators to each of which a reference voltage derived from a respective tapping of a logarithmic resistive chain is applied; each of said arrangements being substantially identical thereby simplifying their manufacture. The detector stage includes a diode coupled to an input of an operational amplifier and in order to minimise the effect of drift in the diode which is forward biased, a replica diode is coupled to a second input of the operational amplifier. The amplifiers of the amplifier chain may conveniently comprise operational amplifiers and each of them has a secondary feedback loop including a diode to provide an external limiting circuit. If internal limiting was applied then there will be a severe increase in the recovery time in the event of an overload. In order to increase the sensitivity of the receiver and its measuring range, there may be provided a duplicate video detector for measuring the signal amplitudes in the lower half of the dynamic range of the input signal. In such an embodiment the input signal is applied to a power splitter which has two outputs one coupled to the first mentioned video detector and the other to an R.F. linear amplifier which is connected to the duplicate video detector. The present invention will now be described, by way of example, with reference to the accompanying drawings, wherein: Figure 1 is a block schematic circuit diagram of an embodiment of the present invention, and, Figure 2 is a detailed circuit diagram of the embodiment shown in Figure 1, In the drawings corresponding reference numerals have been used to indicate the same features. The amplitude measuring receiver is adapted to produce as an output digital signal an indication of the amplitude of the input signal which may have a frequency in the range 400MHz and 40GHz. An input signal is applied to an input terminal 10 which is coupled via an amplitude limiter circuit 12 to a wide band power splitter 14. In the illustrated embodiment the sensitivity of the measuring system allows levels between +10dBm and -70dBm to be measured. As this amplitude range is large it is split so that the higher amplitude signals can be measured in a first video detector 16 and the lower amplitude signals after amplification in an R.F. linear amplifier 18 are measured in a second video detector 20 which is identical in construction to the first video detector 16. The digital ouputs from the video detectors 16, 20 are combined in a combiner 32 which produces as an output a digital signal representing the value of the amplitude of the signal at the input terminal 10. The video detector 16 comprises a detector stage 22 and an amplifier chain formed by three series connected amplifier stages 24, 26 and 28 coupled to the detector stage 22. The outputs of the stages 22 to 28 are connected to a logarithmic analogue-to-digital converter 30 which includes a latch. In this embodiment the converter 30 is able to produce a seven bit output. The converter 30 is sampled at a clock rate of 20 MHz. In order to test the receiver circuit, a "bite" oscillator 34 is coupled by a filter coupler 36 to the line linking the limiter 12 to the power splitter 14. The bite oscillator 34 may comprise a dielectric resonator pulsed oscillator operating near the middle of the band of the signals to be measured. The bite oscillator 34 is operated on demand from a receiver processor (not shown). The limiter 12 includes a switch which will enable the R.F.bite signal to be introduced into the receiver without the risk of resultant radiation out through the input terminal 10. If a receiver with a lower sensitivity is desired then the power splitter 14, the amplifier 18, the second video detector 20 and the combiner 32 can be omitted. Turning now to Figure 2, the limiter 12 serves to protect the receiver from excess power signals which for example may be radiated by a nearby transmitter. The limiter 12 combines the limiter action with a switch which inhibits radiation when the bite oscillator 34 is operative. The limiter 12 comprises a multiple bus bar switch with a design limit of 1KW pulse, 10W mean (CW) which is constructed in such a way that can be operated by a d.c. voltage and thereby constitute an externally controllable switch. In order to operate the bite oscillator 34, a voltage pulse is applied simultaneously to the oscillator 34 and to diodes 40 in the limiter 12. The pulse closes the limiter 12 and produces a supply voltage to the oscillator 34. The oscillator signal thus generated will be directed into the receiver with little leakage back to an antenna (not shown) via the input terminal 10. In the present embodiment the bite oscillator output will be a signal of -10dBm for 0.5µS with a frequency accuracy of better than 1 in 10⁴. The wide band power splitter 14 splits the input signal such that -6dB is directed into the first video detector 16 which, as will be explained, operates between the levels of +4dBm and -36dBm. The remainder of the input signal from the power splitter 14 is applied to the second video detector 20 via the R.F. linear amplifier 18 which has a gain of 36dB over the appropriate bandwidth. The gain between the inputs to the first and second video detectors 16, 20 is 40dB. The second video detector 20 is active over the R.F. input signal amplitude range of -30dBm to -70dBm, whereas the first video detector is active over the R.F. input signal amplitude range of +10dBm to -30dBm. As the two video detectors 16 and 20 are identical only the first one 16 will be described in detail and the corresponding main parts in the second video detector 20 will be indicated with primed reference numerals. The detector stage 22 comprises a diode 42 connected via a resistor 44 to one input of non-inverting amplifier 46 whose gain is adjusted to equalise the sensitivities in the system. The gain generally lies between 1 and 10 and in the present embodiment is taken to be 5. The amplifier 46 is an operational amplifier, such as a Signetics 5539 UHF Operational Amplifier, whose gain is set by resistors 48, 50 having resistance values of R and 5R, respectively. A secondary feedback loop including a diode 52 reduces the gain of the amplifier to 0.5 when the output exceeds +2V, +2V corresponds to a video level of +10dBm R.F. input and 1.001V at -30dBm R.F. input. As the detector diode 42 is forward biased a d.c. level will exist at the diode output, which level will be subject to drift. To minimise the effect of this drift an exact replica of the diode and bias network is used as the amplifier reference voltage VR. The replica diode is referenced 56 and the bias network comprises resistors 58, 60, the junction of these resistors 58, 60 being connected to a reference voltage VR. The zero signal level voltage is arranged at +0.2 volts. The signal from the detector stage 22 is then digitised by a 7 bit logarithmic analogue-to-digital conversion network 30 which is clocked at a high frequency of say 20MHz. In the system being described the input signal is quantised on the basis of 32 levels for every 10dB of the input signal. In order that the analogue to digital converters 30, 30' can operate in their optimum ranges the signal levels in the most significant 40dB of the input signal are digitised in the first video detector 16 and signal levels in the least significant 40dB are the input signal are digitised in the second video detector 20. In each video detector the 4 decades are quantised separately by examining the most significant 10dB, then the next most significant 10dB, then the third most significant 10dB, and finally the least significant or fourth most signficant 10dB of the input signal. Taking the first video detector 16 as an example the output from the detector stage 22 is applied respectively to one input of a group of thirty-two comparators 62 in each of which the output is compared with a respective reference voltage derived from a logarithmic resistive chain 64. If the output from the detector stage 22 lies in the first decade then the analogue-to-digital converter 30 will latch at its required digital value. The output from the detector stage 22 is also supplied to the three series connected identical amplifier stages 24, 26, 28 each having a gain of 10. By increasing the amplitude of the signal in this way then the resistive chains 66, 68 and 70 providing the respective reference voltages to each group of thirty-two comparators 64, can be the same. Each of the amplifiers 24, 26, 28 comprises an operational amplifier 72, such as a Signetics 5539 UHF Operational Amplifier, having the usual gain determining resistors 74, 76 whose values are in the ratio of R and 10R to provide a gain of 10. Additionally a secondary feedback loop including a diode 78 and resistors 80, 82 reduces the gain of the amplifier 12 to less than unity when the output exceeds +2V. By avoiding internal limiting of the amplifier 72 then the risk of saturating the amplifier, which will lead to a severe increase of recovery time, is avoided. In digitising the input signal, a clock or sample pulse is applied to an input 84 of the analogue-to-digital converter 30 at the required instant. In the present example, the digital samples are taken every 50nS (20 MHz) and there is 3 sample periods between the sample time and the latched output. The outputs from each of the analogue-to-digital converters 30, 30' comprise a 7 bit parallel output 86, 86', an overflow output 88, 88', and an underflow output 90, 90'. These outputs are coupled to the combiner 32 which comprises two latches each with a tristate output. The signals from the analogue-to-digital converters 30, 30' are separately latched and held and the selection of the tristate enable is actioned by the signals on the overflow and underflow lines 88, 88', 90, 90'. Thus if the analogue-to-digital converter 30' produces an overflow signal on the line 88' and the analogue-to-digital converter 30 shows no underflow then the read-out from the combiner 32 comprises the 7 bits from the converter 30 and a first digit 1 is added to make 8 bits thus indicating that the input signal had an amplitude in the most significant 40dB. Conversely if the converter 30 indicates an underflow on the output 90 and the converter 30' shows no overflow then the 8 bit output from the combiner 32 is the 7 bit output from the converter 30' with the first bit comprising a digital 0 thus indicating that the input signal had an amplitude in the least significant 40dB. The construction of the described and illustrated circuit can be based on alumina substrate microwave circuits and alumina substrate thin film techniques which enable a very compact unit to be made and by being so compact it is possible to temperature control the entire unit by means of a heater.

Claims: A receiver for amplitude modulated signals, characterised by an input terminal and a video detector coupled to the input terminal, the video detector including a detector stage, an amplifier chain connected to the detector stage and a logarithmic analogue-to-digital converter having a plurality of inputs respectively coupled to outputs of the detector stage and of the amplifiers constituting said amplifier chain. A receiver as claimed in claim 1, characterised in that the amplifier chain comprises a series arrangement of n amplifiers of equal gain, where n is at least two. A receiver as claimed in claim 2, characterised in that the dynamic range of the detected signal is quantised in ( n +1) equal steps. A receiver as claimed in claim 3, characterised in that the detected input signal is quantised in steps of 10dB and the gain of each of the amplifiers in the amplifier chain is 10. A receiver as claimed in claim 3 or 4, characterised in that a corresponding plurality of comparators are connected to the output of the detector stage and to each of the ouputs of the amplifiers, each of the comparators having an input for a respective reference voltage derived from a tapping on a logarithmic resistive chain, and in that each plurality of comparators has a respective chain and each of the resistive chains and the tappings thereon are substantially the same. A receiver as claimed in any one of claims 1 to 5, characterised in that the detector stage includes a gain equalising amplifier having an external limiting arrangement. A receiver as claimed in claim 6, characterised in that the gain equalising amplifier is an operational amplifier having first and second inputs, in that the detector comprises a diode coupled to the first input and in that a replica diode is coupled to the second input to minimise the effect of drift in the detector diode. A receiver as claimed in claim 2, 3, 4 or 5 or claim 6 or 7 when appended to claim 2, characterised in that each of the n amplifiers comprises an operational amplifier having a secondary feedback loop including a diode to provide an external limiting arrangement. A receiver as claimed in any one of claims 1 to 8, characterised by a limiter including a switch coupled to the input terminal and a bite oscillator operatively coupled to the output from the limiter, whereby when the bite oscillator is pulsed, the switch is actuated to block signals from being radiated via the input terminal. A receiver as claimed in any one of claims 1 to 9, characterised by a second video detector, substantially identical to the first mentioned video detector, a power splitter coupled to the signal input terminal, the power splitter having a first output coupled to the first mentioned video detector and a second output coupled to a linear R.F. amplifier which is coupled to the second video detector and a signal combiner coupled to the outputs of each of the video detectors.

Inventors: BOLLARD, DAVID ROOKSEY
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN, PHILIPS ELECTRONICS UK LIMITED
Application year: 1985
Main classification: H04N5
IPC codes: H03G7, H03M1, H04N17, H04N5