Classifications

• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03J—TUNING RESONANT CIRCUITS; SELECTING RESONANT CIRCUITS
  • H03J7/00—Automatic frequency control; Automatic scanning over a band of frequencies
  • H03J7/02—Automatic frequency control
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
  • H04B1/06—Receivers
  • H04B1/16—Circuits
  • H04B1/26—Circuits for superheterodyne receivers
  • H04B1/28—Circuits for superheterodyne receivers the receiver comprising at least one semiconductor device having three or more electrodes

Definitions

• the invention relates to a superheterodyne circuit.
• One field of application of the invention relates to radio frequency communication systems.
• Superheterodyne circuits usually include a channel selection bandpass filter interposed in the signal path between an input receiving a first signal and an output producing a second signal, from which a signal can be produced. base band.
• the filter is determined to pass an intermediate frequency of the superheterodyne circuit.
• the central frequency passed by the filter must remain constant to allow the correct functioning of the circuit.
• one or more other stages at intermediate frequencies can be provided upstream and / or downstream of the stage in which the filter is provided.
• the intermediate frequencies of these stages being determined as a function of each other to obtain the baseband signal, any drift of the center frequency of the filter is unfavorably reflected in the baseband signal obtained.
• This method of stabilizing the center frequency of the bandpass filter is limited to active filters with setting parameters and is therefore not suitable for all types of filters. In addition, this method is complicated to implement.
• the invention aims to obtain a superheterodyne circuit which overcomes the drawbacks of the state of the art and which makes it possible to use a channel selection filter having a frequency drift.
• the subject of the invention is a superheterodyne circuit, comprising at least one input for receiving a first signal, at least one output for producing a second signal, from which a band signal can be produced. base, and at least one channel selection bandpass filter, interposed in the signal path between the reception input and the production output, characterized in that the channel selection bandpass filter is capable of being connected to means for measuring a characteristic signal passage frequency in said channel selection bandpass filter, controllable frequency shift means are arranged in said signal path, and control means are provided, connected to the measuring means and controlling the frequency shifting means for shifting the at least one signal present in said path by an additional signal, which compensates for the deviation of the characteristic frequency.
• any frequency drift of the filter whether of mechanical, thermal or electrical origin can be compensated in the circuit.
• the circuit adapts to variations that may occur in the frequency of passage of the filter. There is therefore no need to intervene directly on the filter itself so that its frequency of passage is always equal to the prescribed value.
• the invention accommodates all types of bandpass filters, and in particular non-ideal filters, and makes it possible in particular to use filters of large quality factors which may be relatively unstable in frequency at maximum gain.
• FIG. 1 is a block diagram of the circuit according to the invention.
• FIG. 2 schematically shows a micromechanical filter that can be used in the circuit according to the invention
• FIG. 3 is a block diagram of the control and measurement means used in the circuit according to the invention.
• the superheterodyne circuit 1 is part of a not shown superheterodyne receiver, comprising for example a reception antenna.
• the superheterodyne circuit 1 comprises one or more stages at intermediate frequency and, for example, as shown, a stage 2 at second intermediate frequency connected between a first stage 3 at first intermediate frequency upstream and a stage 4 in baseband downstream .
• a stage 2 at second intermediate frequency connected between a first stage 3 at first intermediate frequency upstream and a stage 4 in baseband downstream .
• one or more stages at intermediate frequency could also be provided downstream of stage 2.
• the superheterodyne circuit 1 comprises an input 5 for receiving a first signal, which is in fact the output signal from the stage 3 at first intermediate frequency and an output 6 for producing a second signal, which is in fact the baseband input of stage 4 in baseband.
• a signal path 7 is provided between input 5 and output 6.
• the value of the first intermediate frequency applied to input 5 is for example equal to 10.7 MHz, for a receiver operating in the ISM band at a radio frequency of 433.92 MHz.
• the part of the receiver receiving the radiofrequency signal and converting it into the first intermediate frequency upstream of stage 2 is carried out according to a conventional heterodyne architecture, comprising for example an antenna filter not shown.
• a channel selection bandpass filter 8 is provided in the path 7 between the input 5 and the output 6.
• This filter 8 is for example a micromechanical filter of the comb resonator type according to FIG. 2 and as described by document "Micromechanical Resonators for Oscillators and Filters", by Clark T.-C. Nguyen, Proceedings of the 1995 IEEE International Ultrasonics Symposium, Seattle, WA, pages 489 to 499, November 7-10, 1995, shown in Figure 2 of this document.
• This filter is manufactured in thick layer epitaxial technology with a single structural layer of silicon and an underground layer of polysilicon, used for polarization.
• the inlet 9 of the filter is connected to an inlet comb 10, between the teeth 11 of which are provided the teeth 12 of an inlet comb 13 of a transducer 14 also comprising an outlet comb 15 whose teeth 16 are provided between the teeth 17 of a comb 18 connected to the outlet 19 of the filter.
• Means 20 for suspension with respect to anchorages 21 are provided for the transducer 14.
• DC voltages V 1, Vo and V p are provided for respectively polarizing the input 9, the output 19 and the transducer 14.
• the resonant frequency of this resonator is 94510 Hz at room temperature, the bandwidth of the filter is 2 Hz to 30 Hz depending on the air pressure under which the resonator operates, the bias voltage in normal operation is around from 40 to 60 Volts.
• the elements provided outside the filter 8 are described below.
• means 22, 23 for frequency offset are provided in the signal path 7.
• the means 22, 23 are for example of the frequency mixer type.
• the frequency shifting means 22 is provided between the input 5 and the input 9 of the filter 8.
• the frequency shifting means 22 comprises a first signal input 24 connected to the input 5, a second signal input 25 connected to the output of a first local oscillator 26, capable of producing on the second input 25 an additional signal of variable frequency as a function of the signal sent to an input 27 of frequency control thereof.
• the frequency shifting means 22 includes a signal output 28 providing a time signal which is the product of the time signals present on the first and second inputs 24 and 25 thereof.
• the frequency shifting means 23 comprises a first input 29 and a second input 30 connected to the output of a second local oscillator 31 providing an additional frequency signal on the input 30 and comprising an input 32 for controlling the signal frequency supplied on this input 30.
• the frequency shifting means 23 includes an output 33 connected to the output 6.
• Switching means 34 are provided between the output 28 of the first frequency shifting means 22 and the input 9 of the filter 8, and between the output 19 of the filter 8 and the first input 29 of the second shifting means 23 frequency.
• Switching means 36, 37 are provided for connecting the filter 8 to means 38 for measuring the characteristic frequency of signal passage through this filter 8.
• the switching means 36, 37 are for example connected to the input 9 and to the output 19 of the filter 8 on the one hand and at the terminals of a module 39 for positive feedback, forming, when the switching means 36, 37 are closed, a loop with the filter 8 to cause it to oscillate.
• the switching means 36, 37 are controlled in the same way with respect to each other in the closed state of connection or open disconnection, for example simultaneously, as shown by the dashes between them.
• the switching means 34 and 35 are controlled in the same way with respect to each other in the closed state of connection or in the open state of disconnection, as shown by the dashes between them. this. Finally, the switching means 34, 35 are controlled inversely with respect to the means 36, 37 switching, to connect the filter 8 either to the signal path 7 or to the measuring means 38.
• the switching means 34, 35, 36, 37 are each formed by a manually operable switch.
• the switching means 34 and 36 could be formed by a switch proper from the input 9 either to the output 28, or to the module 39, and the switching means 35 and 37 could also be formed by a switch properly said from output 19 either to input 29 or to module 39, these switches being connected to each other to switch at the same time either to output 28 and input 29, or to module 39 .
• the measurement means 38 are connected to a module 40 for controlling the inputs 27, 32 for controlling the frequency of the local oscillators 26 and 31.
• the measurement means 38 comprise a module 41 for determining the characteristic frequency of passage of the filter 8, connected to the module 39, for example by a terminal of the latter connected to the means 36.
• the module 41 for determining the characteristic frequency of passage of the filter 8 comprises for example, as shown in FIG. 3, a first counting member 42 the number of oscillations produced by the filter 8, connected to the module 39 for positive feedback, a second member 43 for counting time, for example for counting clock periods of a computer.
• the members 43 and 44 are connected to a member 44 for calculating the frequency of the oscillations produced.
• the measurement of the characteristic frequency of passage of the filter 8 is obtained by dividing the number of oscillations of the filter 8 counted by the first counting member 42 by the time counted by the second time counting member 43, the means of switching 36, 37 being assumed to be closed for counting, as shown in FIG. 3.
• the member 44 for calculating the frequency of the oscillations produced is connected to the control means 40.
• the control means 40 comprise a first frequency control output 45 connected to the frequency control input 27 of the local oscillator 26 and a second frequency control output 46 connected to the frequency control input 32 the local oscillator 31.
• the control means 40 comprises a first module 47 for calculating the control signal sent to the first frequency control output 45 for the local oscillator 26 and a second module 48 for calculating the frequency control signal sent to the second output 46 of frequency control for the other local oscillator 31.
• the filter 8 is for example provided to let pass the signal having a frequency equal to the difference of the frequencies of the signals present on the first and second input 24, 25 of the means 22 and attenuate with a very important attenuation factor the signal having a frequency equal to the sum of the frequencies of the signals of the first and second inputs 24, 25 of the means 22, the frequency of the signal present on the input 5 being greater than that of the signal present on the second input 25.
• the characteristic frequency of passage of the filter is for example its central frequency fc of passage of its passband, that is to say the half sum of the high and low cutoff frequencies at - 3dB on either side of its band.
• the baseband signal present on the output 6 is formed for example by the signal of frequency equal to the frequency of the signal present on the first input 29 of the means 23, minus the frequency of the signal present on the second input 30 of the means 23 , the theoretical characteristic frequency of passage of the filter 8 being greater than the frequency of the signal present on the second input 30 of the means 23.
• the calculation module 47 is designed to send to the input
• a frequency control signal for subtracting from the frequency signal present at the output of the first oscillator 26 and on the second input 25 the algebraic deviation of the measured characteristic frequency supplied by the means 38 of measurement with respect to a prescribed value of frequency of passage of filter 8.
• This prescribed value of frequency of passage of filter 8 is equal to the theoretical characteristic frequency of passage of filter 8, that is to say the frequency passage characteristic for which the filter 8 has been designed. Ideally, that is to say in the absence of frequency drift of the filter 8, the actual characteristic frequency measured by the means of measurement 38 is equal to the prescribed value of frequency of passage and the frequency difference is zero.
• the frequency of the signal present on the second input 30 is ideally equal to the prescribed value of frequency of passage of the filter.
• the second calculation module 48 is designed to send to the frequency control input 32 of the oscillator 31 a frequency control signal to add to the frequency of the signal present on the first input 29 of the second means 23 of frequency offset, the algebraic deviation of the measured characteristic frequency provided by the measuring means 38 with respect to said prescribed value of frequency of passage of the filter 8.
• the frequency drift which may be present in the filter 8 is it eliminated from the frequency signal equal to the difference in the frequencies of the signals present on the inputs 29 and 30 of the means 23 of the frequency offset, and therefore on the baseband output 6, by a corresponding frequency offset upstream of the output 6, when the latter is connected by means 35 to the outlet 19 of the filter 8.
• the frequency drift that may be present in the filter 8 is not passed on to the output 6 in baseband, while making it possible to pass to the output 6 the information contained in the signal present on the input 5 and in the stages upstream and intended to be coded frequently in a manner corresponding to the theoretical characteristic frequency of passage of the filter 8, thus preventing the information transmitted to the output 6 from being altered by the filter 8.
• the characteristic frequency must be determined by the means 38 with an absolute error not exceeding 1 Hz. It is therefore also the precision with which the center frequency of the filter should be determined.
• the number of pulses that must be received by the body 42 is equal to 2000 (this value depends on the precision with which the organ 43 measure a time interval, which depends on its operating frequency).
• the measurement time required is equal to 20 milliseconds.
• the oscillators 26, 31 are produced for example each by a local resonator.
• a local resonator To adapt the frequency of the local resonator to changes in the central frequency of filter 8, its value must be checked with 1 Hz of precision. Since this frequency is of the same order as the first intermediate frequency (10.7 MHz), the change step required is 0.00001% of the absolute value of the frequency.
• This precision being difficult to achieve by using a basic locking loop (PLL) in oscillator 26 or 31, the signal from the local oscillator was generated with direct digital synthesis DDS (Direct Digital Synthesys).
• DDS Direct Digital Synthesys
• the inventors varied the temperature of the filter, and thereby caused its center frequency to be derived from -400 Hz.
• the measurements proved that the correction of the center frequency of the filter was done with sufficient precision, the second intermediate frequency always coinciding with the central frequency of the filter.
• the duration of the measurement phase is 143 ms. Although this value appears high in a real RF system (during this period reception cannot take place), it should however be borne in mind that the target center frequency of the micromechanical filters in the intermediate frequency stages is l 'order of a hundred megahertz, which will require a much shorter measurement time.
• the filter 8 is switched by the means 34 to 37 on the measurement means 38 to obtain a measurement thereof during a prior calibration phase, then the filter 8 is switched on path 7, input 5 and output 6, to transmit the information contained on input 5 to output 6, during a reception phase.
• the filter 8 is periodically switched over to the measuring means to adjust each time the frequency of the oscillators 26, 31 and the frequency correction in the path 7 for the immediately following reception phase, periodic switching control means being provided for this purpose.
• the communication means 34 to 37 can be formed by electronic switches controlled manually or automatically when the circuit is energized.
• the circuit thus automatically adapts to the frequency drift of the filter used, whatever the origin of this drift and whatever the band-selection filter for channel selection.
• the invention thus makes it possible to use the micromechanical filters mentioned previously, which have the advantage of being very selective in frequency, and of making the circuit in which they are used insensitive to the instability of their central frequency.
• the tolerance to errors of the central frequency due to manufacturing can be increased, the architecture automatically adapting to the filter whatever the error on its central frequency in the range of controlled frequencies.

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Superheterodyne Receivers (AREA)
• Control Of Motors That Do Not Use Commutators (AREA)

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• 2002
  • 2002-03-01 FR FR0202658A patent/FR2836765B1/fr not_active Expired - Fee Related
• 2003
  • 2003-02-21 WO PCT/FR2003/000586 patent/WO2003075474A1/fr active Application Filing
  • 2003-02-21 EP EP03720635A patent/EP1481484A1/de not_active Withdrawn
  • 2003-02-21 US US10/506,163 patent/US20060166639A1/en not_active Abandoned
  • 2003-02-21 JP JP2003573795A patent/JP2005519517A/ja active Pending
  • 2003-02-21 AU AU2003224213A patent/AU2003224213A1/en not_active Abandoned

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