Patent Publication Number: US-10763837-B2

Title: Reference oscillator with variable duty cycle, frequency synthesizer and signal receiver with reference oscillator

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to European Patent Application No. 18174100.0 filed on May 24, 2018, the entire disclosure of which is hereby incorporated herein by reference. 
     TECHNICAL FIELD OF THE INVENTION 
     The invention relates to the field of reference oscillators and, more precisely in this field, reference oscillators comprising a means for suppressing or limiting at least one higher harmonic. The invention finds applications, for example, in the fields of receiving radio frequency signals, analogue-digital conversion, and frequency synthesis. 
     PRIOR ART 
     For transmitting electromagnetic signals, in particular radio frequency signals, appropriate receivers are needed. A conventional receiver is illustrated in  FIG. 1 . The signal receiver  1  comprises an antenna  12  for receiving electromagnetic signals. The antenna  12  is connected to a low noise amplifier  14  (LNA) suitable for amplifying the signals received by the antenna  12 . The LNA is further connected to a mixer  16  for mixing the amplified and received signals with an oscillating signal  80  supplied by a frequency matching circuit  36 , which is generally based on a reference oscillator  20 . The output of the mixer  16  is connected to a band-pass (or low-pass) filter  18  for filtering the intermediate signals  40  of the mixer  16 . In  FIG. 1 , the spectrum of the intermediate signals  40  is represented by the amplitude or the power (p) over the frequency (f). 
     Typically, the reference oscillator  20  uses a signal generator  24  with a reference resonator  22  such as a quartz resonator, supplying a well-defined and stable reference signal. For example, in typical applications of quartz resonators, the signal generator  24  with the reference resonator  22  operate at a reference frequency of 26 MHz. Generally, the reference oscillator  20  supplies a reference signal which is used as the basis for the frequency matching circuit  36 . 
     In the case of a radio frequency transmission, the signal receiver  1  may, for example, operate in a Bluetooth range, therefore in the 2.4 GHz band by using a signal generator  24  with a quartz reference resonator  22  operating at 26 MHz. Through radiation and parasitic coupling, the 93rd, 94th and 95th higher harmonics of the frequency of the oscillator  20  based on the reference resonator  22  may then coincide with the frequency of the useful RF signals captured by the antenna  12 , and end up converted in the band  41  of the band-pass (or low-pass) filter  18 . 
     In general, the particular waveform of the reference signal introduces by parasitic coupling of the higher harmonics of the signal  50  on the input  12  (dashed arrow in  FIG. 1 ) in the intermediate signals of the unwanted and polluting aliasing of the reference signal (present in the frequency band  41  of the filter  18 ). These harmonics have the effect, locally on their specific frequency, of a reduction in the sensitivity of the receiver. The exact frequency of these harmonics, and their amplitude depends directly on the duty cycle of the oscillating signal which, if it is not controlled, depends statistically on the conditions of production (manufacturing method) and use (temperature, voltage) of the integrated circuit comprising the oscillator, as well as typical variations of the reference resonator and its connections (PCB or card on which the integrated circuit comprising the resonator is formed, packaging, etc.). 
     The intermediate signals received by the filter may therefore be severely disrupted by the high frequency harmonic components of the signal from the reference resonator (injected by parasitic coupling on the desired RF signal). This problem becomes even more significant when the signal receiver is implemented in a single integrated circuit. This problem is even more significant in digital circuits, because of the very steep transitions of the reference (or clock) signal and the synchronous consumption peaks of the logic circuits. 
     Patent application EP 2 869 483 A1 describes this problem in detail, and provides for removing at least one of these harmonics by adjusting the duty cycle of the oscillator signal. The adjustment is performed step-by-step, by incremental changes in the duty cycle adjusted according to a measurement of the effect of a previous variation, until a minimum disruption is obtained. 
     This solution is interesting insofar as it greatly minimizes and even suppresses the harmonics polluting the output signal  40  of the receiver. However, this solution requires a setting up time at each activation. This activation time may be long, or even too long for some applications. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to supply an alternative solution to the solution provided in patent application EP 2 869 483 A1, which does not require any setting up time at the startup. 
     To this end, the invention provides a new reference oscillator for a radio frequency signal transmitter and/or receiver. The reference oscillator is suitable for generating a modified reference signal alternating ON times and OFF times with a predefined duty cycle from a signal supplied by a reference resonator. The invention is characterized in that the reference oscillator comprises an adjustment circuit suitable for adjusting the duty cycle of the reference signal modified according to at least one predefined adjustment parameter so as to minimize at least one harmonic component of the modified reference signal. 
     Thus, by appropriately and deterministically adjusting the duty cycle of the modified reference signal produced by the reference oscillator according to the invention, it is possible to minimize the pollution generated by certain harmonics of the modified reference signal, e.g. around a particular frequency. In addition, as the duty cycle is adjusted by predetermined parameters, notably according to the rank of the harmonic or harmonics to be minimized, and not by successive adjustments, the duty cycle is adjusted as soon as the oscillator is activated. 
     According to one embodiment, the oscillator may also comprise a reference generator coupled to the resonator for producing an initial reference signal, the adjustment circuit being coupled to the reference generator for generating the modified reference signal with a predefined duty cycle from the initial reference signal. 
     The adjustment circuit of the oscillator may comprise a circuit for synchronizing the modified reference signal on one flank of the initial reference signal. This makes it possible to overcome a possible phase drift which could appear in the adjustment circuit. 
     According to one implementation, the receiver comprises a Man/Machine interface for enabling a user to supply the adjustment circuit with at least one adjustment parameter of the duty cycle. The adjustment of the duty cycle is thus immediate, as soon as the reference oscillator is activated, and the duty cycle may be adjusted in real time in case of a change in the frequency around which the harmonics have to be minimized. 
     The invention also provides a frequency synthesizer comprising a reference oscillator, as described above and a frequency matching circuit for producing a high frequency oscillating signal from the modified reference signal. By adjusting the duty cycle of the reference oscillator adjustment circuit, the high frequency oscillating signal offers the advantage of being rid of its harmonics around a particular frequency, as soon as the reference oscillator is activated. 
     According to one embodiment, the frequency matching circuit comprises a PLL loop arranged for receiving the modified reference signal of frequency f ref  and producing an oscillating signal of higher frequency x·f ref . 
     The invention further provides a signal receiver, comprising: 
     an antenna for receiving electromagnetic signals, 
     at least one low noise amplifier for amplifying the signals received by the antenna, 
     at least one reference oscillator, as described above or at least one frequency synthesizer also described above, arranged for producing a modified reference signal or a high frequency oscillating signal with a predefined duty cycle, 
     a mixer for mixing the amplified and received signals with the modified reference signal or with the high frequency oscillating signal for generating intermediate signals, 
     a filter for filtering the intermediate signals, the reference oscillator comprising an adjustment circuit suitable for adjusting the duty cycle according to a frequency of the electromagnetic signals received for minimizing an amplitude of at least one higher harmonic component of the modified reference signal and present in the filtered intermediate signals. 
     Thus, in the receiver according to the invention, the duty cycle of the modified reference signal is predefined according to a frequency of the signals received on the antenna, for minimizing the influence of any possible harmonics present around the frequency of the signals received on the antenna. And the optimum duty cycle is set as soon as the receiver is activated, and no setting up time is necessary at each activation of the receiver for having an optimized reception. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention will be described below in more detail with the aid of the appended drawings, given by way of non-restrictive examples, in which: 
         FIG. 1  shows a diagram of a signal receiver according to the prior art, 
         FIG. 2  shows a diagram of a receiver according to the invention, 
         FIG. 3  shows a diagram of a reference oscillator according to the invention producing a modified reference signal with a controlled duty cycle, and 
         FIG. 4  shows a diagram of a frequency synthesizer according to the invention producing a high frequency signal with a controlled duty cycle. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As stated earlier, the invention provides a new reference oscillator  20 . The invention also provides a frequency synthesizer  70  and a signal receiver comprising a reference oscillator  20 . 
       FIG. 3  details a reference oscillator  20  according to the invention, e.g. a MEMS or quartz crystal oscillator. It comprises a reference generator  24  locked onto a reference resonator  22 . The resonator  22  may be a MEMS resonator, a quartz crystal resonator, or a similar resonator. Examples of implementation of the resonator  22  and the reference generator  24  are also described in patent application EP 2 869 483 A1. The signal generator  24  supplies an initial reference signal  26  of constant and stable reference frequency f ref , typically in the range of several MHz, e.g. 26 MHz. 
     In a reference oscillator  20  according to the invention, as represented in  FIG. 3 , the generator  24  is connected to an adjustment circuit  30  of the duty cycle. The circuit  30  generates, on the basis of the initial reference signal  26  from the signal generator  24 , a modified reference signal  50  having, for example, a rectangular waveform having an alternating sequence of ON times  51  and OFF times  52 . The duty cycle r defines the relationship between the duration of the ON times  51  and the duration of the OFF times  52  of the modified reference signal  50 . 
     In the oscillator  20  according to the invention, the adjustment circuit  30  of the duty cycle is arranged for supplying a modified reference signal having a predefined duty cycle r according to at least one adjustment parameter so as to minimize at least one harmonic component of the modified reference signal  50 . In the example in  FIG. 2 , the reference oscillator  20  is used for producing a signal receiver and at least one adjustment parameter is selected according to a frequency of the electromagnetic signals received on the antenna or amplified, as will be seen better later. 
     For a rectangular signal, the amplitude of a rank harmonic h is proportional to [sin(pi·h·r)]/(pi·h·r), where pi is Archimedes&#39; constant, r is the duty cycle of the rectangular signal, and h is the rank of the harmonic considered. Setting the duty cycle r thus makes it possible to minimize the amplitude of at least one higher harmonic component of the modified reference signal  50  injected parasitically into the spectrum of the desired RF signal and therefore to minimize the harmonic pollution in the vicinity of the received frequency. As shown by the dashed arrow in  FIG. 2 , the modified reference signal  50  introduces higher harmonics of the signal  50  on the input  12  via parasitic coupling. 
     According to one embodiment, the reference oscillator  20  comprises a Man/Machine interface MMI (not represented) for enabling a user to supply the adjustment circuit with at least one adjustment parameter of the duty cycle. According to an actual implementation, the MMI may comprise a selection button enabling the user to select a frequency of the signals to be received, and a memory, storing a table comprising, for each frequency of signals liable to be received, the parameter or parameters necessary for adjusting the duty cycle. 
     According to the embodiment in  FIG. 3 , the adjustment circuit  30  comprises a circuit for generating the modified reference signal, a circuit comprising a phase-locked loop or PLL  31 , a counter  32 , a comparator  33  and a pulse generator  34 . The PLL  31  is connected to the reference generator  24 , and produces a signal of frequency n times greater than the frequency f ref  of the reference signal (f=n·f ref ). The counter  32  is connected to the PLL and counts the pulses of the signal produced by the PLL. The counter is chosen for being capable of counting beyond the number n. The comparator  33  is connected to an output of the counter  32  and compares a number of pulses counted to a predefined value m. The comparator  33  produces an ON signal if the number of pulses counted is less than the predefined value m or an OFF signal otherwise. Finally, the pulse generator  34  is connected to the reference generator  24 . It produces an initializing signal of the same frequency f ref  as the initial reference signal, the initializing signal here is a pulse signal, comprising a sequence of pulses, one pulse being produced, for example, at each rising edge of the reference signal  26 . The initializing signal is applied to an initializing input of the counter for resetting the counter to zero. The adjustment parameters m, n or m/n are supplied according to the application envisaged, e.g. via the MMI interface. 
     The modified reference signal obtained at the output of the comparator is a rectangular signal, of frequency f ref  and which, over a period T, is equal to ON during a first time T 1 =m/f ref  and equal to OFF otherwise. The duty cycle of the modified reference signal obtained in this embodiment is thus equal to r=m/n=T 1 /T and is imposed by the parameters m and n. According to the invention, the parameters m and n are predefined as explained above. 
     According to the embodiment in  FIG. 3 , the adjustment circuit also comprises a circuit  35  for synchronizing the modified reference signal on one flank of the initial reference signal  26 . The synchronization circuit makes it possible to overcome a possible phase drift of the adjustment circuit  30 . It thus makes it possible, for example, to use a PLL  31  having rather poor noise characteristics, but with the advantage of being of reduced power and size. 
     The synchronization circuit  35  is in the example of  FIG. 3  an RS flip-flop, a SET input of which is connected to the pulse generator  34  and a RESET input of which is connected at the output of the comparator  33 , the synchronized modified reference signal  50  being available on an output of the flip-flop  35  which may be connected to a frequency matching circuit  36  of an RF receiver. Whenever it receives the initializing signal on its SET input, the output of the RS flip-flop changes from OFF to ON. The output signal from the flip-flop is thus synchronized on one flank of the initial reference signal  26 . 
       FIG. 4  details one embodiment of a frequency synthesizer according to the invention comprising a reference oscillator  20  and a frequency matching circuit  36  for producing a high frequency oscillating signal OUT  80 . The reference oscillator  20  is, for example, identical to that in  FIG. 3 . The frequency matching circuit here is a PLL loop  36  comprising a voltage controlled oscillator VCO  64 , a divider circuit  65 , a phase detector  61 , a differential charge pump  62 , a low-pass filter  63 , and a parameterizing circuit  66 . 
     It should also be noted that the modified reference signal  50  may also be introduced by parasitic coupling of the higher harmonics of the input signal  50  at the input of the VCO  64 . 
     The oscillator  64  supplies the high frequency oscillating signal OUT in a determined frequency band. The divider circuit  65  divides the frequency of the signal OUT by a factor N in order to supply a divided frequency signal f div . The circuit  66  adjusts the factor N according to a desired frequency and the divided frequency f div . The phase detector  61  compares the modified reference signal  50  at a reference frequency f ref  with the divided frequency signal supplied by the divider circuit. The phase detector  61  supplies two control signals UP, DOWN according to the result of the comparison to the differential charge pump  62 . The charge pump  62  injects or withdraws the charges in the filter  63  according to the UP or DOWN signals. The low-pass filter  63  filters the output signal S 1  of the charge pump for supplying a filtered control signal S F  to the voltage-controlled oscillator  64 . 
     The spectrum of the high frequency oscillating signal  80  shows the frequency signal supplied by the oscillator  64 , a signal comprising a main component f out    81  and a harmonic pollution f p    82  generated by a harmonic of the initial reference signal  26 . When the main component  81  and harmonic pollution  82  are too close to each other, the oscillator  64  produces a series of additional harmonics of frequency f out ±k·(f out −f p ), a phenomenon known as “VCO pulling.” The adjustment circuit  30  advantageously makes it possible here to limit the harmonic  82  of the initial reference signal  26 , and thus limit the phenomenon of “VCO pulling”. 
     Finally,  FIG. 2  details a signal receiver  10  according to the invention. The receiver comprises an antenna  12 , a low noise amplifier LNA  14 , a mixer  16  and a band-pass or low-pass filter  18 . The antenna  12  is suitable for receiving electromagnetic signals. According to the invention, the receiver also comprises a reference frequency synthesizer comprising a reference oscillator  20  according to the invention and a frequency matching circuit  36 . The amplifier  14  is connected to the antenna  12  for amplifying the signals received by the antenna  12 . The reference oscillator  20  comprises a reference resonator  22  for generating a modified reference signal  50  alternating ON times and OFF times with a predefined duty cycle. The mixer  16  is connected to the amplifier  14  and to the frequency matching circuit  36  for mixing the amplified and received signals with the high frequency signal OUT  80  and for generating intermediate signals  40 . Finally, the band-pass (or low-pass) filter  18  is connected to the output of the mixer for filtering the intermediate signals  40 . Implementations of the antenna  12 , the amplifier  14 , the mixer  16  and the filter  18  are described in detail in patent application EP 2 869 483 A1, incorporated herein by reference. The reference oscillator  20  and the frequency matching circuit  36  may be produced in accordance with  FIGS. 3 and 4 . It should be noted that the frequency matching circuit  36  is useful here only if the signals received on the antenna have a frequency distant from the frequency of the reference signal. Otherwise, the receiver may be produced without a frequency matching circuit  36 . 
     From the description that has just been given, multiple variations of the reference oscillator, the frequency synthesizer and the signal receiver may be devised by the person skilled in the art without departing from the scope of the invention defined by the claims.