Abstract:
A frequency generator for providing one or more clock signals with reduced phase jitter can include a phase-locked loop (NI) configured to couple with a crystal and to provide a first clock signal, a multiplier circuit configured to receive the first clock signal and to provide a second dock signal, the second clock signal having a higher frequency than the first clock signal, wherein the multiplier circuit includes a second PLL, and wherein the second clock signal is an output frequency signal of the frequency generator.

Description:
RELATED APPLICATION INFORMATION 
       [0001]    This application is a continuation of and claims the benefit of priority to, Maerzinger et al., U.S. patent application Ser. No. 14/516,188, entitled, “TRANSCEIVER”, filed on Oct. 16, 2014, which is a continuation of U.S. patent application Ser. No. 11/685,288, filed on Mar. 13, 2007, now issued as U.S. Pat. No. 8,880,005, which claims priority to German Patent Application DE 10 2006 011 682.8-35, filed on Mar. 14, 2006, each of which is hereby incorporated by reference herein in their entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present subject matter relates to a frequency generator. In particular, the present subject matter relates to a frequency generator, for example, for use in a communication system. 
       BACKGROUND 
       [0003]    A transceiver is used to transmit information in a communication system, e.g. a mobile radio system or a wire based communication system. The transceiver architecture for implementing broadband transmission standards, such as UNITS (Universal Mobile Telecommunications System), requires the generation of suitable clock signals for use in the respective standard. In a common communication system the clock signal is too high to be generated directly using a crystal oscillator. Therefore, oscillator circuitry is provided to generate the clock signal. 
         [0004]    While transmission frequencies used in a communication system are becoming higher, the interference between the transmission signal and clock signals is getting more important. The interference may cause degradation in the quality of signal transmission. In consequence, increased demands are imposed on a spectral purity of the clock signals. In particular, the clock signal should have a minimum amount of phase noise (jitter). 
         [0005]    As soon as dock signals at a higher frequency are required, a DLL (Delay-Locked Loop) circuit may be used to multiply the frequency of the crystal oscillator. A DLL circuit is a control loop in which an oscillator is stabilized by means of a reference frequency signal. The spectral purity of the clock generated in this manner is sufficient to supply a clock signal to digital blocks such as DSP (Digital Signal Processing) blocks and digital/analogue converters (DACs), for example. However, the quality of a clock signal generated in a DLL is insufficient in terms of the spectral purity required. It may not be used as a reference signal for a so-called phase-locked loop (PLL) to generate the transmission signal. For this and other reasons, there is a need for the present invention. 
       OVERVIEW 
       [0006]    A frequency generator for providing one or more clock signals with reduced phase jitter can include a phase-locked loop (PLL) configured to couple with a crystal and to provide a first clock signal, a multiplier circuit configured to receive the first clock signal and to provide a second clock signal, the second clock signal having a higher frequency than the first clock signal, wherein the multiplier circuit includes a second PLL, and wherein the second clock signal is an output frequency signal of the frequency generator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification. The drawings illustrate the embodiments of the invention and together with the description serve to explain the principles of the invention. Other embodiments of the invention and many of the intended advantages of the invention will be readily appreciated as they become better understood by reference to the following description. Like references numerals designate corresponding similar parts. 
           [0008]      FIG. 1  illustrates a block diagram of a polar modulator of a mobile radio; 
           [0009]      FIG. 2  illustrates a block diagram of a phase-locked loop with an external crystal oscillator for generating a reference; and 
           [0010]      FIG. 3  illustrates an embodiment of a transceiver according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]      FIG. 1  illustrates the block diagram of a polar modulator of a mobile radio. The polar modulator comprises blocks, which impose increased demands on clock signals used in the system. A digital signal processor DSP, to which input signals I and Q are applied, requires a high operating frequency DSPCLK. In this case, a signal I indicates the real part of a complex baseband signal. A second signal Q represents the imaginary part of said baseband signal. 
         [0012]    A digital/analogue converter DAC requires both a high operating frequency and a high spectral purity of the applied clock signals DAC CLK . A phase-locked loop PLL, which is used as a phase modulator also requires a high operating frequency with a high spectral purity of the clock signals PLL REF . 
         [0013]    An output signal from the digital signal processor DSP makes use of a polar representation having an amplitude component A and a phase component F. The amplitude signal A is injected into a digital/analogue converter DAC and the phase signal F is injected into a phase-locked loop PLL. 
         [0014]    For use as a polar modulator, the amplitude signal A is applied to the corresponding amplitude signal input A of an amplitude modulator AM via the digital/analogue converter DAC and a filter Fat, while the output signal from the phase-locked loop is injected into the phase signal input F of the amplitude modulator AM. The amplitude modulator generates a radio-frequency signal RF out  and provides it to its output. 
         [0015]      FIG. 2  shows a phased-locked loop in which the reference frequency PLLREF is fed into a phase detector PD from an external crystal oscillator XO. An output signal of the phase detector PD is applied to a voltage-controlled oscillator (VCO) via a loop filter LF. The VCO generates an output signal PLL out . The output signal PLL out  is fed back into the phase detector PD via a divider element DIV. Furthermore the output signal PLL put  is provided at an output of the phase-locked-loop for further data processing. 
         [0016]    The reference signal PLL REF  may be simultaneously fed into a digital signal processor DSP or a digital/analogue converter DAC as a clock signal. 
         [0017]      FIG. 3  shows the block diagram of a cascaded phased-locked loop circuit. An external crystal oscillator XO generates a reference frequency signal RX REF  which is injected into a phase detector PD 1  of the first phase-locked loop PLL 1 . The signal is injected into a voltage-controlled oscillator VCO 1  via a loop filter LF 1 . The output signal RX OUT  from said oscillator is then used, on the one band, as a clock signal for the reception path of the mobile radio circuit. In radio-frequency technology, the signal RX OUT  is also referred to as a local oscillator signal. The signal RX OUT  is fed back to the phase detector PD 1  again via a divider element RX DIV . 
         [0018]    The output signal RX OUT  from the reception phase-locked loop PLL 1  is injected into a second phase-locked loop The injection may be effected directly or indirectly, e.g. via a frequency divider element RDIV as shown in  FIG. 3 . The frequency divider element RDIV provides a reference frequency signal TX REF  derived from the output signal RX OUT . The reference frequency signal TX REF  received by the second phase-locked loop PLL 2  is injected into a second phase detector PD 2 . The signal TX REF  may be used as a clock signal for the digital signal processor DSP or for the digital/analogue converter DAC. Furthermore, the signal TX REF  is injected via a loop filter LF 2 , into a voltage-controlled oscillator VCO 2 . The voltage-controlled oscillator VCO 2  generates an output signal TX OUT  from this second phase-locked loop PLL 2 . The output signal TX OUT  fed back to the second phase detector PD 2  via a divider element TX DIV . Furthermore, output signal TX OUT  is provided at an output of the second phase-locked loop PLL 2  for further signal processing. 
         [0019]    The respective control loops of the two phase-locked loops PLL 1  and PLL 2 , which are connected in series in cascaded fashion, are each used to ensure that the respective frequencies generated are kept stable. This results in the clock signal jitter being minimized. 
         [0020]    A concept of the embodiment shown in  FIG. 3  is that the first frequency signal generator is used to generate a spectrally pure reference frequency signal for the second frequency signal generator. A greater demand is imposed on the spectral purity and bandwidth of the second frequency signal generator, which provides a dock signal for a transmission path of the transceiver circuit arrangement, than on the first frequency signal generator which provides a clock signal for a reception path of the transceiver circuit arrangement. These demands can be resolved by using a correspondingly high and stabilized reference frequency signal. This may be effected by virtue of the reception frequency signal being used as a reference frequency signal for the second frequency signal generator. 
         [0021]    In other words, the transceiver comprises a cascaded frequency signal generator circuit, which provides a spectrally pure Clock signal at frequencies higher than a frequency of an external crystal oscillator with a sufficiently high spectral purity. 
         [0022]    The second frequency signal generator is a phase-locked loop. The second frequency signal generator thus generates the transmission frequency signal by comparing the latter or a multiple or divided value of the latter with the reference frequency signal. The phase-locked loop can generate a stable output signal, that is to say a stable transmission frequency signal, at different frequencies. 
         [0023]    The first frequency signal generator may be a phase-locked loop. The first frequency signal generator can advantageously generate a stable output signal, that is to say a stable reception frequency signal, at different frequencies. 
         [0024]    The transceiver includes a crystal oscillator, which is coupled to the first frequency signal generator in order to supply a frequency signal to the latter as a first reference frequency signal. The first frequency signal generator thus has a very stable first reference frequency signal as a reference for generating the reception frequency signal. 
         [0025]    An output of the first frequency signal generator connects to an input of the second frequency signal generator via a frequency divider in order to inject the reception frequency signal into the second frequency signal generator as a reference frequency signal. The reference frequency signal for the second frequency signal generator may thus be supplied to the second frequency signal generator at different frequencies and in particular, at a frequency that differs from the frequency of the reception frequency 
         [0026]    The frequency divider is arranged, so that the reference frequency signal is divided down to a frequency depending on the use of the output signals, e.g. between 100 MHz and 300 MHz. The transceiver is thus operable in mobile radio systems, for example UNITS or in a wireline communication system. 
         [0027]    The reception frequency signal is simultaneously a clock signal for a reception path of the transceiver circuit arrangement. The transmission frequency signal is a second clock signal in a transmission path of the transceiver circuit arrangement. 
         [0028]    For use of the cascaded phased-locked loop circuit shown in  FIG. 3  in a transceiver, or in more general terms in a Modulator/Demodulator circuitry, the output of the first phase-locked loop PLL 1  couples to a demodulator in a receiption. path of the transceiver. The second phase-locked loop PLL 2  couples to a modulator in a transmit path of the transceiver.