FREQUENCY SYNTHESIZER

Abstract: Frequency synthesizers are used for down conversion of RF signals in a lot of applications such as digital and analog radio and television receivers, car radios id. By combining the voltage-controlled oscillator of the phase locked loop demodulator with the voltage controlled oscillator of the frequency synthesizer a major cost reduction can be achieved but measures have to be made to prevent that the two PLLs try to lock the same VCO to different frequencies. The invention provides a frequency synthesizer (FS3) whereby switching means (33) between the phase frequency comparator (31) and the charging means (35) are controlled by using a frequency window detector (371, 373, 375). The phase frequency detector (31) is disconnected from said charging means (35) when it is detected that the frequency difference between the frequency synthesizer input signals is lower than a predetermined value, whereafter said voltage-controlled oscillator (VC03) is only driven by an AFC signal.

Description: Frequency synthesizer. The invention relates to a frequency synthesizer as defined in the preamble of claim 1. The invention further relates to a receiver comprising such a frequency synthesizer. Such frequency synthesizers are known and can be used for down conversion of RF signals in a digital or analog satellite receiver, car radios, digital or analog (cable) TV receivers, cordless or wireless telephones id. By combining the voltage-controlled oscillator of the phase locked loop demodulator with the voltage controlled oscillator of the frequency synthesizer a major cost reduction can be achieved in systems wherein direct demodulation of FM signals is employed. Such a receiver is for example known from US-A-5, 446,411, wherein a phase locked loop for use as a FM demodulator. The frequency synthesizer is used here to set up the phase locked loop demodulator to run at a predetermined frequency. A standard PLL frequency synthesizer is not suitable in systems were the tuning voltage controlled oscillator is used for demodulation. The reason is that two PLLs, the frequency synthesizer and the demodulation loop (FM) or the synchronization loop (AM), will try to lock the same VCO to different frequencies (the former to the multiplied crystal, and the latter to the carrier frequency), what leads to a non functional system. To overcome the disadvantages mentioned above the output of the frequency synthesizer is coupled via switching means and a resistive divider to the input of the voltage controlled oscillator. Disadvantages of this frequency synthesizer is that the switching means at the output of the frequency synthesizer can cause transients, loss of lock and spikes during switching. Further causes such a resistive divider has thermal noise. Power dissipation is high due to the presence of a frequency measurement device operating at the highest frequency. An object of the invention is to overcome the disadvantages of the prior art and further to provide a frequency synthesizer with lower costs, lower dissipation, lower noise and with an improved performance and wider application range. To this end a first aspect of the invention provides a frequency synthesizer as defined in claim 1. A second aspect of the invention provides a receiver as defined in claim 5. The invention is based on the recognition that by using a frequency window detector and switching means between the comparator and the charging means, the frequency synthesizer can be turned of. There are two ways to implement this. Firstly its effect is zero, but the circuitry stays active (watch dog function), and secondly completely "turn off (low power dissipation). In this way the influence of the frequency synthesizer during normal operation, that is within the frequency window, is reduced to (nearly) zero. Because the charging means are turned of before the charging means and at the moment where it is inactive no transients or spikes can occur at the output of the frequency synthesizer. A further advantage of the frequency synthesizer according to the invention is that the accuracy of the frequency of the output signal is not dependent on the accuracy of the reference frequency. This receiver structure with the combined tuning system enables the use of cheap crystal oscillators because when the voltage controlled oscillator is "in-window" the frequency synthesizer is disabled. Therefore the accuracy of the VCO frequency is not depending on the accuracy of the crystal frequency but on the AFC signal. Another advantage is that the AFC has taken over control of the VCO, saving a substantial amount of power dissipation when switching off the power of the frequency synthesizer. It is to be noticed here that from US-A-4,787,097 a phase-locked loop having a phase detector and a frequency detector, with associated monitor and recovery circuitry is known for data and clock extraction from N Z (Non Return to Zero) data streams. After detecting that the phase locked loop is outside a narrow frequency window the phase detector is turned off and the frequency detector is turned on. After determining that the phase locked loop is (again) within the narrow frequency window the phase detector is turned on and the frequency detector is turned off. Further this phase locked loop comprises an EXOR (exclusive OR) and analog elements to obtain a first input signal for the phase frequency comparator. The frequency synthesizer of the invention contains no analog elements and is therefore robust with relation to aging, spread in component values, id. An embodiment of a frequency synthesizer according to the invention comprises the features of claim 2. The division values of the dividers can be chosen depending on the input signals and/or on the crystal oscillator used. Another embodiment of a frequency synthesizer according to the invention comprises the features of claim 3. In this way a very efficient way to obtain the switching signal for the switching means is obtained. A further embodiment of a frequency synthesizer according to the invention comprises the features of claim 4. To further improve the switching signal the logic circuit of the frequency window detector comprises three D Flip-FIop to overcome possible unwanted (extra) switching signals. The invention and additional features, which may optionally be used to implement the invention to advantage, will be apparent from and elucidated with reference to the examples described below hereinafter and shown in the figures. Herein shows: Figure 1 a block schematic example of a frequency synthesizer according to the invention, Figure 2 a block schematic example of a frequency synthesizer in more detail according to the invention, Figure 3 a block schematic example of a receiver comprising a frequency synthesizer according to the invention, Figure 4 a block schematic example of a receiver comprising a frequency synthesizer according to the invention, Figure 5 a block schematic example of a receiver comprising a frequency synthesizer according to the invention, and Figure 6 a block schematic example of a frequency window detector according to the invention. Throughout the description corresponding elements will have corresponding reference numerals. Figure 1 shows an example of a frequency synthesizer FS according to the invention, comprising a first input II and a second input 12 for receiving a first frequency input signal si and a second frequency input signal s2 respectively. The inputs II and 12 are coupled to a phase frequency comparator 1 for comparing the two frequency input signals. Depending on the frequency difference between the two frequency input signals si and s2 the phase frequency comparator supplies in operation a signal sc. The output of the phase frequency comparator 1 is coupled via switching means 3 to charging means 5. The charging means supply a charging signal for example via a loop filter to control a voltage controlled oscillator (not shown). The two inputs II and 12 are also coupled to a frequency window detector 7 for detecting whether the two frequency input signals si and s2 are within a predetermined window or not. Depending on the frequency difference between the two frequency input signals the frequency window detector 7 supplies a control signal nwl to the switching means for opening or closing respectively the signal path between the phase frequency comparator 3 and the charging means 5. When the two frequency input signals are within a predetermined frequency window the signal path will be opened resulting in no output signal at the output O. Figure 2 shows an example of a frequency synthesizer FS2 according to the invention in more detail. At the input 121 the frequency synthesizer receives the frequency input signal s21, being in this example the VCO frequency signal. After dividing this input signal in a divider D21 by a predetermined factor N the output signal of the divider fa is supplied to the phase frequency comparator 21. At the input 122 the frequency synthesizer receives a frequency input signal s22 from a crystal oscillator Xtal. After dividing this input signal in a divider D22 by a predetermined factor M, the output signal fr of the divider is supplied to the other input of the phase frequency comparator 21. The outputs of the dividers D21 and D22 respectively are also coupled to the frequency window detector 27. The frequency window detector comprises in this example a D Flip Flop 271, a programmable divider 273 and a phase frequency detector 275.The programmable divider 273 receives a signal ws depending on the required window size. The D Flip Flop 271 supplies a frequency signal fd which is equal to |fa-fr|. The divider 273 supplies a signal fwb whose frequency defines the size of the frequency window. These two signals are supplied to the phase frequency detector 275 for supplying an enable-signal nw to the switching means 23 when the frequency of the signal fd is larger than the frequency window. In this example the switching means are implemented as two AND-stages. At the other inputs receive the AND-gates respectively a signal UP or a signal DN from the phase frequency comparator 21 depending on the frequency or phase difference between the signals fa and fr. The outputs of the AND gates are coupled each to a current source being part of the charging means 25 supplying a positive or negative current signal lp2 respectively at the output 02 of the frequency synthesizer FS2. Figure 3 shows an example of a receiver R3 having a frequency synthesizer FS3 according to the invention. At a receiver input RI3 the receiver receives a RF signal Rfin. This signal is supplied to an input amplifier IA3, which amplifier also receives an automatic gain control signal AGC. The output signal of the input amplifier is supplied to a mixer M3. At the other input the mixer receives a signal from a voltage-controlled oscillator VCO3. The output of the mixer is supplied via a bandpath filter BPF3, a further amplifier A3, a frequency demodulator FD3 and an output amplifier OA3 to supply a baseband output signal for further processing. This is well known in the art, and needs no further explanation. The input signal of the output amplifier OA3 is also supplied via a low pass filter LPF3 as an automatic frequency control signal AFC3 to an input of a summing device SUM3. At the other input receives the summing device via a loop filter LF3 the output signal lp3 from the frequency synthesizer FS3. The frequency synthesizer FS3 has in this example the same structure as in figure 2; all elements have corresponding reference numerals. This receiver structure with the combined tuning system enables the use of cheap crystal oscillators because when the voltage controlled oscillator is "in-window" the frequency synthesizer is disabled. Therefore the accuracy of the VCO frequency is not depending on the accuracy of the crystal frequency but on the AFC signal. Another advantage is that the AFC has taken over control of the VCO, saving a substantial amount of power dissipation if the frequency synthesizer is turned off. Figure 4 shows an example of a digital satellite receiver R4. At the input RI4 the receiver receives the RF signal RFin. This signal is supplied to an input amplifier IA4, which amplifier is controlled with an automatic gain control signal AGC. The output signal of the input amplifier is supplied to a first mixer M41 and to a second mixer M42. The first mixer receives an I -signal at the other input, and supplies via an amplifier A41, and a low pass filter F41 a baseband I-signal bbl. The second mixer receives at its second input a Q-signal and supplies via an amplifier A42 and a low pass filter F42 a baseband Q-signal bbQ. This is generally known in the art and needs no further explanation. The baseband I-signal and the baseband Q-signal are also supplied to a frequency detector FD4. The output of the frequency detector is coupled to a low pass filter LPF41, supplying an analog automatic frequency control signal AFC. This automatic frequency control signal is supplied via a low pass filter LPF42 to a low noise voltage- controlled oscillator VCO41. The output of the voltage-controlled oscillator is supplied to the input 141 of the frequency synthesizer FS4. At the input 142 receives the frequency synthesizer the frequency signal from the crystal oscillator Xtal4 via a programmable divider PD41. The output 04 of the frequency synthesizer is coupled to the input of the low pass filter LPF42. A voltage controlled oscillator VCO42 supplies the I-signal and the Q-signal to the first mixer M41 and the second mixer M42 respectively. The Q-signal is also supplied to a programmable divider PD42. The output of the programmable divider PD42 is supplied to a phase frequency detector PFD4. At the other input receives the phase frequency detector the output signal of the voltage-controlled oscillator VCO41. The output of the phase frequency detector is supplied via a low pass filter LPF43 to the input of the voltage-controlled oscillator VCO42. In this way a wide band loop WBL is created, reduction of phase noise of the integrated quadrature oscillator VCO42. The use of the AFC function enables frequency drifts of the LNB (not shown) to be compensated in a smooth and continuous way avoiding cycle slips. In the standard way the division ratio of the divider D41 has to be switched (discontinuous), The frequency synthesizer FS4 has in this example the same structure as in figure 2 and needs here no further explanation. Figure 5 shows an example of a receiver R5 comprising a demodulator phase locked loop DPLL5 and a frequency synthesizer FS5 for use in a direct conversion analog satellite receiver. In this figure is shown an example of a direct conversion analog satellite receiver. At the input RI5 receives the receiver the RF input signal RFin. The input RI5 is coupled to the input amplifier IA5, which amplifier is controlled by an automatic gain control AGC. The output of the input amplifier is coupled to a mixer M5, for mixing this signal with a signal from a voltage-controlled oscillator VCO5. The output of the mixer is coupled via an output amplifier OA5, also controlled by automatic gain control AGC, to charging means CP5. The charging means CP5 are coupled to an input of summing means SUM5 for supplying a current Icp5. At the other input receives the summing means the output signal from the frequency synthesizer FS5. The output of the summing means SUM5 is coupled via a loop filter LF5 to the input of the voltage controlled oscillator VCO5. The frequency synthesizer corresponds with earlier described examples. Because of the switching means of the frequency synthesizer according to the invention a direct conversion is possible. Figure 6 shows an example of a frequency window detector 67, wherein the D Flip Flop 571 (see figure 5) has been replaced by a logic circuit 671 comprising three D Flip Flops DFF61, DFF62, and DFF63, a multiplexer MUX6, an OR-gate OR6 and an EXOR-gate EXOR6. The signal fa from the divider D61 is supplied to a first input of D Flip-FIop DFF61, and the signal from the divider D62 is supplied to the other input of DFF61. In figure 5 the output of this D Flip-FIop (571, in figure 5) is supplied to the phase frequency detector 675 (575 in figure 5). Here the output signal of D Flip-FIop DFF61 is supplied to the multiplexer MUX6. The output signal of the multiplexer is supplied to the D Flip-FIop DFF63, which receives at the other input the signal fr from the divider D62. The output signal of D Flip Flop DFF63 is supplied to the phase frequency detector 675 and also coupled back to the other input of the multiplexer MUX6. The multiplexer has further a select input, which receives a signal from the EXOR-gate EXOR6. The clock signal fx from the crystal oscillator Xtalό is supplied to an input of the OR-gate OR6. At the other input receives the OR-gate the signal fr from the divider D62. The output of the OR-gate is supplied to the D Flip-FIop DFF62. At the D-input this D Flip- FIop receives the signal fa from the divider D61. The output signal of the D Flip-FIop DFF62 is supplied to the other input of the EXOR-gate EXOR6. By replacing the D Flip-FIop (571, figure 5) by the logic circuit 671, the detection of 'out of window' is further improved. To detect (extra) transitions of the signal fa which lie within a given distance from the sampling moment which could cause 'wrong' decisions these transitions are then disregarded. The safety window is achieved by use of the D Flip-FIop DFF62, that is sampling the signal fa with a higher frequency fx. The OR-gate OR6 let D Flip-FIop DFF62 be clocked by signal fx when signal fr is low. The rising edge of the signal fr not only clocks the D Flip Flop DFF61, sampling fa, but also freezes the state of the D Flip Flop DFF62. To assess a 'dangerous' transition in the signal fa the output of the D Flip-Flops DFF61 and DFF62 are combined in the EXOR-gate EXOR6. If the state of the two D Flip Flops DFF61 and DFF62 is not the same, it means that a transition in the signal fa happened in between the sampling moments of the D Flip-Flops DFF61 and DFF62. This makes the output of the EXOR-gate EXOR6 a high signal, which in turn forces the multiplexer MUX6 to redirect the output of the D Flip Flop DFF63 to its input, in this way disregarding the transition in the signal fa. For example when the output of the EXOR-gate is low will the signal at input b supplied to the output and when the output of the EXOR-gate is high the signal at the input a is supplied to the output. The D Flip-FIop DFF63 is then clocked on the falling edge of the signal fr. For proper operation it is preferred that the signal fx has to have its rising edge before the rising edge of the signal fr. It is to be noticed that above the invention has been described on the basis of some examples. The man skilled in the art will be well aware of a lot of variations, which fall within the scope of the present invention. For example can the frequency window detector be amended as will be known to the man skilled in the art using the same idea to create the required window. Further can the frequency synthesizer according to the invention be used for example in all kind of receivers, pagers, and mobile phones.

Claims: CLAIMS: 1. Frequency synthesizer comprising a first input for receiving a first frequency signal and a second input for receiving a second frequency signal, a comparator coupled to the first and second input for comparing the first and second signal, which comparator is coupled to charging means, which charging means are coupled to an output of the frequency synthesizer for supplying an output signal, characterized in that the inputs of the frequency synthesizer are also coupled to a frequency window detector, which detector supplies a signal depending on whether or not the signals are within a predetermined frequency window, and the frequency synthesizer comprises switching means between the comparator and the charging means which switching means are controlled by the output signal of the frequency window detector. 2. Frequency synthesizer according to claim 1, characterized in that the frequency synthesizer comprises a first divider for dividing the first frequency input signal by a first predetermined value and a second divider for dividing the second frequency input signal by a second predetermined value. 3. Frequency synthesizer according to claim 1, characterized in that the frequency window detector comprises logic circuit coupled with inputs with the first and the second input of the frequency synthesizer, and with an output to a phase frequency detector for supplying a frequency difference signal, whereby the frequency window detector further comprises a programmable divider coupled with an input with the second input of the frequency synthesizer and with an output with a second input of the phase frequency detector which phase frequency detector supplies an output signal depending on the frequency difference between the two input signals as control signal to the switching means. 4. Frequency synthesizer according to claim 3, characterized in that the logic circuit comprises a first D Flip Flop coupled with inputs to the inputs of the frequency synthesizer, with an output to an input of a multiplexer, the first input of the frequency synthesizer is also coupled to a second D Flip Flop, which D Flip Flop is with the other input coupled to an output of an OR gate, which OR gate is coupled with an input to the second input of the frequency synthesizer, and with the other input to a clock signal, the output of the second D Flip Flop is coupled to a first input of an EXOR gate, and the second input of the EXOR gate is coupled to the output of the first D Flip Flop, and the output of the EXOR gate is coupled to the multiplexer, which multiplexer is coupled with an output to a third D Flip Flop, and the output of the third D Flip Flop is coupled to the phase frequency detector. 5. Receiver for receiving a RF signal, comprising a mixer for mixing the RF input signal to baseband signals, a voltage controlled oscillator coupled with an output to the mixer, characterized in that receiver comprises a frequency synthesizer according to claim 1 coupled to the voltage controlled oscillator. 6. Receiver for receiving a RF signal, a first and a second mixer for mixing the input signal to a baseband I signal and a baseband Q signal respectively, a wide band loop comprising a first voltage controlled oscillator for supplying a I respectively Q signal to the first and second mixer, characterized in that the receiver comprises a frequency synthesizer according to claim 1 coupled to a second voltage controlled oscillator.

Inventors: KASPERKOVITZ WOLFDIETRICH G, VAUCHER CICERO S, KASPERKOVITZ, WOLFDIETRICH, G., VAUCHER, CICERO, S.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1999
Main classification: H03L7
IPC codes: H03J5, H03J7, H03L7, H04B1