Modulators – Frequency modulator – Including stabilization or alternatively distortion – noise...
Reexamination Certificate
1999-05-28
2001-04-03
Grimm, Siegfried H. (Department: 2817)
Modulators
Frequency modulator
Including stabilization or alternatively distortion, noise...
C455S113000
Reexamination Certificate
active
06211747
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a frequency synthesiser and in particular to a frequency synthesiser suitable for generating wideband modulated radio frequency signals for use in, for example, portable radio communication devices or Wireless Local Area Network (WLAN) systems.
BACKGROUND OF THE INVENTION
Frequency synthesisers have been developed in recent years which permit direct modulation of a carrier signal generated by a Phase Locked Loop (PLL) circuit by rapidly varying the instantaneous value of a variable divider forming part of the PLL circuit; the variable divider is controlled by a digital signal output from a sigma-delta type multi-accumulator digital circuit which acts to shape the noise generated by such a system so that it mostly occurs at higher frequencies where it can be more easily filtered out (by the natural Low Pass Filter behavior of the closed PLL) before transmission of the signal. Such frequency synthesisers are known as direct modulation multi-accumulator fractional-N synthesisers and an example of such a synthesiser is described in U.S. Pat. No. 5,166,642 which is incorporated herein by way of reference.
Such frequency synthesisers are able to produce very well controlled modulation with a low enough Signal to Noise Ratio (SNR)—and in particular Signal to Phase-Noise Ratio—at relatively low modulation frequencies to find many practical applications. However, as a result of the type of noise shaping employed by the multi-accumulator digital circuit, it is difficult to maintain such a low SNR where a higher bandwidth is required for the modulation signal. This is because the noise shaping reduces the amount of low frequency noise at the expense of increasing the amount of high frequency noise. For this reason, multi-accumulator fractional-N frequency synthesisers have not found application in large modulation bandwidth applications to date.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a direct modulation multi-accumulator fractional-N frequency synthesiser for generating a carrier signal modulated by a modulation signal, the frequency synthesiser comprising a Voltage Controlled Oscillator, VCO, having a tuning port for controlling the frequency of the signal output by the VCO, a variable divider and a multi-accumulator sequence generator for controlling the variable divider, a reference signal generator, a phase detector and a low pass filter, these elements being arranged to form a Phase Locked Loop arrangement, the directly modulated output signal of which is taken from the output of the VCO, wherein in-band modulation is performed by varying the variable divider and out-of-band modulation is performed by directly applying the modulating signal to the VCO tuning port.
It will be understood by a person skilled in the art of radio frequency synthesisers that in-band modulation refers to modulation of the carrier frequency by components of the entire modulation signal whose frequency does not exceed the corner frequency of the low pass filter, while out-of-band modulation refers to modulation of the carrier frequency by components of the entire modulation signal whose frequency does exceed the corner frequency of the low pass filter. Actually, this is only an approximate definition since other components within the PLL arrangement will have low pass filtering characteristics in addition to the low pass filter itself, and corner frequencies themselves represent an arbitrary cut-off point for what is actually a smooth transition. None-the-less, as will be understood by persons skilled in the art, the precise definition of in-band or out-of-band modulation is especially unimportant in the present invention, provided the two definitions are complimentary, since the transfer characteristics of the out-of-band modulation will be complimentary to the transfer characteristics of the in-band modulation as a direct result of the nature of the present invention.
It is significant that all in-band modulation is performed directly by varying the variable divider (i.e. varying the instantaneous value of the divisor of the variable divider), since this is a purely digital process and can thus be very accurately controlled. Furthermore, the fact that the in-band modulation is performed in a purely digital manner is also significant for enabling accurate control of the relative delays between the in-band and out-of-band modulation paths which is a significant advantage of the present invention.
Preferably, the modulating signal is provided as a digital signal both to the sequence generator and to a Digital to Analogue Converter, DAC, the output of which is combined with the output from the low pass filter before being applied to the VCO tuning port. Preferably the DAC has a variable gain which is preferably controllable by means of a suitable digital control signal.
This enables the modulating signal to be transmitted to be maintained in a digital format for a desirably large number of the signal processing steps prior to transmission.
Preferably, the digital modulating signal is provided by a look-up table to which a bit stream is input. Such an arrangement not only provides a much better control over the modulation signal than can be achieved by simply passing the bit stream through a low pass filter. Additionally, in a preferred embodiment of the present invention, it enables a very fine control over the relative delays of the modulation signal along the in-band modulation path and the out-of-band modulation path which enables very close matching of these delays to minimise phase errors which result from a poor matching of these delays. The fine control over the relative delays may be advantageously provided by having two address pointers for accessing data from the look-up table one of which supplies data from the look-up table to the in-band modulation path while the other supplies data from the look-up table to the out-of-band modulation path, whereby one pointer may access the same data from the look-up table at a slightly later or earlier period in time than the other pointer. The relative delay between the address pointers may either be set during initial design of the frequency synthesiser, during factory calibration or it may be dynamically set and adjusted under the control of a suitable digital controlling unit.
Where two pointers are used, the minimum resolution for delay compensation may be the period or cycle time used to read the look-up table. However, in a preferred embodiment, the minimum resolution can be further reduced down to the period of the comparison frequency or the reference frequency by storing the output values of the look-up table into FIFO-type stacked registers.
Additionally, according to a preferred embodiment of the present invention, there is further provided an Analogue to Digital Converter, ADC, connected between the input to the VCO and a digital controlling unit such as a MicroController Unit, MCU, or a Digital Signal Processor, DSP. By further providing a digital control signal from the digital controlling unit to the DAC it is possible to vary the gain of the DAC so that the transfer functions of the in-band and out-of-band modulations are well matched so as to provide an all pass transfer function for the synthesiser as a whole with a constant group delay versus frequency, regardless of the closed loop cut off frequency of the PLL (or in-band modulation bandwidth), which can thus be chosen to take optimum advantage of the noise shaping provided by the sequence generator regardless of the modulation bandwidth required.
REFERENCES:
patent: 4308508 (1981-12-01), Sommer et al.
patent: 5021754 (1991-06-01), Sheperd et al.
patent: 5166642 (1992-11-01), Hietala
patent: 5834987 (1998-11-01), Dent
patent: 5983077 (1999-11-01), Dent
Fourtet Christophe
Trichet Jacques
Grimm Siegfried H.
Motorola Inc.
Parker Lanny L.
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