Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Synthesizer
Patent
1997-09-26
1999-05-18
Wells, Kenneth B.
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Synthesizer
327159, 36471802, H03B21/00
Patent
active
059053887
DESCRIPTION:
BRIEF SUMMARY
The application relates to a circuit for frequency synthesis.
Such circuits are generally known. Frequency synthesizers are thus known which comprise a digital controlled oscillator followed by a filter. When the digital controlled oscillator is adapted for generating more than one frequency, the relevant filter must of course be a tracking filter. Such so-called tracking filters are expensive and bulky.
Such prior art circuits are moreover only suitable for large frequency steps. Nor are they suitable for integration into a modulator.
The object of the present invention is to provide a circuit for frequency synthesis wherein the above mentioned drawbacks are avoided.
This object is achieved by such a circuit which is characterized by: digit to the adder circuit; and adder circuit and which is provided with a phase detector, a low-pass filter and a controlled oscillator, wherein the carry output terminal is connected to the phase detector. It is noted here that in the literature such a digital oscillator is also known as a fractional rate multiplier, digital controlled oscillator, or as accumulator circuit. The frequency of the output signal, the carry signal, satisfies the following formula: ##EQU1## wherein: N.sub.max --is the largest number which can occur in the adder circuit;
Although a circuit according to the above mentioned claims can easily be embodied in integrated form and no tracking filter is necessary due to the use of a PLL, the wave shape is found to be not entirely optimal. This is due to the fact that--as a result of the operation of the digital controlled oscillator--the frequency of the output signal is not regular.
The adder circuit is adapted to generate a remainder-representing signal. Preferably arranged is a correction circuit for deriving a correction signal from this remainder. According to a first embodiment an output terminal of the correction circuit is connected to a combination circuit which is connected to the input terminals of the phase detector.
The results are however improved even more when the correction circuit comprises three controlled accumulators and a D/A converter.
The very best results are however obtained only when the correction circuit comprises N digital controlled accumulator circuits, wherein each of the accumulators is adapted to receive the remainder of the preceding accumulator and each of the accumulators is connected via a plurality of difference determining circuits equal to its order number--1 to an adder circuit, the output terminal of which is connected to the D/A converter.
The above described frequency synthesizer is suitable for generating a sine-shaped signal, a saw-tooth signal, a square wave signal or other type of signal of which the frequency is externally controlled, for instance for generating a signal for use in measuring equipment or in modulators and/or demodulators.
In such an application it is therefore attractive to integrate the modulation process or the demodulation process in the circuit. It is assumed here that by the term "modulate" in the accompanying claims is also understood demodulate; both after all comprise making a non-linear combination of the signal for processing with a carrier wave signal.
The circuit according to the invention can be used in frequency or phase modulation and even in amplitude modulation. In this latter case a combination is made of two or more phase modulators. It is of course possible to use the circuit according to the invention in a combination of frequency or phase modulation with amplitude modulation.
The invention will be further elucidated hereinbelow with reference to the annexed drawings, wherein:
FIG. 1 shows a diagram explaining the operation of a digital controlled oscillator;
FIG. 2 shows a phase spectrum of the signal generated by the digital controlled oscillator;
FIG. 3 shows a frequency spectrum of the signal generated by the digital controlled oscillator;
FIG. 4 shows a diagram of a first embodiment of a circuit according to the invention;
FIG. 5 shows a diagram of a second embo
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patent: 4746880 (1988-05-01), McCune, Jr.
patent: 5552727 (1996-09-01), Nakao
patent: 5708687 (1998-01-01), Powell et al.
Proceedings of the 40th Annual Frequency Control Symposium, 1986 (CAT. No. 86CH2330-9), pp. 355-365.
Proceedings of the 40th Annual Frequency Control Symposium, 1986 (CAT. No. 86CH2330-9), pp. 373-378.
Patent Abstracts of Japan, vol. 16, No. 389, Aug. 1992.
De Rijk Johannes H. A.
Dequesnoy Robertus J.
Spijker Menno T.
Van Der Valk Robertus L.
Nguyen Hai L.
Wells Kenneth B.
X Integrated Circuits B.V.
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