GB2239748A

GB2239748A - Direct frequency synthesiser

Info

Publication number: GB2239748A
Application number: GB9000317A
Authority: GB
Inventors: Peter Henry Saul
Original assignee: GEC Marconi Ltd; Plessey Co Ltd; Marconi Co Ltd
Current assignee: Plessey Co Ltd; BAE Systems Electronics Ltd
Priority date: 1990-01-06
Filing date: 1990-01-06
Publication date: 1991-07-10
Anticipated expiration: 2010-01-06
Also published as: GB2239748B; GB9000317D0

Abstract

A direct frequency synthesiser includes an accumulator 2 having an input receiving a digital word representative of the desired frequency, a clock controlling the successive adding of the digital word to the contents of the accumulator 2 to provide a periodically varying value at the output of the accumulator, means 10, 12, 14 for providing from the periodically varying value, values representing a waveform of desired shape, a digital to analog converter (DAG) 8 providing the waveform in analog form, and a clock-operated filter (not shown) coupled to the input or the output of the DAC 8 having means for deriving a clock signal from the waveform at the input or output of the filter, which clock signal is employed to enable operation of the filter. <IMAGE>

Description

DIRECT FREQUENCY SYNTHESISER This invention relates to a direct frequency synthesiser.

Direct frequency synthesisers are known and are for example described in IEEE Journal of Solid-State Circuits, Vol. 23 No. 3 June 1988 "A Direct Digital Synthesiser with 100 MHz Output Capability" P.H. Saul, M.S.J. Mudd pp 819 to 821. Such a synthesiser is shown in Figure 1 as comprising an accumulator 2 for receiving a digital word at an input 4 representative of the desired frequency, which word is successively added to the contents of the accumulator under the control of the system clock to provide a periodically varying output value having the form of a sawtooth wave. The output value is converted by means of a ROM 6 to digital values representing a desired waveform, e.g. triangular, sine, square, which digital values are applied to a Digital to Analog converter 8 which provides an analog waveform at its output.

Figure 2 shows a modification of the known synthesiser of Figure 1 and forms the subject of our copending application < (F20778). Similar parts are indicated by similar reference numerals.

The output of accumulator 2 is output via gate system 10 which provides an equal-sided triangular waveform to first and second integrators 12, 14 which are equivalent to low pass filters in order to generate an output sine waveform.

In both the known systems of Figure 1 and the system of Figure 2 forming the subject of our copending application < (F20778) there may be a need for further filtering to produce a more pure waveform. More specifically, there may be a sign:ficant amount of spurious radiation at frequencies harmonically related to the clock frequency of the synthesiser. It is an object of the invention to overcome this problem.

The present invention provides a direct frequency synthesiser comprising an accumulator having an input for receiving a digital word representative of the desired frequency, clock means for controlling the successive adding of the digital word to the contents of the accumulator to provide a periodically varying value at the output of the accumulator, means for providing from said periodically varying value, values representing a waveform of desired shape, digital to analog conversion means (DAC) for providing .said waveform in analog form, and a clock-operated filter means coupled to the input or the output of the DAC having means for deriving a clock signal from the waveform at the input/output of the filter means, which clock signal is employed to enable operation of the filter means.

Thus in accordance with the invention, a clock-operated filter means is provided to further filter the output waveform of the synthesiser. The filter may either be coupled to the input of the DAC, in which case the filter will be a digital filter of any type, or it may be coupled to the output of the DAC, in which case it will be a clockoperated analog filter, for example a switched capacitor type. In any event, the clock signal for operating the filter is derived from the filter input signal by providing a frequency multiplication device for generating a higher harmonic of the input waveform. The advantage is that such a clock signal will in general be different from the system clock frequency and thus the filtering effect will be operative against spurious radiation created by the system clock and will not have "blind-spots" at harmonics of the system clock frequency.

It would in principle be possible for the clock-operated filter means to perform whole or part of the function of the waveform providing means.

A preferred embodiment of the invention will now be described with reference to the accompanying drawings, wherein: Figure 1 is a block diagram of a known frequency synthesiser; Figure 2 is a block diagram of a frequency synthesiser according to our copending application < > (F20778); Figure 3 is a first embodiment of a clock operated filter to be used in accordance with the invention; Figure 4 is a second embodiment of a clock operated filter to be used in accordance with the invention; Figure 5 is a third embodiment of a clock operated filter to be used in accordance with the invention; and, Figure 6 is a fourth embodiment of a clock operated filter to be used in accordance with the invention.

Referring now to the drawings, the simplest variant proposed here is to add analogue filtering after the DAC 8. This is shown diagrammatically in figure 3. The nominally sinewave output from the DAC 8, including some spurious frequencies, is passed into two circuits, one circuit 20 of which squares it up and frequency multiplies to some small factor times the input, minimally two, but preferably eight or sixteen. This frequency is used to drive the clock input of an analogue filter 22, a switched capacitor type, preferably of the type known as a "commutating" or "N-path" filter. This class of filter has a capability for operation at relatively high frequencies and can have very high Q and very narrow passband.They have the disadvantage of some clock related breakthrough on the output frequency, but in this case, such a signal is of no real consequence since it is at the desired output frequency or a harmonic of it. More important will be the suppression of the off-frequency spurious lines.

This should be very substantial, and will depend in each case on the parameters chosen.

A further improvement of the scheme is shown in figure 4.

Since it is generally undesirable to use a clock containing jitter in the filter, the squarer/multiplier 20 is fed from the filter 22 output (referred to as "recursion"). It is necessary to provide a filter which will pass some signal to the output in the absence of a clock. This leads to an overall lower spurious level than figure 3.

Both the circuits of figure 3 and 4 could be realised in digital form prior to the DAC (figures 5 and 6). The filters 24 and digital squarers and frequency multipliers 26 will be of conventional digital form and several types could be used. Again, feedback from the system output to the clock could be used as shown in figure 6.

Claims

1. A direct frequency synthesiser comprising an accumulator having an input for receiving a digital word representative of the desired frequency, clock means for controlling the successive adding of the digital word to the contents of the accumulator to provide a periodically varying value at the output of the accumulator, means for providing from said periodically varying value, values representing a waveform of desired shape, digital to analog conversion means (DAC) for providing said waveform in analog form, and a clock-operated filter means coupled to the input or the output of the DAC having means for deriving a clock signal from the waveform at the input/output of the filter means, which clock signal is employed to enable operation of the filter means.

2. A synthesiser as claimed in claim 1 wherein the clock-operated filter means comprises a clock operated analog filter of the switched capacitor type coupled to the output of the DAC.

3. A synthesiser as claimed in claim 1 wherein the clock-operated filter means is a digital filter coupled to the input of the DAC.

4. A synthesiser as claimed in claim 1 wherein the clock signal deriving means comprises a squarer/frequency multiplier in digital or analog form.

5. A synthesiser as claimed in claim 1 wherein the clock signal deriving means is coupled to receive the output signal from the clock-operated filter means.

6. Frequency synthesisers as claimed in claim 1 and substantially as described with reference to the accompanying drawings.