GB2142197A - Oscillator - Google Patents

Abstract

A low noise wide bandwidth oscillator includes two delay lines (2, 4) arranged in parallel. The outputs of the parallel delay lines are added and amplified in a limiting amplifier (8) and fed back to the inputs of the delay lines via respective phase shifters (12, 14). A tuned circuit (10) is included in the feed-back loop to select the required oscillation mode of the circuit. The circuit oscillates at any frequency where the phase change at the oscillation frequency around each of the alternative paths is equal to an integral multiple of 2 pi .The phase shifters included in each of the paths allow tuning to frequencies between the normal modes provided by the delay lines. <IMAGE>

Description

SPECIFICATION Oscillator The present invention relates to oscillators and, more specifically, to oscillators of the delay line type.

A delay line oscillatortypically includes a delay line, an amplifier and a bandpass filter arranged in a series loop. The circuit oscillates when the total electrical path length around the loop is equal to an integral number of wavelengths. The main contributiontothe electrical path length around the loop is provided by the delay line. Such an oscillator has a number of modes. Each mode has a wavelength which divides into the path length ofthe circuit by a different integer.

A bandpass filter is provided to attentuate the unwanted modes For relatively long delay lines,the Q ofthe bandpassfilter must be relatively high in order to differentiate between adjacent modes permitted by the delay line. For this reason it is difficult to produce such an oscillator which is capable of being tuned over a wide frequency range. Moreover, if theoscillator is to have good stability and low phase noise, it is necessary to provide a relatively long delay line.

A phase shifter may also be provided in the circuit in orderto enabletheoscillatorto betunedto any particularfrequencywithinthe range. The phase shifter allowsthe circuit to oscillate with non-integral wavelength paths around the circuit provided that the phase shifter ensurethatthe total phase change around the path is equal to 2nn, where n is any integer.

The present invention provides an oscillator circuit comprising at least two delay lines of different lengths connected in parallel such thatthe outputs of the respective delay lines are summed, an amplifier and a bandpass filter connected in series between the outputs and the inputstothe delay lines.

Because this new delay line oscillator has two delay lines of different lengths there are two different electrical path lengths around the circuit. Therefore, for the circuit to oscillate at any given frequency, the two different path lengths must each be equal to an integral number of wavelengths corresponding to that frequency. This results in the modes of the circuit being more widely spaced than in a simple single delay line oscillator circuit Therefore, the 0 of the bandpass filter is not required to be so high to provide the same selectivity between modes.

Preferably a phase shifter is connected in series with each delay line to enable the oscillator circuit to be tuned to any frequency within its range. With phase shifters present the condition for oscillation is that the combined phase change along each path due to both the delay line and the phase shifter in that path should be equal to 2nowhere n is an integer.

In another embodiment, the delay lines are dispersive and a limiter is connected in series with the amplifier and filter in ordertoimprovethesuppres- sion of unwanted modes and, therefore, frequency modulation (FM) noise.

A delay line oscillator according to the present invention will now be described, byway of example only, with reference to the accompanying diagram my tic drawings, in which: Figure 1 represents a block circuit diagram of the oscillator; and Figure 2 is a diagram illustrating the mode spacing produced bythe oscillator of Figure 1.

The oscillatorcircuit illustrated in Figure 1 includes delay lines 2 and 4 arranged in parallel. The outputs of the delay lines are fed to an adderwhich produces the vector sum of the input signals from the delay lines 2, 4. The output of the adder is fed to a limiting amplifier 8 and then to a bandpass filter 10. The bandpass filter is in turn connected to the inputs ofthetwo delay lines 2,4viarespectivevariablephaseshiftersl2, 14.

Therearetwosignal paths around the circuit of Figure lone via the delay line 2 and phase shifter 12 and the other via delay line 4 and phase shifter 14.

Each path also includes the adder 6, limiting amplifier 8 and bandpass filter 10. These two alternative paths have different electrical path lengths due to the differentelectrical lengthsof delay lines2 and 4. For example, if the delay line 2 has a length 2L and then delay line 4 has a length 3L, and assuming the phase shifters 12 and 14 are both setforzero phase shift, the modes of possible oscillation aroundthetwodifferent paths are illustrated in Figures 2a and 2b respectively.

These Figures show the frequency modes of oscillation around the two paths. It will be noted that the spacing of modes via the shorter path including delay line 2 is 3fwhereas the spacing of the modes via the longer path including delay line 4 is 2f. The circuit will therefore only resonate if the frequency of oscillation is at a mode of both paths simulataneosuly. Figure 2c illustrates these coincident modes which will be the modes of the circuit of Figure 1. It will be noted that these modes are more widely spaced at a frequency separation of 6f. Therefore, the presence of the two parallel delay lines has resulted in awiderspacing of the modes.

Although only two delay lines 2, 4 have been illustrated, it will be appreciated that any number of delay lines may be provided in parallel. Each ofthese delay lines will be selected to have a different path length. The largerthe number of delay lines provided, the largerthe spacing will be between the modes of the circuit. Good results have been achieved in the 40-80 MHz range using two delay lines in parallel with different delays ofthe order of 100 nanoseconds.

The delay lines 2, 4 may be of any suitable type, depending upon the range of oscillation required of the circuit. For example, surface acoustic wave (SAW) devices may be used. SAW devices which are dispersive may be used. In this case it is necessary to provide a limiter in series with the amplifier and bandpass filter. This may be done by the use of limiting amplifier 8. The use of a dispersive delay line The drawing(s) originally filed was (were) informal and the print here reproduced is taken from a later filed formal copy.

The claims were filed later than the filing date within the period prescribed by Rule 25(1) of the Patents Rules 1982.

and limiting provide improved rejection of FM noise.

The dispersion ensures that equally spaced modes either side ofthe desired mode are displaced by relatively different amounts thus preventing the build up of components of frequencies otherthan the desired oscillation frequency.

The adder 6 produces the vector sum of the outputs of the tWO delay lines. ltmayforexampleconsistofa conventional summing hybrid.

The output ofthe adder6 is fed via a limited amplifier8 to a bandpass filter 10, which is provided to selectthe mode of oscillation required. Because the modes of this circuit are more widely spaced than with a single delay line oscillator circuit, the Q of the bandpass filter 10 is not required to be as high as in the case of such a single delay line oscillator, in orderto provide the same amount of attenuation of adjacent modes.

The phase shifters 12 and 14are provided to enable the phase changes over the paths including delay lines 2, 4to be varied by an amount corresponding to a fraction of 2n. These phase shifters 12, 14 allow the circuitto be tuned to any frequency between the normal modes provided by the delay lines 2, 4.

The phase shifters may be of any known type. The phase shifters may include a 90 phase shifter which produces two 90 phase related components of the incoming signal and multiply these respectively by the sine and cosine of the desired angular phase shift in bipolarvoltage controlled attenuators. The suitably modified components are then summed to produce the desired phase shifted signal.

It is possible to construct both delay lines, their associated 900 phase shifters and the bandpass filter as a single hybrid circuit using SAW devices, thereby enabling an extremely compact oscillatorto be produced.

Claims (8)

1. An oscillatorcircuit comprising at least two delay lines of different lengths connected in parallel such that the outputs ofthe respective delay lines are summed, and an amplifier and a bandpass filter connected in series between the outputs andthe inputs to the delay lines.

2. An oscillator circuit as claimed in claim 1, wherein a phase shifter is connected in series with at least one ofthe delay lines.

3. Anoscillatorcircuitasclaimed in claim 7 or2, wherein the delay lines are dispersive, and a limiter is connected in series with the amplifier and filter.

4. An oscillator circuit as claimed in any one ofthe preceding claims, wherein the delay lines are surface accousticwave (SAW) devices.

5. An oscillator comprising two signal paths each containing a phaseshifterand a respective delay line, the outputs of both signal paths being added and amplified and passed via a bandpassfiltertothe inputs of both signal paths,the delay introduced by each ofthe delay lines being different.

6. An oscillator as claimed in claim 5, wherein the amplifier is a limiting amplifierandthe delay lines are dispersive.

7. An oscillator circuit as claimed in any one of the preceding claims, adapted to operate in the 40 to 80 MHz range and the delay line have different delays of the order of 100 nanoseconds.

8. An oscillator circuit substantially as herein described with reference to the accompanying drawings.