GB2044025A - Standard frequency generator - Google Patents

Abstract

A standard frequency generator includes a phase-locked oscillator (1) which is adjustable to the m<th> harmonic of a reference frequency (fr) which is the output from a synthesizer (3) adjustable in frequency steps of delta fa/m by means of a logic circuit (4) in such a way that the reference frequency is that one which gives an output frequency from the phase- locked oscillator closest to the required output frequency. <IMAGE>

Description

SPECIFICATION Standard frequency generator This invention relates to a standard frequency generator.

Standard frequency generators with a controllable output frequency are known (e.g.

German Patent Specification No. 25 02 477 and British Patent Specification No. 1 ,303,63 1).

In a known standard frequency generator, a reference frequency f,, which is adjustable in steps, is generated via a synthesizer having a decade adjustment, the synthesizer being followed by an adjustable electronic frequency divider.

Simultaneously with the selection of the mth harmonic of a phase-locked output oscillator, the frequency divider is adjustable by a division factor m and the synthesizer has a correspondingly higher frequency position, so that the reference frequency value f, is adjustable exactly in order to produce the output frequency fa exactly in accordance with the equation fa = m. In such a generator, there is therefore a precise mathematical relationship between the reference frequency and the adjusted output frequency. This result, however, can only be obtained at considerable expense, because both the frequency divider and the synthesizer which have to have the same range of frequency variation and accuracy as the output frequency, have very complicated circuits.

It is an object of this invention to provide a signal generator which is simple and cheap.

According to the invention, a standard frequency generator comprises a phase-locked oscillator, which is adjustable to the mth harmonic of an adjustable reference frequency (for) and the output frequency (fa) of which is to be tunable with an accuracy of Sofa, the reference frequency being the frequency (for) of the output from a synthesizer which has a decade adjustment, and being adjustable in frequency steps of Sfa/m in the lowest decade, and comprises adjustment means including a logic circuit (for example a microprocessor), by means of which the synthesizer is adjusted to that reference frequency value (Vr) which gives an output frequency value (via = m V) closest to the required set-value output frequency (fa) Such a signal generator makes use of the discovery that the reference frequency fr of the phase-locked oscillator need only be adjustable with an accuracy Sfa/m corresponding to a required frequency accuracy Sfa of the complete generator.For example, if the generator is to be adjustable in 100 Hz steps in the range between 10 MHz and 100 MHz, and a 1 MHz,basic frequency is provided for the reference frequency Vr so that the phase-locked oscillator must be adjustable to between the 1 oth and 1 00th harmonic, and if the generator is in addition required to have an adjustment accuracy of Sfa of 1 Hz, then according to the invention it is sufficient to use an ordinary synthesizer for providing the reference frequency, the lowest decade Sfa/m of the synthesizer being 1 Hz/100 = 0.01 Hz, i.e. a synthesizer need only be adjustable between 1 MHz and 1.1 MHz in 0.01 Hz steps.To minimise errors in the output frequency, the adjustment means is also provided with an appropriate computer which calculates that frequency value to which the synthesizer must be set at any time to give the required output frequency with the maximum accuracy. A signal generator according to the invention can therefore be constructed very easily and cheaply. An ordinary synthesizer is all that is needed for adjustment of the reference frequency. An expensive complicated frequency divider is unnecessary and the synthesizer requires a very small range of variation, in fact only a fraction of the range of variation of the output frequency, i.e. only a variation of 1:1.1 in the above numerical example.Nevertheless, according to the invention, the required output frequency can be adjusted with the same frequency constancy according to the required accuracy of adjustment as a known more expensive generator.

The cost of the synthesizer can be further reduced if there is disposed between the synthesizer and the following phase-locked oscillator a long time-constant phase locked buffer oscillator of high spectral purity which acts as a narrow-band filter, because in this way it is possible to use a synthesizer which is not only greatly restricted in the frequency variation range, but which does not need to have high spectral purity.

The invention may be carried into practice in various ways, and one embodiment will now be described by way of example with reference to the accompanying drawing, in which the single figure is a block diagram of a variable frequency signal generator.

The standard frequency generator comprises a phase-locked output oscillator 1, the output frequency fa of which is adjustable to a selected harmonic m, and the input of which receives a highly stable reference frequency fr The phase-locked oscillator 1 comprises in known manner, a high-frequency oscillator 6 which is tunable via Varaktor diodes, a series filter 7, a phase comparison network 8 with an associated phase control loop 9, and a harmonic generator 10 connected in series on the input side, which harmonic generator enables the spectrum of all the harmonics of the input frequency fr to be generated. Thus, in the case of fur=1 MHz, for example, the spectrum between 1 MHz and 100 MHz at 1 MHz intervals is available at the output of the harmonic generator 10.

The oscillator 6 can be pre-set in known manner to any desired harmonic of the harmonic spectrum produced by the harmonic generator 10.

By means of a control voltage supplied via the line 1 5 to the Varaktor diodes of the oscillator 6, the latter can, for example, be approximately set to any desired harmonic m between 10 fr and 100 fr and the oscillator 6 is then synchronised via its control loop 9 and the phase comparison network 8 to the corresponding mth harmonic of the spectrum produced by the harmonic generator 10.

The control voltage on the line 1 5 is generated in the adjustment means 2. The latter comprises a plurality of decade switches 16 on which the desired output frequency can be set in digital form. Instead of the ten-digit knobs shown for each decade, press buttons or the like may be provided. A corresponding analog control voltage is generated from the frequency set in digital form at the decade switches 16, by means of a suitable digital-analog converter. According to the following numerical example, the oscillator 6 has to oscillate, for example, on the 27th harmonic, and this means that the 10 MHz and 1 MHz decade switches are set to 2 and 7 respectively.

The digital analog converter converts this digital adjustment "27" of the two top decade switches in known manner to produce a control voltage on the line 1 5 of a magnitude such that the Varaktor diodes of the oscillator 6 are set to a value by means of which the oscillator 6 generates an output frequency of about 27 MHz. This output frequency produced solely by the adjustment of the Varaktor diodes is naturally not accurate.

The output frequency does not become accurate until there is phase synchronization via the phase control loop 9 and the phase comparison in the network 8. Assuming that the harmonic generator 10 is fed with a reference frequency fr of exactly 1 MHz, the oscillator tuned roughly to 27 MHz is exactly synchronised in known manner via the phase control loop to the 27th harmonic, i.e. exactly 27 MHz. If a reference frequency of 1.01 MHz is fed to the harmonic generator, the oscillator is synchronised accurately at 27.27 MHz.The reference frequency fr in the synthesizer 3 is also adjusted simultaneously by the adjustment means 2 by means of the decade switches 1 6. The synthesizer 3 is adjustable in a plurality of decades, to an appropriate frequency position of the frequency raster required for the reference frequency fr The lowest decade is adjustable in frequency steps of Nfa/m.

In accordance with the numerical example given above, the output frequency fa of the phaselocked output oscillator 1 is to be adjustable, for example in the range between 10 and 100 MHz in 100 Hz steps, and the adjustment means 2 therefore has adjustment decades for 100 Hz, 1 KHz, 10 KHz etc. to 10 MHz. The first two MHz decades 10 MHz and 1 MHz are used for adjusting the factor m between 10 and 100, i.e. the required harmonic of the oscillator 6. The other decades are used to adjust the synthesizer 3 between 1 and 1.1 MHz, the lowest decade being 0.01 Hz.

Also associated with the adjustment means 2 for adjustment of the frequency of the synthesizer 3, is a logic circuit 4, e.g. a microprocessor, to determine which reference frequencyfr adjustable at the synthesizer 3 will at any time give an output frequency closest to the required set-value frequency fa to which the adjustment facility 2 has been exactly adjusted. Let it be assumed in the above numerical example, for example, that the output frequency fa = 27.5433 MHz is to be provided. The 27th harmonic is then selected in the phase-controlled output oscillator 1 via the adjustment means 2.In accurate mathematical terms, the reference frequency itself should be; fur = farm = 1.020122222222222..MHz. However, this frequency value cannot be adjusted with this accuracy by use of the synthesizer 3, because its lowest adjustable decade is or course adjustable only in 0.01 Hz steps. The logic circuit 4 therefore determines that frequency value fir which has as its 27th harmonic a frequency fta closest in frequency terms to the desired value output frequency fa In the above numerical example this it the frequency value fir = 1.02012222 MHz, i.e.

the lowest decade (0.01 Hz) is set to "2". In the numerical example therefore, the actual output frequency fr is rounded down, and in other numerical examples rounding up of the frequency value may be necessary via the logic circuit 4. This frequency value f'r gives an output frequency f a = 27.54329994 MHz so that the accuracy is within the required tolerance range of 6f, = 1 Hz referred to the set value frequency fa The output frequency is therefore sufficiently accurate despite the simplified arrangement. In some cases, calculation by way of the logic current 4 may shown that it is sufficient accurately to adjust the synthesizer only in its penultimate or even the antepenultimate decade (e.g. in the 0.1 Hz of 1 Hz decade) to obtain a required output frequency.

Then the last decade 0.01 Hz is no longer necessary to maintain a required accuracy Nfa = 1 Hz of the output frequency. The logic circuit automatically calculates in each case that reference frequency which is necessary - in the light of the given frequency tolerance Afa of the output frequency a - to come closest to the set value frequency.

The construction of the synthesizer 3 can be much simplified by providing a buffer oscillator 5 between the synthesizer 3 and the output oscillator 1. This is shown in broken lines and acts as a narrow band filter and may, for example as shown in FIGURE 2, also consist of a phase locked oscillator comprising an oscillator 11 tunable via Varaktor diodes, a series filter 12, a phase comparison network 13 and a phase control loop 1 4 leading from the output to this phase comparison network 1 3. Because the range of variation of the reference frequency fed to the buffer oscillator from the synthesizer is only small (only 1-1.1 MHz), the buffer oscillator can easily be constructed to have high spectral purity and its output therefore delivers, within the narrow frequency variation range, an output frequency fr which also has high spectral purity. The synthesizer 3 itself can therefore be made even simpler and cheaper because in that case it does not need to have an output of high spectral purity.

Claims (3)

1. A standard frequency generator comprising a phase locked oscillator, which is adjustable to the mth harmonic of an adjustable reference frequency (for) and the output frequency (fa) of which is to be tunable with an accuracy of Sofa, the reference frequency being the frequency (for) of the output from a synthesizer which has a decade adjustment, and being adjustable in frequency steps of f8/m in the lowest decade, and comprising adjustment means including a logic circuit (for example a microprocessor), by means of which the synthesizer is adjusted to that reference frequency value (fir) which gives an output frequency value (Va = m.f'r) closest to the required set-value output frequency (fa)

2. A generator according to Claim 1 including a phase-locked buffer oscillator of high spectral purity acting as a narrow-band filter connected between the synthesizer and the phase-locked oscillator.

3. A signal generator arranged substantially as herein specifically described with reference to the accompanying drawing.