GB2163019A - Frequency synthesizer - Google Patents

Abstract

A frequency synthesizer has a reference crystal oscillator 50 connected to apply output to a comb generator 51 and a multiplier 52. The output of the multiplier 52 is connected to one input of a mixer 53, the other input of the mixer 53 being connected to receive input from a voltage control oscillator 54. The mixer 53 produces a difference signal output and applies the difference signal to one input of a further mixer 55, the other input of the mixer 55 being derived from a switchable filter 56. The mixer 55 produces a difference frequency signal to one input of a phase sensitive detector 59, the other input of the phase sensitive detector being connected to receive signals which are a sub-harmonic of the frequency outputted from switchable filter 56. The output of the phase sensitive detector is applied in a feedback path through an integrator 64 to the input of the voltage control oscillator 54 such that the output of the voltage controlled oscillator is stabilised in frequency. The comb generator 51 supplies switchable filter 56 to generate harmonic M.fr and supplies switchable filter 60 and divider 62 to generate sub-harmonic fr.L/K. The values of L and M are controllable 57, 61. <IMAGE>

Description

SPECIFICATION Frequency synthesizer This invention relates to a frequency synthesizer and particularly, although not exclusively, to a synth esizerfor a frequency-agile radar system. The present invention is therefore particularly applicable to a microwave source capable of generating several stable, selectable, frequencies over a given radio frequency band and which is able to be switched rapidly between these frequencies.

A radar having such frequency agility has wide application including the reduction of sea clutter,the reduction of interference between aircraft in the squadron or as an electronic counter-measure.

One known form of frequency synthesizer is that known as a phase-locked or indirect synthesizer and one such synthesizer will now be described with referenceto Figure 1.

Acrystal controlled frequency source 1 producing an outputfrequency F1 is connected as one inputto a frequency mixer 2 which produces an output to a programmable frequency divider 3 in dependence upon the difference in input signals to the mixer 2. The divider3 may be constructed using emitter coupled logic devices and in dependence upon a signal from a channel control signal device 4 so the divider 3 divides the output from the mixer 2 by an integer n in the range 16to 32, the 2to 1 ratio being chosen to minimise the change of loop behaviourwith change oi output frequency.

The outputfrom the divider 3 is applied to one input of a phase sensitive detector the other input of which is provided from a channel spacing crystal oscillator6 arrangedto producea sub-harmonic, F2, of the output from source 1. The phase sensitive detector is a digital circuitwhich produces both frequency and phase information, the output phase ofthe phase sensitive detector 5 being proportional to the sine of the differnce in phase at its inputs. The frequency information derived from the phase sensitive detector assists the loop to acquire lock under large error conditions.The output from the phase sensitive detector 5 is applied to a low pass filter7 to remove ripple atthefrequency F2 and a low frequency signal containing a component proportional to the phase difference between the input signals is amplified in an active integratorformed by an operational amplifier configured as an integrator. The integrator8 has a lead/lag low pass filter characteristic to determine the dynamic characteristics of the loop and the output of the integrator8 is applied to the tuning port of a voltage controlled oscillator (VCO) 9 to correct the VCO frequency.The output ofthe VCO 9 is applied to a multiplier 10 for utilisation to an output port 11 and the outputofthe VCO is also tapped to provide the other input to the mixer 2 and to thereby form a feed back loop.Themultiplier 10 may be a step recovery diode multiplier which converts the VCO outputto the required local oscillatorfrequency by multiplying the VCOfrequency by an appropriate factor M.

By utilising such a circuit arrangementthe VCO output is phase locked to the frequencies F1, F2 of the oscillators 1,6 respectively in the relationship Fvco = F1-nF2.

The FM noise characteristic of such an indirect synthesizer is shown with reference to Figure 2 in which the abscissa is the offset frequency, orthe displacementfrom the carrierfrequency, and the ordinate is FM noise in db's below the carrier frequency per Hz. In the Figure 2the dashed line a is the free running VCO characteristic, the dotted line b is the characteristicforthe crystal source 1 ,the chain dotted line c is the characteristic for the channel spacing oscillator 6, the phase sensitive detector 5 and the divider3 and the solid lined is the loop resultant characteristic.It will be noted that the loop characteristic discomposed ofthethree elements shown by characteristics a, band cato minimisethesynthesizer noise the loop natural frequency is chosen to coincide with the cross over of VCO and loop noise characteristics.

Thetime taken to change from onefrequencyto another and to settle within the specified tolerance is determined primarily bythe loop natural frequencyso thatthe higherthis frequencythe shorter is the switching time. As will be seen from Figure 2 however, this parameter is fixed by the required noise performance.

The output of the phase sensitive detector is a stream of pulses at the reference frequency, the width of which is proportional to the phase difference between the input signal. The phase sensitive detector output signal is equivalentto a DC voltage, proportional to the phase difference ofthe input signal suppliedthereto,with asuperimposed high frequency ripple. Although this ripple is filtered bythe low pass filter7thereisa residual level which modulatesthe VCO 9 and produces spurious sidebands. The permitted level of such sidebands is normally specified such that adjacent channel interference and intermodulation products within the radar receiver are acceptable.

Itis often necessaryto introducefurtherfiltering within the loop to reduce the transfer ofthe ripple but such extra filtering introduces additional phase shifts which modifythestabilityoftheloop.Theeffectofthe extra phase shift is to cause the damping factor to reduce togetherwith the loop natural frequency resulting in an increase in switching time. The value of the loop time constant may be adjusted to produce an acceptable loop damping factor and natural frequency butthere is a limitto this adjustment since for very large values of phase shift negative time constants may be necessaryto achieve the desired loop characteristics. This is clearly not possible and there is a minimum ratio of phase sensitive detector operating frequency to loop natural frequency empirically found to be 5:1.

Because output power is generated bytheVCO, at the output frequency, or at a sub-multiple thereof, the efficiencywith which the input DC power is converted to microwave output is high. It will be realised that if the VCO wereto operateattheoutputfrequency efficiencywould be maximised and this would also act to reduce the noise in spurious sidebands. However the intermediate frequency circuits would then nesd tocopewiththefull output bandwidth, rather than the bandwidth divided by M, butthis is usually not allowed due to the bandwidth limitations of the digital programmable divider.

Afurther known synthesizer is that known as a direct frequency synthesizer and such a configuration for operation in the L band will now be described with reference to Figure 3 ofthe accompanying drawings.

In Figure 3, three crystal oscillators 21,22,23 producing output frequencies f21,f22 and f23 respectively. The outputs from the oscillators 21,22,23 are each applied to fourthree-way switches 27 which are used to selectthe outputs from the oscillators and to applytheoutputstothe inputs of amplifiers 28 and thence asfarasthe output switches 24,25,26 are concerned to up converter mixers 29 which are serially connected with bandpass filters 30 and dividers 31 having a divide by three factor. The output from the chain is further amplified byan amplifier32 and multiplied in a x 16 multiplier 33 to provide an output at terminal 34. The switches 24-27 may be PIN diode switches.

The FM noise characteristic of a direct synthesizer is given bythe noise ofthe crystal oscillator and is as shown by characteristic bin Figure 2. Since there are no feed back paths containing dividers the only enhancement of this noise is due to the multiplier chain. The time to change frequency is dependent on the PIN diode switches 24-27 and the subsequent settling ofthe crystal oscillator. Typical switching times of a few microseconds are readily obtained.

To achieve a desirable minus 70dBc maximum level of spurious sidebands atthe output terminal 34 due to breakthrough ofthe basic crystal oscillators, a minimum isolation of 106dB is required in the switches 24-27 in the off position.Asimilardegree of RF screening between the oscillators 21-23 is also required.

The efficiency with which the output power is generated in a direct synthesizer is low due to the conversion loss in the mixers and RFamplification is required.

Fromtheforegoing itwill be realised that direct synthesis techniques provide workable synthesizers at the expense of increased bulk due to the filters and low efficiency due to the inherently low output of the mixers arising from the need to operate linearly and thus reduce spurious outputs and that care in construction to ensure adequate screening, particularly in the switches 24-27 is necessary.

The advantages and disadvantages of the indirect and direct known synthesizers may be summarised as followsforthefollowing parameters: Noise - is better in the indirect synthesizer because ofthe absence ofthe noise floor of characteristic c in Figure 2.

Switching time- istypically five microseconds and is very rapid forthe direct synthesizer dueto the rapid switching faciiitated by the PIN diode switches 24-27 whereas the indirect synthesizer loop takes a relatively long time to settle and has a typical switching time of twenty five microseconds.

Spurious-is betterforthe phase locked loop system of the indirect synthesizer since not only is high isolation required in the switches 24-27 but also intermediate intermodulation products are produced in the multiplier chain ofthe direct synthesizer. The phase locked loop acts as a narrow band filter two such products.

Efficiency-- is betterforthe in direct synthesizer than the direct synthesizer since only relatively low powersignalscan be handled in the diode mixers of the mixer chain in the direct synthesizer.

Ease ofconstruction-dueto the difficulty in providing adequate screening forthe directsynthesizerthe indirect synthesizer is easierto construct.

Size-the indirect synthesizer may be made more compact A discussion of indirect and direct synthesizers is given in IEE PROCEEDINGS, Vol.130, Pt. H, No. 7, DECEMBER 1983, pages 430-436 and pages 456-462.

The present invention seeks to combine the advantages of direct and indirect synthesizers.

According to this invention there is provided a frequency synthesizer comprising reference frequency means having the outputthereof arranged to be multiplied by a multiplier means, the output of the multiplier means being connected to one input of a mixer, a further inputofthe mixer being connected to an output of a voltage controlled oscillator, the mixer being arranged to produce a difference signal output fromthetwo inputs thereto and to apply said difference signal output to one input of a further mixer, the other input of said further mixer being derived from a predeterminedly variable frequency selection means arranged to produce an inputtosaid furthermixerwhich is lower in frequency than the difference signal output from the first mixer, the second mixer being arranged to produce a difference frequencysignalto one input a phase sensitive detector, another input of the phase sensitive detector being connected to receive signals produced by meansfor producing a sub-harmonic ofthe other inputtothefurther mixer, the output of the phase sensitive detector being arranged to be fed in a feed back path through integratorto an input ofthe voltage control oscillatorwhereby output ofthe voltage control oscillator is substantially stabilised in frequency.

Preferably the predeterminedly variable frequency selection means comprises means for producing a plurality of signal frequencies and a first switchable filter connected thereto forselecting one of said frequencies.

Convenientlythe means for producing a subharmonic comprises a second switched filter substantiallythe same as said first switched filter and a divider for dividing the output of said second switched filter, said second switched filter also being connected to said means for producing a plurality of signal frequencies.

The invention will now be described byway of example with referenceto the accompanying Figure 4 which shows a synthesizer in accordance with this invention in blockschematicform.

Referring to Figure 4 a reference crystal oscillator 50 producing an outputfrequencyfris connected to a comb generator 51 and a xN multiplier 52 The multiplier 52 is connected to one input of a mixer 53, the other input of the mixer 53 being connected to receive output from a voltage control oscillator { D) 54 having similar characteristics to the VCO 9 discus- sed with reference to Figure 1 The mixer 53 produces a difference signal outputwhich is amplified in amplifier 66 and applied as one input to a further mixer 55. The other input to the mixer 55 is derived from a switched filter 56 connected to the output of the comb generator 51 and the switched filter 56 is arranged to be controlled by a channel control signal device 57 to select a particularfrequencyfrom the comb generator 51 output. The switched filter 56 has M selectable positions but the input applied to the mixer 55 from the switchable filter 56 is arranged to be a selected harmonic ofthe reference oscillator 50 and lower in IFrequencythan thelFrequencyof ofthe signal applied to the mixer 55 from the mixer 53 The mixer 55 produces a difference frequency which is applied to a further amplifier 58 the output of which is connected as one input to a phase sensitive detector 59 which is similar in characteristic to the phase sensitive detector5 described with reference to Figure 1. The other input ofthe phase sensitive detector 59 is obtained from the output ofthe comb generator 51 which is applied through a switchable filter 60 which is controlled by a channel control signal device 61, the switchablefilter having L selectable positions. The output from the switchable filter 60 is applied through a divider 62 having a division factor K and the output of the divider is applied to the other input ofthe phase sensitive detector 59.The frequency applied to the phase sensitive detectorfrom the mixer 55 is thus compared in the phase sensitive detector 59 with the reference frequency derived via switchable filter 60 and divider 62, the reference frequency being a selected harmonic of the crystal oscillator50which has been divided bye fixed integer K.

The output ofthe phase sensitive detector is passed through a lovv pass8lter63 and thence through an operational amplifier configured as an integrater64, the output of which is then applied to the tuning port of the VCO 54. The output ofthe VCO 54, fo, is applied to an outputterminal 65.

The output frequency fo is given by the following expression: L fo = fr(N + M + -) K fr The channel spacing frequence equals K For equally spaced channels, the switchable filter 60 has a number of values of L equal to K The number of channels equals (number of values of L) x (numberofvalues of M).

With the reference frequencyfrtypically in the range 60-120 MHz and M, Land K in the range 2-10, the phase sensitive detector operates in the range from 35-135 MHz. Such an arrangementallowsthe loop bandwidth to be as high as 8 MHz although in practice 5MHz is sufficientto achieve a switching time of 5 microseconds.

With a loop bandwidth of typically 5 MHz the output noise is dominated by that ofthe crystal oscillator multiplied by N and outside the loop bandwidth the noise of the VCO is predominant. In the embodiment of Figure4the sampling rate of the phase sensitive detector is 50MHz compared with a 4MHz rate obtainable with an indirect synthesizer, that is an increase by a factor of 12.5 or 22 dB. This factor of 12.5 results in a net improvement of plus 28 dB's on the indirect synthesizer in a region dominated by the phase sensitive detector and amplifier noise.

The switchablefilters 56,58 which may be PIN diode switched orvaractortuned filters or digital rate multipliers can be switched in less than 5 microseconds and the phase locked loop has a wide bandwidth permitting switching times of 5 microseconds. The frequence synthesizer of Figure 4 therefore has a switching time comparable with that of a known direct synthesizer.

As far as spurious sidebands are concerned the principal spurious components occur displaced from the main signal by multiples ofthe channel spacing frequency. Since the ratio of phase sensitive detector operating frequency to loop bandwidth is approximately 10:1 in both the indirect synthesizer and the synthesizer of Figure 4, the filters may be scaled to ensure that the 50 MHz ripple from the phase sensitive detector is attenuated sufficiently to meet the minus 70 dBs spurious output requirement normally associ- ated with frequency agile radars.The channel spacing frequency K also needs to be rejected, although less attentuation is required atthisfrequencysincetheswitchedfilters will contribute to the rejection.

Therefore the partial filtering of the unwanted harmonics produced by the comb generator 51 are at offset frequencies much higherthan the bandwidth of phase locked loop and so will be rejected bythe loop.

The spurious sideband performance ofthe invention shown in Figure 4 should therefore exceed that over a conventional, known synthesizer. Because the power is generated directly at the output frequency so better efficiency is obtained from the present invention than that of an indirect synthesizer. Moreover because simple filtering only is necessary, ease of construction and improvement in size is improved over conventional direct synthesizers.

From the foregoing it will be understood bythose skilled in the artthatthe frequency synthesizer of this invention provides improved switching time and noise performance compared to known indirect synth esizers without the spurious outputs, low efficiency and size penalties associated with direct synthesizers.

It will also be appreciated by those skilled in the art that a SAW oscillator may be employed instead ofthe crystal oscillator 50.

The present invention has the following advantages: (a) high output powerthrough the use of a VCO rather than a mixer and amplifier of the direct synthesizer arrangement (b) rejection of spurious signals since the phase locked loop acts as a narrow band tracking filter (c) there is no requirement for high output powers from mixers so that they m mav be operated linearly to minimise intermodulation products (d) rapid switching is possib!e since wide loop bandwidths may be employed-the phase sensitive detector operating in the 50 MHz region (e) noise performance is improved due to the elimination of the final multiplying frequency divider preceding the phase sensitive detector Ij) reduction in size is facilitated compared with a direct synthesizer since simple filtering only is required.

Claims (4)

1. Afrequency synthesizer comprising reference frequency means having the output thereof arranged to be multiplied bv a multiplier means, the output of the multiplier means being connected to one input of a mixer, a further input of the miter being connected to an output of a voltage controlled oscillator, the mixer being arranged to produce a difference signal output from the two inputs thereto and to apply said difference signal output to one input of a further mixer, the other input of said further mi::er being derived from a predeterminedly variable frequency selection means arranged to produce an input to said further mixer which is lower in frequency than the difference signal output from the f first mier, the second mixer being arranged to produce a difference frequency signal to one input of a phase sensitive detector, another input of the phase sensitive detector being connected to receive signals produced by means for producing a sub-harmonic of the other input to the further mixer, the output of the phase sensitive detector being arranged to be fed in a feed back path through an integrator to an input of the voltage control oscillator whereby output of the voltage control oscillator is substantially stabilized in frequency.

2. A frequency synthesizer as claimed in claim 1 wherein the predeterminedly variable frequency selection means comprises means for producing a plurality of signal frequencies and a first switchable filter connected thereto for selecting one of said frequencies.

3. A frequency synthesizer as claimed in claim 2 wherein the means for producing a sub-harmonic comprises a second switched filter substantially the same as said first switched filter and a divider for dividing the output of said second switched filter, said second switched filter also being connected to said means for producing a plurality of signal frequencies.

4. A frequency synthesizer substantially as herein described with reference to and as shown in Fig. ss of the accompanying draqings.