FR2573540A1 - Frequency synthesiser with high frequency agility, low phase noise and with coherent direct synthesis - Google Patents

Abstract

The synthesiser comprises a frequency standard controlled by a single reference oscillator at a frequency f0, a fine-step channel with one output, a coarse-step channel with two, or more generally M, outputs, a final synthesis channel CHF with three inputs supplying a control output of frequency fp, and a local oscillator output of frequency fOL offset from the value f1, where f1 is the intermediate frequency. The final synthesis channel receives at the input En of its switch C5 the fine-step frequencies, at the input E1 of its first mixer M1 N coarse-step frequencies, and at the input E2 of its second mixer M2 N other coarse-step frequencies offset by N intervals of frequency Nf0. Application to control circuits of Doppler radars, with pulse compression, with high frequency agility.

Description

 The invention relates to a frequency synthesizer with high frequency agility and very low phase noise, produced by direct synthesis, comprising a single reference oscillator and intended for use as a pilot of frequency-agile pulse-compression Doppler radar. .

 The performance of a Doppler radar is closely linked to the quality of the sources it uses, the frequency variations obtained by the Doppler effect being relatively small. It is therefore necessary to # use sources with very short-term phase stability. This phase stability will condition the quality of the rate of elimination of fixed echoes obtained. It should be of the same order of magnitude as that encountered in fixed frequency sources constituted by a quartz oscillator followed by multiplier stages.

In addition, in the context of frequency agility radar, it may be necessary to be able to change the transmission frequency from one pulse to the next. The switching time of the synthesizer must therefore be low enough to allow rapid access to any frequency during this time interval.

 On the other hand, it is also necessary to obtain high spectral purity. This requires that the harmonic or non-harmonic lines be located sufficiently below the main line. These parasitic lines are particularly troublesome when they are offset from the main line by a value equal to the intermediate frequency plus or minus the modulation band. It will therefore be necessary to make two parallel chains synthesized and offset from each other by the intermediate frequency and comprising parasitic lines, offset from the intermediate frequency, as low as possible.

 The synthesis techniques commonly used for the realization of low frequency and high frequency synthesizers cannot all be applied to microwave synthesizers, and, above one gigahertz, the synthesis chain must be entirely analogical; In addition, direct synthesis chains are easier to realize than indirect synthesis chains with phase loop if the number of frequencies required is high and if it is desired to obtain low switching times and high phase stability at short term.

This direct synthesis will moreover have to be coherent, that is to say be carried out using a single basic oscillator, which makes it possible to obtain fewer parasitic lines, in particular in the medium frequency band, and thus facilitates filtering. This frequency synthesizer with direct synthesis will also be of better quality in terms of phase noise if it only performs additive synthesizers.

 There are synthesizers to obtain a large number of frequencies. they generally use several circuits, based on phase loops or performing direct synthesis; each of these circuits makes it possible to make more or less fine frequency jumps, the sum of the jumps of one being at least equal to the smallest jump of the other.

 Thus, the object of the present invention is to provide a frequency synthesizer with direct synthesis comprising a final synthesis chain capable of supplying on its pilot output IMN different frequency values and on its local oscillator output LMN other lower or higher frequency values, offset by the intermediate frequency fi with respect to the frequency supplied on the pilot output, comprising a coarse pitch chain which can supply NN frequency values and a fine pitch chain which can successively supply L different frequency values, these elements being such that the switching from any of the frequencies supplied to another is done quickly, and further comprising a pilot frequency standard by a single reference oscillator providing a reference frequency fg, this frequency standard providing continuously and simultaneously to the final synthesis chain, to the large pitch chain and to the fine pitch chain, all s useful frequencies.

According to the invention, this coarse pitch chain comprises first means for developing, from the reference frequency, a set of N frequencies at the pitch A ~ 1 = fg and for selecting one of these frequencies, second means comprising M outputs can each supply, successively, N different frequency values at step Bfl = fg, the M frequencies simultaneously supplied on these M outputs being offset from each other by the frequency difference df, = Nfo and these different frequencies being produced from the frequency selected by the first means. The fine-pitch chain comprises K similar stages of indices k = I to K, connected in cascade, each receiving two predetermined frequencies and mixing the output signal of the preceding stage with l one of the predetermined frequencies and further comprises input selection means for applying to the input of the first stage a frequency selected from among several predetermined frequency values ce, the values of all these predetermined frequencies supplied to this chain being chosen so that the output of the K stage, which constitutes the output of the fine pitch chain, can provide L different frequencies spaced by the same
f0 step #f = L0 included in a frequency band of width fO. The final synthesis chain comprises third means for supplying identical signals at two outputs at a frequency chosen from
LMN frequencies spaced from step df, from the M output signals supplied on the M outputs of the coarse pitch chain and the output signal of the fine pitch chain, a local oscillator channel connected to the first output of the third means supplying by mixing with a frequency f1 the LMN output frequencies of this local oscillator channel and a pilot channel connected to the second output of the third means, to supply, by mixing with a frequency f1 + fi, the
LMN output frequencies of this pilot channel.

The invention will be better understood and other characteristics will appear with the aid of the description below and of the accompanying drawings in which: - Figure 1 represents the channe with large pitch of the synthesizer according to
the invention; - Figure 2 shows the fine pitch chain; - Figure 3 shows the final synthesis chain; - Figure 4 shows the modulation chain; and - Figure 5 shows the frequency standard and the oscillator
pilot.

 -In Figure 1, the large pitch chain # is shown. This chain receives on the input of its amplifier AHI a signal at the frequency f0 supplied, from a reference oscillator G, by a frequency standard Sf (FIG. 5). This signal at the frequency of the reference oscillator, or reference frequency, amplified by the amplifier AHI, is supplied to a comb or generator of harmonics GH which supplies a multitude of harmonics of this reference frequency to a divider. of power D1 to N outputs. -On each of these outputs is placed a bandpass filter intended to select one of the harmonics of this reference signal. This filter wire battery consists of filters Fil1, FH2, ..., FH. FHN. FHN ~ These filters are such that they allow a single harmonic of the reference signal to pass.

et et il permet donc le passage de toutes les combinaisons additives de la fréquence f# avec l'ensemble des harmoniques sélectionnées, R étant le rang de la première harmonique sélec tionnée. Le deuxième mélangeur N#2 reçoit d'autre part, du standard de fréquence, un signal à fréquence f#2 = f# + Nfo. Sa sortie est filtrée par le filtre E fréquence fQ +

de bande Nf0 centrée sur la il permet donc le passage de toutes les combinaisons additives de la fréquepce fQ + Nf0 avec l'ensemble des harmoniques sélectionnées. La première sortie S1fournira donc la suite des N fréquences à pas gros les plus basses et la deuxième sortie fournira la suite des N fréquences plus élevées suivantes.Pour chaque harmonique sélectionnée par le- commutateur- Cl, on obtient donc deux fréquences de sortie décalées entre elles de l'intervalle de fréquence Nfo. On peut considérer que cette extrémité de la channe à pas gros constitue une chaîne de sortie à pas très gros égal à Nf0 et à M = 2 sorties. La généralisation à M sorties conduit à utiliser
M filtres FA
m la fréquence d'indice m = I à M, de la rgeur de bande Nf0 centrée sur

If R is the rank of the first harmonic chosen, the selection of the ith
N contiguous harmonics required requires that the n Fil filter
n has its passband centered on the frequency (R + n-1) fO and of width fO. The CI switch receiving the N outputs of these N filters therefore makes it possible to select a harmonic from N successive successive harmonics. The signal supplied by this switch is first amplified in the amplifier AH2 then supplied to a power divider with two outputs D2. The first of these outputs is intended, after amplification in the amplifier A1, for the first mixer NA1. The second of these outputs is intended, after amplification in the amplifier Ay to the second mixer MA2. The first mixer M # 1 also receives from the frequency standard a signal at frequency f # 1 = f. Its output is filtered by the filter F # 1 of bandwidth Nf0 centered on the frequency

and and it therefore allows the passage of all the additive combinations of the frequency f # with the set of selected harmonics, R being the rank of the first selected harmonic. The second mixer N # 2 receives on the other hand, from the frequency standard, a signal at frequency f # 2 = f # + Nfo. Its output is filtered by the filter E frequency fQ +

of band Nf0 centered on the so it allows the passage of all the additive combinations of the frequency fQ + Nf0 with all the selected harmonics. The first output S1 will therefore provide the sequence of the N lowest pitched frequencies and the second output will provide the sequence of the following highest frequencies. For each harmonic selected by the -change-Cl, two output frequencies are thus shifted between them of the frequency interval Nfo. We can consider that this end of the chain with large pitch constitutes an output chain with very large pitch equal to Nf0 and to M = 2 outputs. The generalization to M outputs leads to using
M FA filters
m the frequency of index m = I to M, of the band feeder Nf0 centered on

Figure 2 shows the fine-pitch chain #. It receives the frequency standard, controlled by the reference oscillator, three frequency fs. The first, F0, 11 f frequency groups. The first, ~ 0, it 1, 12 f, is received on the
f0 three inputs of the three position switch C # 0. The second, 50
f0 55 10, is received on the two inputs of the switch two f C # 1. The third, 48 f
f0 f0 positions C # 1. The third, 48 f0 50 f0, is received on the two inputs of the two-position switch Co2. This fine-pitch chain further comprises a first mixer M # 1 receiving on its two inputs the output signals of the switches ebo and C6 and whose output, amplified by an amplifier Ad1, is filtered by the filter Fd1 associated with this mixer. The bandwidth of the associated filter Fd1 is adjusted to allow the six frequencies obtained by additive synthesis # from the first and second group of frequencies to pass. The output signal of this first filter is supplied on the first input of a second mixer M62, the second input of which receives the frequency of the third group which has been selected by the third switch Cd2. The output of this second mixer is amplified by the amplifier Ad2 then filtered by the filter Fd2 associated with this mixer. The bandwidth of this filter is adjusted to allow the ten different frequencies obtained by additive synthesis from the six output frequencies to pass. of the first filter and of the two frequencies of the third group. These
f0 ten frequencies are the frequencies between 108 10 and 117 10 in steps of 10 For frequencies 110 f0 and 115 10} there are two configu
Vo For the frequencies 110 Vo and Vo 'rations possible of the positions of the switches allowing to obtain these frequencies. It is thus possible to use a C # oa switch
J inputs and add additional stages identical to the first two. The stage of index k will therefore consist of a Cak switch with two inputs, a mixer M # k, an amplifier # Adk and a filter F6k. The output of the F6K filter of the last stage will then constitute the output signal of this fine-pitch chain allowing to provide
L successive frequencies. it is then necessary, within the framework of this synthesizer, to replace, in the expressions of the input frequencies of the first three groups of frequencies, the value f0 by a value suitably chosen. Additional frequencies to this new
f no df = must, in addition to these new values, be provided
L by the frequency standard.

 FIG. 3 represents the final synthesis chain CHF.

This comprises two inputs with large pitch El, E2 receiving, from the chain with large pitch Q, the output signals supplied respectively on the outputs S1, S2 of this chain, and an input with fine pitch E6 receiving the output signal of the chain is not fine. This chain further comprises two mixers M1, M2, the first M1 receiving the low frequency input E1 and the second M2 receiving the higher frequency input E2.

These mixers receive, on the other hand, on their other input, each one of the outputs of the two-position switch C5, the input of which is connected to the output of the fine-pitch chain. The selection of the input at low frequency or of reentry at higher frequency is carried out starting from the switch C5 of this chain and by filtering. Indeed, this input switch selects the mixer, MI or M2, which will receive the frequency at the end f6 of the chain at the end 6. The other mixer receiving nothing on its second input will therefore transmit on the filter which follows the coarse frequency received on its first input. Filters F1 and F2 are adjusted to allow all the frequencies corresponding to the additive synthesizers to pass between the frequencies with fine pitch and respectively the low frequencies with large step and the higher frequencies with step big, and only those frequencies. The mixer not selected by the input switch will therefore provide, at its output filter, a signal at a frequency lower than the lowest frequency of the passband of this filter, regardless of the frequency at large pitch . This signal will therefore not be transmitted, at least without attenuation.

9 10
Après passage dans un isolateur li, I2, les signaux à fréquence basse et plus élevée sont envoyées dans un diviseur de puissance D3 qui dédouble le signal fourni, comportant 10 NM raies distinctes, en une voie pilote et une voie oscillateur local décalée de la fréquence inter médiaire fi. Ce décalage en fréquence est obtenu au niveau des derniers mélangeurs MOL, N .Le signal fourni par le diviseur de puissance D3 au mélangeur MOL de la voie oscillateur local, après son amplification dans l'amplificateur A3, est mélangé au signal à fréquence fl, associé à ce mélangeur et élaboré par le standard de fréquence. Le signal fourni par le diviseur de puissance au mélangeur HP de la foie pilote, après son amplification dans l'amplificateur A4, est mélangé au signal à fréquence f1 + fi, associé à ce mélangeur et élaboré par la chaîne de modulation. il contient les éventuelles modulations de phase et d'amplitude utiles a la compression d'impulsions et est obtenu par synthèse additive a partir d'un signal a la fréquence f1 et d'un signal a la fréquence intermédiaire fi, tous deux élaborés par le standard de fréquence.Ce signal à fréquence f1 + fi est élaboré par une chaîne de modulation telle celle représentée à la figure 4.In the more general case of a large pitch chain with M outputs, there will be a switch C5 with M outputs and M mixers M1 a #k each followed by a filter. The filter Fm of index m will have a width of band Nfn centered on the frequency

9 10
After passing through an isolator li, I2, the signals at low and higher frequency are sent in a power divider D3 which splits the signal supplied, comprising 10 NM distinct lines, into a pilot channel and a local oscillator channel offset by the frequency intermediate fi. This frequency offset is obtained at the level of the last mixers MOL, N. The signal supplied by the power divider D3 to the mixer MOL of the local oscillator channel, after its amplification in the amplifier A3, is mixed with the signal at frequency fl, associated with this mixer and developed by the frequency standard. The signal supplied by the power divider to the HP mixer of the pilot liver, after its amplification in the amplifier A4, is mixed with the signal at frequency f1 + fi, associated with this mixer and produced by the modulation chain. it contains the possible phase and amplitude modulations useful for pulse compression and is obtained by additive synthesis from a signal at the frequency f1 and a signal at the intermediate frequency fi, both developed by the frequency standard.This signal at frequency f1 + fi is produced by a modulation chain like the one shown in Figure 4.

de largeur de bande MNfo centrée sur + 107,5 ffio Le signal de sortie du
10 mélangeur Mp de la voie pilote est filtré par le filtre Fp de bande MNF0 centrée sur la fréquence

The output signal of the MOL mixer of the local oscillator channel is filtered by the filter F01 the frequency fl + c +

MNfo bandwidth centered on + 107.5 ffio The output signal from the
10 Mp mixer of the pilot channel is filtered by the Fp filter of band MNF0 centered on the frequency

width of
fg + 107.5 10.
The two chains are then conventionally made up of an amplifier AS, A6, a limiter LOL, LP and an isolator I3, i 4
The modulation chain represented in FIG. 4 comprises a modulation input Em making it possible to control a phase modulator 0, w M #, a high frequency input, receiving from the frequency standard a signal at frequency f1 and which constitutes the input of the mixer Mm of the chain, and an intermediate frequency input receiving an intermediate frequency signal f. through an isolator Im. It is the mixer Mm which makes it possible to obtain the signal at the frequency f1 + f. after filtering by the narrow band Fm filter centered on this sum frequency. The output signal of this filter is then modulated in phase by the phase modulator M #, then amplified by the amplifier Am and modulated in amplitude by the gate P controlled by modulation pulses received on the input Ei. The signal on the Sm output can then be used as a signal at the frequency ~ l + f.

for the pilot channel Mp mixer.

 FIG. 5 shows the frequency standard Sf controlled by the reference generator G and supplying all the frequencies necessary for the three synthesis chains.

 Although the present invention has been described in the context of a particular embodiment, it is clear that it is not limited to said example and that it is capable of modifications or variants without departing from its field.

Claims (7)

 1. Direct synthesizer frequency synthesizer comprising a final synthesis chain capable of providing on its pilot output LMN different frequency values and on its local oscillator output LMN other lower or higher frequency values, offset by the intermediate frequency f. with respect to the frequency supplied on the pilot output, comprising a coarse pitch chain which can supply MN frequency values and a fine pitch chain which can successively supply L different frequency values, these elements being such as switching, from one any of the frequencies supplied to another, is done quickly, and further comprising a frequency standard, controlled by a single reference oscillator supplying a reference frequency fg, this frequency standard supplying the chain continuously and simultaneously final synthesis, to the coarse step chain and to the fine step chain all the useful frequencies, characterized in that this coarse step chain (A) comprises first means (AH1, GH, D1, FHI to FHN, Cl , AH2) to develop, from the reference frequency f0, a set of N frequencies at the step Af 1 = #O and to select one of these frequencies, and second means (D2, A1, A2, MA1, MA2, Fbl, FA2) including M sor S2) which can each successively supply N different frequency values at the step Af1 = ~ 0, the M frequencies simultaneously supplied on these M outputs being offset from one another by the frequency difference Af2 = Nfo and these different frequencies being developed from the frequency selected by the first means, in that the fine-pitch chain (.6) comprises K similar stages of indices k = 1 to s connected in cascade, each receiving two predetermined frequencies and mixing the output signal of the preceding stage with one of these predetermined frequencies, and further comprises input selection means for applying to the input of the first stage a frequency selected from several predetermined frequency values, the values of all these predetermined frequencies supplied to this chain being chosen so that the output of the Kth stage, which constitutes the output of the chain with fine pitch, can supply L frequencies  not even 6f = different spaced by the same pitch df = L0 included in a frequency band of width fg, and in that the final synthesis chain (CHF) comprises third means (M1, M2, C5, F1, F2 , li, 12, D3) to supply identical signals on two outputs, at a frequency chosen from 1MN frequencies spaced apart by the step #f, from the M output signals supplied on the M outputs (S1, S2) of the large pitch chain (A) and of the output signal of the fine pitch chain (6), a local oscillator channel (A3, MOL, FOL, AS , - LOL, I3), connected to the first output of the third means for supplying, by mixing with a frequency f1, the LMN output frequencies of this local cilator channel and a pilot channel (A4, Mp, Fp, A6, LP , 14), connected to the second output of the third means, to supply, by mixing with a frequency f1 + fi, the LMN output frequencies of this pilot channel.  2. Direct synthesizer frequency synthesizer according to claim 1, characterized in that the first means of the coarse pitch chain (#) comprise an amplifier (AH1) receiving from the frequency standard a signal at the frequency # nce of reference f0 and supplying an amplified signal to a harmonic generator (GH) connected to a power divider (D1) with N outputs, on each of these outputs a battery (FR) of N filters ("1 to F #) selecting N contiguous harmonics , the outputs of these N filters being connected to the inputs of a switch (C1) with N inputs making it possible to select one of these harmonics, this harmonic then being amplified in an amplifier (AH2) before being supplied to the second means, and in that these second means include a power divider (D2) at M outputs receiving from the first means the selected harmonic and supplying on each of its outputs this harmonic to H mixers (M # 1, MA2) after it has been amplified by M amplifiers (A1, A2), each preceding a mixer, these mixers receiving on the other hand, from the frequency standard, respectively signals at frequency f ~ 1 = f, fA2 = f # + Nf0, ... f # m = f # ...

 f = C + MNfo and being each connected, through a filter (FA1, F # 2) of passband (Nf0), to a corresponding output (S1, S2) of the chain with large pitch, the filter of index m being centered on the frequency J entries.  3.Frequency synthesizer with direct synthesis according to one of claims 1 or 2, characterized in that each stage of the fine pitch chain comprises a mixer (M61, M62) receiving from the stage which precedes the frequency provided by this stage, and a switch (C61, C62) with two inputs, supplying the mixer, one of the two predetermined frequencies present at the input of this switch and further comprising, at the output of this mixer, an amplifier ( Ad1, A62) followed by a filter (Fd1, F62) whose output constitutes the output of this stage of index k, and in that the input selection means consist of an input switch (C60 ) at  4. Direct synthesis frequency synthesizer according to claim 3, characterized in that the fine pitch chain comprises two stages and an input switch (C60) with three inputs, this input switch receiving the frequency fO frequencies ,  f0 f0 f + 1, fo + 2 1, the first stage switch (C61) receiving the  10 'O 10'  f f0 frequencies 50 10 and 55 1 and the second stage switch (cl2)  10 10  f0 f0 receiving the frequencies 48 fO and 50 Vo  5.Frequency synthesizer with direct synthesis according to any one of claims 1 to 4, characterized in that the third means, forming part of the final synthesis chain, comprise N inputs with large pitch (E1, E2), each connected at an output (S1, S2) of the coarse pitch chain, these N inputs each being the input of a mixer (M1, M2), these mixers receiving, on their other input, one of the outputs of a switch ( C5) at N outputs, the input of this switch, which constitutes the fine pitch input of this chain, being connected to the output of the fine pitch chain (6), in that each of these mixers is followed by a filter (F1, F2) which authorizes the passage of the output signal from its mixer only if the latter has carried out the additive mixing of its coarse pitch frequency received with the fine pitch frequency, and in that the N outputs of these filters are combined in a power divider (D3) after passing through an isolator (I1, I2). 12).  6. Frequency synthesizer with direct synthesis according to any one of claims 1 to 5, characterized in that the local oscillator channel, receiving one of the two outputs of the third means, comprises, at the input, an amplifier (A3) followed a mixer (MoL) this mixer receiving at its second input a signal at frequency f 1 coming from the frequency standard (Sf), this mixer being followed by a filter (FOL) letting only the frequency LMNs obtained only by additive synthesis , this filter itself being followed by an amplifier (A5), then a power limiter (LOL) and an isolator (13), and in that the pilot channel, receiving the other output of the third means, comprises, at the input, an amplifier (A4) followed by a mixer (#), this mixer receiving at its second input a signal at frequency f1 + f. provided by a modulation chain (CHM), this mixer being followed by a filter (Fp) letting only pass the frequency LMNs obtained only by additive synthesis, this filter itself being followed by an amplifier (A6), then by a power limiter (LP) and an isolator (I4).  7. Frequency synthesizer with direct synthesis according to claim 6, characterized in that the modulation chain comprises a mixer (Mm) receiving from the frequency standard (Sf) a signal at frequency flet a signal at frequency f. previously passing through an isolator (Im), this mixer being followed by a bandpass filter (Fm) allowing only the sum frequency f1 + fi to pass, this modulation chain further comprising a phase modulator 0, r (Mm) receiving filters (Fm) the signal at frequency f1 + f. and comprising a modulation control input (Em), this modulator supplying a phase modulated signal to an amplifier (Am) followed by an amplitude modulator (P) comprising a modulation control input (Ei) and the output of which constitutes the output (Sm) of the modulation chain.