Abstract:
A low-noise sampling PLL for high-resolution radar systems wherein the reference frequencies are taken from a line spectrum (SPK), which has been generated from the mixture of a low quartz oscillator frequency (F 1 ) with a high quartz oscillator frequency (F 2 ), with the quartz oscillator frequencies (F 1 , F 2 ) being synchronized. The invention is employed in connection with sampling PLLs for high-resolution radar systems with a controllable frequency oscillator, whose output signal is partially coupled out into a feedback branch and compared with reference frequencies in a phase comparator.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority of German Application Ser. No. 197 48 608.8, filed Nov. 4, 1997, which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The invention relates to a sampling Phase Locked Loop (PLL) for high-resolution radar systems, having a controllable frequency oscillator, whose output signal is partially coupled out into a feedback branch and compared with reference frequencies in a phase comparator, for whose generation at least one quartz oscillator is provided. 
     BACKGROUND OF THE INVENTION 
     In connection with high-resolution radar systems, it is often not possible to employ PLLs with a frequency divider in the feedback branch. The divider noise occurring in such PLLs too greatly impairs the frequency stability of the output signal. 
     In another known embodiment variant of a PLL which offers an improvement in regard to phase noise at the PLL output, the phase comparison is performed on a defined harmonic of the reference signal. The increased technical outlay for this variant makes possible the transfer of the quartz oscillator stability, which can be realized, to the output signal of the PLL even for the frequency multiplications. In this variant the selection of a defined upper harmonic wave, in particular in connection with large multiples of the reference frequency, takes place indirectly by the PLL. The latter can be considered to be a bandpass filter with a bandwidth corresponding to the bandwidth of the control loop. The selection of a defined frequency is performed by a pre-tuning process which takes place, for example, by a conventional digital control loop with a frequency divider described at the outset. When using a YIG oscillator with a digital driver, the preselection can take place by means of the driver. The actual control of the PLL takes place via the fine tuning of the YIG oscillator. 
     The reference frequency of such a sampling PLL is determined by the required step size. Therefore very large multiplication factors at the reference oscillator are required for generating high output frequencies. Besides the problem of achieving a sufficiently high output yield at high multiples of the reference frequency, which is necessary for maintaining a sufficient distance from the thermal noise, each doubling of the reference frequency raises the phase noise by 6 dB. Therefore, even with this concept the frequency stability is impaired, if high output frequencies are demanded with small step sizes in high-resolution radar systems. 
     It therefore is the object of the invention to provide a low-noise sampling PLL for high-resolution radar systems. 
     SUMMARY OF THE INVENTION 
     The above object generally is achieved according to the present invention by a sampling PLL for high-resolution radar systems, which has a controllable frequency oscillator, whose output signal is partially coupled out into a feedback branch and compared with reference frequencies in a phase comparator, with the reference frequencies being taken from a line spectrum (SPK), which has been generated from the mixture of a low quartz oscillator frequency (F 1 ) with a high quartz oscillator frequency (F 2 ), and with the quartz oscillator frequencies (F 1 , F 2 ) being synchronized. 
     Further embodiments and modification of the invention are disclosed. 
     The invention makes advantageous use of a property of quartz oscillators which is, that it is possible to realize quartz oscillators with approximately the same phase noise performance in a frequency range between 5 MHZ to 100 MHZ. Based on this property, in accordance with the invention, the reference signal for the sampling PLL is generated in the form of a line spectrum by means of frequency mixing. It is possible in this way to generate high output frequencies with small multiplication factors. When using the attainment of the object in accordance with the invention, a clear improvement in stability of the output signal occurs because of the given square dependency of the phase noise on the multiplication factors. 
    
    
     Exemplary embodiments of the invention will be explained in greater detail by means of the drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic block circuit diagram representing a sampling PLL with reference frequency generation in accordance with the prior art, 
     FIG. 2 is a schematic block circuit diagram of a circuit for generating a reference frequency for the PLL of FIG. 1 by multiplication of a line triplet according to a first embodiment of the invention. 
     FIG. 3 is a schematic block circuit diagram of a circuit for generating a reference frequency for the PLL of FIG. 1 which is created by the mixing of a comb spectrum with a higher single frequency according to a further embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The sampling PLL represented in FIG. 1 consists of a controllable oscillator  1 , a quartz oscillator  2 , a multiplier  3 , a mixer  4 , an outcoupler  5  and a low pass filter  6 . 
     In this sampling PLL, the phase comparison takes place in a known manner on a predetermined upper harmonic F 3  of the reference signal REF. To this end, the reference signal REF passes through the multiplier  3  prior to being fed to the mixer  4  for the phase comparison with an outcoupled portion of the output signal of the oscillator  1 , and thus of the PLL. Following filtering in the low pass filter  6 , the output signal of the mixer  4  is used as a corrective signal for the controllable oscillator  1 . This oscillator  1  is a voltage-controlled oscillator VCO. But a YIG oscillator, for example, could also be used for the oscillator  1 . 
     To improve the phase noise at the output of a sampling PLL embodied in this way, in accordance with the invention the reference generation is altered in such a way that large multiplications are avoided here. Two variants or embodiments are provided for attaining the object on which this is based. With both embodiments, the desired respective reference frequencies used for the comparison are taken from a line spectrum generated from a mixture of two quartz oscillator frequencies. The embodiment according to FIG. 2 requires little hardware outlay, along with possibly potential limitations regarding the completeness and uniformity of the generated line spectrum. The embodiment according to FIG. 3 is a more elaborate method and apparatus and provides a complete and uniform line spectrum. 
     The reference frequency generation circuit represented in FIG. 2 replaces the reference signal generation portion of the sampling PLL represented in FIG. 1, i.e., the oscillator  2  and the multiplier  3 . The reference frequency signal generation circuit in accordance with the invention consists of a quartz oscillator  7 , a quartz oscillator  8 , a mixer  9 , an amplifier  10 , a step recovery diode multiplier  11  and a bandpass filter  12 . 
     The quartz oscillator  7  generates a low quartz oscillator frequency F 1  of a value of, for example, 5 MHZ, and the quartz oscillator  8  generates a high oscillator frequency F 2  of a value of, for example, 100 MHZ. It is also possible to employ frequencies F 1  and F 2  with values differing from these for this purpose, if a sufficient short time stability (phase noise performance) can be realized. The frequencies F 1  and F 2  are supplied to the mixer  9 , which, in a known manner, generates a line triplet TRI therefrom. 
     To maintain the coherence between the individual lines, it is necessary for the two quartz oscillators  7  and  8  to be synchronized as indicated. Circuit arrangements for synchronizing the two frequencies F 1  and F 2  are known to those skilled in the art and have not specifically been represented in FIG. 2 for reasons of showing a clear representation of the embodiment in accordance with the invention; for example, both quartz oscillators  7  and  8  can be included in a separate PLL. In this connection, the narrowest loop bandwidth should be attempted, so that uncorrelated side bands are present even with low offset frequencies. It would otherwise be possible that in place of output additions, voltage additions could occur in the mixer  9 , which in turn could result in a loss of maximally 3 dB of the phase noise. 
     The line triplet TRI is sufficiently amplified in the amplifier  10  and supplied to the step recovery diode multiplier  11 , which generates, in a known manner, a line spectrum SPK of its output. It might be necessary to limit the edges of the line spectrum by a downstream-connected bandpass filter  12 . The mostly continuous distribution of the line spectrum SPK generated in this manner can be explained by the generation of higher order inter-modulation products. This can be proven by a sort of “FM marking” of a source. The effective degrees of multiplication result from this in turn, which for their part determine the phase noise properties. The line spectrum SPK is then fed to a line selection circuit  19 , which in a known manner, selects the desired frequency line as a reference frequency and supplies same to the mixer  4 . 
     The upper and the lower side bands of the line triplet TRI generated in this manner have different phase positions. In connection with some combinations of the quartz oscillator frequencies F 1  and F 2 , this can result in individual lines in the spectrum being nearly eradicated, because of which the completeness and uniformity of the line spectrum SPK can be impaired. 
     The reference frequency generation circuit in accordance with FIG. 3 again consists of the two quartz oscillators  7  and  8 , the mixer  9  and the bandpass filter  12  at the output. Different from the previously described embodiment in accordance with FIG. 2, the synchronized quartz oscillator frequencies F 1  and F 2  are not directly supplied to the mixer  9  for generating a line triplet TRI. 
     Rather, with this embodiment, a comb spectrum KAM 1  is generated from the low quartz oscillator frequency F 1 , which spectrum KAM 1  is supplied to the mixer  9  together with an upper harmonic F 3  of the high oscillator frequency F 2  in order to generate the line spectrum SPK. To generate the comb spectrum KAM 1 , following an amplification of the low quartz oscillation frequency F 1 , the comb spectrum KAM 1  is generated from the latter by means of a step recovery diode multiplier  16  and a downstream-connected low pass filter  14 . The upper limit of the comb spectrum KAM 1  should extend at least to F 2 / 2 . The upper harmonic F 3  to be fed to the mixer  9  is filtered out of a comb spectrum KAM 2 , which has been generated by a step recovery diode multiplier  15  from the high quartz oscillator frequency F 2  after being amplified by an amplifier  18 , by a bandpass filter  13 . 
     With a low oscillator frequency F 1  of, for example, 5 MHZ, a high oscillator frequency F 2  of, for example, 100 MHZ, an upper limit of the comb spectrum KAM 1  at, for example, 50 MHZ, and a multiplication of the high oscillator frequency F 2  by a factor of, for example, 10, it is possible in accordance with the embodiment of FIG. 3 to generate a line spectrum between 950 MHZ to 1050 MHZ with a step width of 5 MHZ. At the effective multiplication of 10, the phase noise would be increased by maximally 20 dB. If with the prior art in accordance with FIG. 1 it were intended to generate a line spectrum with a step width of 5 MHZ at 1 GHz, it would be necessary to multiply the 5 MHZ reference by a factor of 200, and therefore the phase noise would be increased by 46 dB. With the embodiment according to FIG. 3 of the invention, the phase noise performance is thereby improved by at least 20 dB. 
     If a line spectrum which is wider than F 2  is required, the bandpass filter  13  must be switchable, so that adjoining upper harmonics F 3  can be supplied to the mixer  9 . 
     The specific manner of selecting the individual frequency lines from the line spectrum SPK for performing the phase comparison in the mixer  4  of the sampling PLL has not been specifically addressed in the specification and the drawings, but is shown only schematically of 19 since any arbitrary means for this are available to one skilled in the art. 
     The invention now being fully described, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the invention as set forth herein.