Method for Eliminating Subharmonic False Locking in Sampler and Frequency Multiplier Based Source Locking Systems

1. Field of the Invention

The present invention relates to vector network analyzers in general and in particular to a method for eliminating subharmonic false locking in a sampler and frequency multiplier based phase-locked-loop signal source locking system which is used for generating an r.f. signal for use in such analyzers.

2. Description of the Prior Art

Vector network analyzers require an accurate and stable signal source to generate the r.f. signals used for testing and analyzing the electrical characteristics of an electrical circuit. When being analyzed the circuit is commonly called a device-under-test or DUT.

A sampler based source locking system, of the type used as a signal source in a vector network analyzer, allows an open loop source with a maximum frequency error of .+-.F.sub.error to be phase-locked to a more accurate and stable reference signal. For example, in a typical prior known sampler based source locking system there is provided a source comprising a voltage-controlled oscillator (VCO) having an output coupled to a frequency multiplier for providing an output signal having a nominal fundamental frequency F.sub.s. The output of the source is coupled to a sampler which is driven by a first local oscillator having a frequency F.sub.L01. On the output of the sampler there is produced a first intermediate frequency signal having an intermediate frequency IF.sub.1 which is equal to the difference between the frequency of the source F.sub.s and a harmonic frequency or comb line of the first local oscillator. For convenience, the harmonic frequency is often defined by a harmonic number (H). The harmonic number H is the integer by which the frequency F.sub.L01 of the oscillator is multiplied to obtain the harmonic frequency, e.g. 531.75 MHz.times.H=60087.75 MHz, where H=113.

The first intermediate frequency signal IF.sub.1 is then fed to a mixer and mixed with the output of a second local oscillator to produce a second intermediate frequency signal having an intermediate frequency IF.sub.2. The output of the mixer is then fed to a phase detector along with a reference signal having a frequency F.sub.ref. The output of the phase detector, which comprises an error signal having a magnitude corresponding to the difference between IF.sub.2 and F.sub.ref, is then used to control the frequency of the VCO so as to maintain the second intermediate frequency IF.sub.2 at F.sub.ref.

Alternatively, the mixer stage may be omitted. In this event the first intermediate frequency IF.sub.1 is compared in the phase detector with an appropriate alternative reference frequency F.sub.ref to maintain the frequency IF.sub.1 at the alternative reference frequency F.sub.ref.

Heretofore it has been the practice to select for the first local oscillator the lowest usable harmonic number (H) and the highest usable local oscillator frequency F.sub.L01 in order to maximize sampler efficiency and minimize local oscillator phase noise due to frequency multiplication. A typical algorithm which has been used for this purpose is as follows

wherein

INT is the integer value of the parenthetical terms associated therewith

F.sub.s is the fundamental output frequency of the source

H is the harmonic number of the first local oscillator

F.sub.L01 and F.sub.L02 are the frequencies of the first and second local oscillators

IF.sub.1 is the first intermediate frequency and

F.sub.1, Z, A and K are parameters used in calculating the above quantities

Using the above algorithm and assuming a fundamental source frequency F.sub.s of 60 GHz, i.e. 60000 MHz, the lowest harmonic number H, the highest first and second local oscillator frequencies F.sub.L01 and F.sub.L02, respectively, and the first intermediate frequency IF.sub.1 in megahertz which can be used to lock the source within its search range of .+-.F.sub.error, typically .+-.60 MHz, is calculated as follows:

If the source uses a frequency multiplier to generate the final r.f. energy, as many of them do, its output will also contain subharmonic frequencies which will mix with lower harmonic comb lines of the sampler. This can result in subharmonic false locking of the phase-locked-loop. For example, a source comprising a frequency tripler having a nominal fundamental output frequency F.sub.s of 60 GHz, i.e. 60000 MHz, a maximum frequency error signal, i.e. search range, of .+-.60 MHz, a first local oscillator frequency F.sub.L01 of 531.75 MHz and a first intermediate frequency IF.sub.1 of 87.75 as calculated above, can, during searching for a lock, shift through a frequency range of from 59540 MHz to 60060 MHz. When this is done, it has been found that a false lock can occur at an output frequency F.sub.s of 59953.5 MHz producing a subharmonic false lock error of 46.5 MHz which is clearly within the permissible search range, i.e. .+-.60 MHz of the source. This is due to the fact that the second subharmonic of the source, i.e. 39969 MHz, will mix with the 75th harmonic, i.e. H=75, of the first local oscillator, i.e. 39881.25 MHz, in producing the intermediate frequency IF.sub.1 of 87.5 MHz.

In view of the foregoing, a principal object of the present invention is a method for eliminating subharmonic false locking in a sampler and frequency multiplier based phased-locked-loop source locking system having a first local oscillator which is operated at a frequency such that the allowable source search range before subharmonic false locking occurs can be significantly increased.

In practice, the maximum source search range to guarantee proper locking must be equal to or greater than 2.times.F.sub.error, where .+-.F.sub.error is the maximum open loop frequency error of the source.

In accordance with the present invention, to maximize the allowable source search range before subharmonic false locking occurs, it was found that an initial harmonic number H should be determined using the prior known above-described algorithm or a comparable algorithm which produces the lowest usable harmonic number H and the highest usable local oscillator frequency F.sub.L01 which maximizes sampler efficiency and minimizes local oscillator phase noise. Thereafter, the initial number H as determined by the algorithm is divided by M, where M is the multiplication factor used for the frequency multiplier, e.g. 3. If the remainder of the division step is equal to 0, then H is increased by 1. If the remainder of the division step is equal to 2/3, then H is increased by 2. If the remainder of the division step is equal to 1/3, then the magnitude of H as initially calculated is not adjusted. After H has been calculated and, if necessary, adjusted as described above, it is then used in the remaining steps of the algorithm to calculate a new frequency F.sub.L01 for the first local oscillator as well as the other parameters described.

In this manner, a sampler and frequency multiplier based phase-locked-loop source locking system can be built and operated free of subharmonic false locks over its entire source search range.

The above and other objects, features and advantages of the present invention will become apparent from the following detailed description of the accompanying drawings, in which:

FIG. 1 is a generalized block diagram of a conventional source locking system;

FIG. 2 is a block diagram of a prior known sampler and frequency multiplier based phase-locked-loop source locking system in which the first local oscillator comprises a frequency which produces a lock at the fundamental frequency of the source using the 113th harmonic of the oscillator;

FIG. 3 is a block diagram of the apparatus of FIG. 2 showing how the apparatus of FIG. 2 can operate to produce a subharmonic false lock;

FIG. 4 is a block diagram of a sampler and frequency multiplier based phase-locked-loop source locking system which is being operated in accordance with the present invention; and

FIG. 5 is a block diagram of the apparatus of FIG. 4 which shows the elimination of subharmonic false locking when the first local oscillator is operated at a frequency which has been determined according to the present invention.

Referring to FIG. 1, there is provided in a conventional phase-locked-loop source locking system designated generally as 1 a source 2, comprising a voltage controlled oscillator (VCO) for generating an output signal comprising a fundamental frequency F.sub.s. Coupled to the output of the source 2 there is provided a sampler 3. Coupled to the sampler 3 there is provided a local oscillator 4. The local oscillator 4 drives the sampler 3 by means of a sampling signal having a local oscillator/sampling frequency F.sub.L0. The output of the sampler which comprises an intermediate frequency signal having a frequency IF is compared in a phase detector 5 with a reference signal having a frequency F.sub.ref from a reference signal source 6. In practice, the intermediate frequency IF is equal to the difference between F.sub.s and a harmonic frequency/comb line of the local oscillator 4. The harmonic frequency, as defined above, is the product of the local oscillator frequency F.sub.L0 and the harmonic number (H) of the oscillator 4. A difference between the intermediate frequency IF and the reference source frequency F.sub.ref produces an error signal in the form of an output control voltage V.sub.C on the output of the phase detector 5 which has a magnitude corresponding to the difference between the intermediate frequency IF and the reference frequency F.sub.ref. The control voltage V.sub.C is then used for adjusting the frequency of the source 2 such that the error signal is reduced to zero.

Referring to FIG. 2, there is shown a conventional sampler and frequency multiplier based phase-locked-loop source locking system 10. In the system 10 there is provided a source 11 comprising a source oscillator 12 and a frequency multiplier 13 having a multiplication factor (M) of 3 for tripling the frequency of the oscillator 12. The output of the source 11, which is provided by the frequency multiplier 13, provides an output signal having a fundamental frequency F.sub.s of 60 (GHz) and subharmonic frequencies of 20 GHz and 40 GHz. The output of the source 11 is coupled to a sampler 14. The sampler 14 is provided with a first oscillator 15 having a first local oscillator frequency of 531.75 MHz. The output of the sampler is coupled to a mixer 16 and provides to the mixer 16 a first intermediate frequency (IF.sub.1) of 87.75 MHz. The intermediate frequency IF.sub.1 is equal to the difference between the fundamental frequency F.sub.s and the 113th harmonic of the first local oscillator 15. The intermediate frequency IF.sub.1 is then mixed with a signal from a second local oscillator 17 having a local oscillator frequency F.sub.L02 of 90 MHz for producing a second intermediate frequency IF.sub.2 of 2.25 MHz. The second intermediate frequency IF.sub.2 is then compared in a phase detector 18 with a signal from a reference signal source 19. The reference signal comprises a frequency of 2.25 MHz. The output of the phase detector 18 comprises an error signal V.sub.C having a magnitude corresponding to the difference between the frequency of the reference signal and the magnitude of the second intermediate frequency IF.sub.2 for controlling the source oscillator 12 so as to keep the second intermediate IF.sub.2 at 2.25 MHz.

The lowest usable harmonic number H equal to 113 and the highest usable local oscillator frequency F.sub.L01 of 531.75 MHz was determined using the above described prior known algorithm in order to maximize sampler efficiency and minimize local oscillator phase noise due to frequency multiplication. As pointed out, however, the use of the prior known algorithm permits a subharmonic false lock over the typical search source range of the source 11. As is well known, the maximum source search range to guarantee proper locking must be equal to or greater than 2 times F.sub.error, where .+-.F.sub.error is the maximum open loop error of the source.

Referring to FIG. 3, which comprises the same components as the apparatus of FIG. 2, it will be seen that with a first local oscillator frequency F.sub.L01 of 531.75 MHz there exists a harmonic frequency of 39881.25 MHz having a harmonic number of 75 which when mixed with the second harmonic frequency of the source 11, namely 39969.0 MHz, produces the first intermediate frequency signal IF.sub.1 of 87.75 MHz which in turn produces a subharmonic false lock of the system.

Referring to FIG. 4, there is provided in accordance with the present invention a sampler and frequency multiplier based source locking system in which the harmonic number and the frequency of the first local oscillator 15 is determined in accordance with the present invention from a modification of the previously described prior known algorithm. That is to say, after calculating the harmonic number H which produces the lowest usable harmonic number H and the highest usable local oscillator frequency F.sub.L01 which maximizes sampler efficiency and minimizes local oscillator phase noise as described above, the initial number H as so determined is divided by M where M is the multiplication factor used for the frequency multiplier, e.g. 3. If the remainder of the division step is equal to 0, then H is increased by one. If the remainder of the division step is equal to 2/M, then H is increased by two. If the remainder of the division step is equal to 1/M then the magnitude of H as initially calculated is not adjusted. After the harmonic number H has been calculated and, if necessary, adjusted as described above, it is then used in the remaining steps of the algorithm to calculate a new frequency F.sub.L01 for the first local oscillator as well as the other parameters described above. For example, using the number H=113 as calculated above for the apparatus of FIG. 2, it is seen that the remainder of the division of 113 by the multiplication factor 3 is equal to 372/3. Increasing H by two in accordance with the above-described algorithm as modified by the present invention increases H to 115. Thereafter, using H with a value of 115 and recalculating the parameters Z, A, K and F.sub.L01 as defined in steps 30, 40, 50 and 60 of the prior known algorithm produces a frequency F.sub.L01 of 522.5 MHz, as follows:

With the first oscillator frequency set at 522.5 MHz it can be seen that the 115th harmonic of the first local oscillator frequency will produce an IF frequency IF.sub.1 of 87.5 MHz at the fundamental frequency F.sub.s of 60 GHz. More importantly, as shown in FIG. 5, the frequency of the first local oscillator as thus calculated eliminates a subharmonic false lock within the source search range of .+-.60 MHz.

Referring to FIG. 5, it will be seen that with an F.sub.L01 of 522.5 MHz the closest harmonic lock that can occur is when the source is tuned during a search for a lock up to a frequency F.sub.s of 60217.50 MHz. If this were to occur, the lock would result from the second subharmonic 40145.0 MHz of the source and the 77th harmonic, 40232.50 MHz, of oscillator 15. However, since the frequency of 60217.50 MHz is much more than the maximum source search range to guarantee proper locking, e.g. .+-.60 MHz of the source 11, subharmonic false locking has been eliminated.

From the foregoing, it can be seen that the algorithm of the present invention clearly extends the maximum source search range while at the same time eliminating subharmonic false locks.

While a preferred embodiment of the present invention is described above, it is contemplated that various modifications may be made thereto without departing from the spirit and scope of the present invention. Accordingly, it is intended that the embodiment described be considered only as an illustration of the present invention and that the scope thereof should not be limited thereto but be determined by reference to the claims hereinafter provided and their equivalents.