Instantaneous Frequency Measurement, IFM, is an integral part of any Electronic Countermeasures System, ECM, that is required to counter a threat whose effectivity is dependent on Pulsed emissions. Most often, these threats take the form of pulsed radars. Since modern radars tend to frequency hop, it is difficult for an ECM system to know, a priori, what is the frequency of the incoming pulse. This knowledge becomes crucial where set-on jammers or non-coherent repeaters must be quickly slewed to the incoming pulse frequency. Even where Digital RF Memories, DRFMs, are utilized, they tend to be narrow band and it is necessary to accurately determine frequency in order to select the correct up conversion frequency. The preferred implementation of the invention is stated for an application requiring IFM over the 2-GHz frequency range with an acquisition time of 100 nanoseconds although the invention is generally applicable to other frequency ranges and acquisition times. The frequency range of 2-6 GHz is a common IF frequency for ECM systems. Typically these systems take the entire microwave spectrum of interest and convert it to 4 GHz spans which are converted to the 2-6 GHz range. 100 nanoseconds is the length of a typical short Radar pulse.
As a result of the importance of IFM, a number of techniques have been previously developed to help localize the incoming frequency. The description of the prior art which follows enumerates the main approaches to achieving IFM as previously implemented.
Ratio Measurements
In this type of frequency measurement, the carrier frequency is compared to a reference clock frequency to determine the ratio between two frequencies. This is accomplished by counting how many complete cycles of one frequency fit within one complete cycle of the other, all partial cycles discarded. It can be shown that by utilizing this method the frequency measurement resolution is limited to the inverse of the pulse width, or as in the case of 100 nanoseconds, to 10 MHZ. This is due to the fact that only complete cycles are counted. This technique is equivalent to determining the period of the frequency by counting the number of cycles within a given time period. This is the classical method employed by commercial frequency measuring instruments.
Correlators
This type of frequency measurement equipment is based on a correlator or an arrangement of a power splitter, 1, a fixed length delay line, 2, and a mixer, 3 as shown in FIG. 1. The signal RF in is split and then mixed with itself delayed by the delay, xcfx84, causing a phase shift xcex94"psgr".                               Correlator          ⁢                      xe2x80x83                    ⁢          as          ⁢                      xe2x80x83                    ⁢          a          ⁢                      xe2x80x83                    ⁢          frequency          ⁢                      xe2x80x83                    ⁢          discriminator                ⁢                  
                ⁢                  MixerOutput          =                                                    sin                ⁡                                  (                                      2                    ·                    π                    ·                    Fin                                    )                                            ·                              xe2x80x83                            ⁢              sin                        ⁢                          xe2x80x83                        ⁢                          (                                                2                  ·                  π                  ·                  Fin                                +                Δφ                            )                                      ⁢                  
                ⁢                              Choose            ⁢                          xe2x80x83                        ⁢            delay            ⁢                          xe2x80x83                        ⁢            Δφ            ⁢                          xe2x80x83                        ⁢            such            ⁢                          xe2x80x83                        ⁢            that            ⁢                          xe2x80x83                        ⁢                                          π                2                            ·                              xe2x80x83                            ⁢                              Fin                fo                                              =          Δφ                ⁢                  
                ⁢                  MixerOutput          =                                                    1                2                            ·                              [                                                                            sin                      ⁡                                              (                                                  2                          ·                          π                          ·                          Fin                                                )                                                              ·                                          xe2x80x83                                        ⁢                    sin                                    ⁢                                      xe2x80x83                                    ⁢                                      (                                                                  2                        ·                        π                        ·                        Fin                                            +                                                                        π                          2                                                ·                                                  Fin                          fo                                                                                      )                                                  ]                                      =                                                            1                  2                                ·                                  cos                  ⁡                                      (                    Δφ                    )                                                              +                              cos                ⁡                                  (                                                            2                      ⁢                      Fin                                        +                    Δφ                                    )                                                                    ⁢                  
                ⁢                  MixerOutput          =                                    cos              ⁡                              (                                                      π                    2                                    ·                                      Fin                    fo                                                  )                                      =                                          sin                ⁡                                  (                                                                                    π                        2                                            ·                                              Fin                        fo                                                              -                                          π                      2                                                        )                                            ≈                              const                ·                                  (                                                            Fin                      fo                                        -                    1                                    )                                                                                        Eq        .                  xe2x80x83                ⁢        1            
The actual signal is obtained by only considering the low frequency component at the output of the mixer, cos(xcex1). At the frequency for which the delay line equals exactly a quarter wave length, xcex1=xc2xdxcfx80, the output of the delay lines is in quadrature to the non delayed input and thus the output of the mixer will be zero volts. As the input frequency is moved up or down, the output of the mixer is a DC voltage which can be shown to vary as sin(xcex94"psgr"), here xcex94"psgr" is the phase difference through the delay line. This voltage is digitized and used as a means to indicate the input frequency based on the fact that this system is basically a frequency discriminator. The shorter the delay, the wider the frequency range of unambiguous output. The corollary to this is that the sensitivity is low. Longer delay lines suffer from ambiguity but have greater sensitivity to frequency change. This problem can be avoided by combining the outputs of several of these correlators as shown in FIG. 2. Each correlator, M0 through M4 employs a delay of twice the prior correlator and hence twice the frequency resolution. Shorter delay correlators provide gross frequency resolution and resolve ambiguities of longer delay correlators in a fashion often analogized to a xe2x80x9cgas metersxe2x80x9d. The digitizer, 5, encodes the amplitudes from each correlator when the threshold signal, 6, is received. The digitizer in conjunction with the PROMs 7-9 resolve frequency alignment issues and provides a binary representation of frequency, 10, over a broad range.
The frequency accuracy of a correlator based IFM is ultimately dependent on the transfer properties of the mixers used in the correlators. Variations in DC offset as well as slope due to mixer variations, parameter shift over temperature, and characteristic changes over frequency and input power greatly reduce the accuracy. Typical systems covering an input frequency range of 2-6 GHz can be expected to have no better than 5 MHZ of accuracy assuming that temperature is stabilized and also that the power level of the input signal is constrained to within a 1 dB range.
Acousto-Optic Methods
An example of this category is the Bragg cell which deflects the light generated by a laser into an array of photo diodes. The magnitude of the deflection depends on the input frequency, and thus by detecting which diode of the array is illuminated one can determine the input frequency. The frequency span of such a cell is limited and the frequency resolution depends on the number of photo diodes in the array. But as the number of diodes increases, their size and decreases and the required illumination time increases, which make them impractical. Frequency resolution is a function of the number of discrete photo cells; the beamwidth of the laser beam, and the distance of the photo cells from the Bragg cell. In general, this technology is not practical for resolving the frequency accuracy or resolution to meet the stated requirement.
Dispersive Delay Lines
A common technique for determining the frequency of pulsed signals is the use of surface acoustic wave dispersive delay lines. The frequency of the pulse is determined by the time delay through the dispersive delay line. It is this inherent property that results in this technology not being applicable to modern day requirements. Typical delay times are in the microsecond region such that it would take considerable amount of time to make a frequency detemination. Additionally, SAW devices tend to be narrowband.
Other Methods
Other methods to measure frequency exist but have obvious problems. Examples are filter banks which are bulky and lack resolution; stepped synthesizers which are slow; or bulk dispersive delay lines which suffer the same difficulties as surface dispersive delay lines.
Emerging ECM systems require an IFM which is capable of measuring the carrier frequency of pulsed signals, with a frequency resolution of about 1 MHZ and a small size as the prime concern. At this time, there is no small sized technique for IFM within the stated requirements of an ECM system. The closest technique is that of using binary correlators but this ultimately lacks the required accuracy.
It is an object of this invention to provide a new method of Instantaneous Frequency Measurement which will overcome the deficiencies in the prior art IFMs. The new method is based on extracting the instantaneous phase of a received signal, and obtaining the instantaneous frequency by differentiating the instantaneous phase over time:                               Definition          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          frequency                ⁢                  
                ⁢                              Frequency            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            a            ⁢                          xe2x80x83                        ⁢            signal            ⁢                          xe2x80x83                        ⁢            E                    =                      sin            ⁡                          (                              Φ                ⁡                                  (                  t                  )                                            )                                      ⁢                  
                ⁢                  is          ⁢                      xe2x80x83                    ⁢          given          ⁢                      xe2x80x83                    ⁢                      by            :                          xe2x80x83                        ⁢                                          1                                  2                  ⁢                  π                                            ⁢                                                ⅆ                  Φ                                                  ⅆ                  t                                                                                        Eq        .                  xe2x80x83                ⁢        2            
It is a further object of this invention to provide a means to measure frequency which much faster than the methods of the prior art provides a higher frequency resolution and accuracy and is independent of device characteristics.