Digital frequency discriminators (DFD) and instantaneous- frequency-measurement (IFM) receivers are still considered to be the preeminent advanced components for real-time, tactical signal identification and analysis. Widely used in many military electronics systems, they remain the essential component in a variety of applications including airborne and shipboard ESM, ground mobile ELINT collection, defense avionics, and RWR systems. DFDs and IFMs are critical for capturing high-speed pulsed signals over wide instantaneous bandwidths, particularly in dense pulsed-signal environments, as well as to provide digitized data for analysis.

However, as the functionality of military electronics systems has grown more sophisticated over time, the requirements for IFM and DFD subsystems have also changed. Manufacturers are now responding to various new customer requests for a stateof- the-art IFM, or DFD, that exceeds the performance specifications of its predecessor.

The IFM receiver is a fully integrated, wideband electronic warfare (EW) system receiver. The IFM receiver design from Wide Band Systems, Inc. combines a DFD, RF amplitude digitizer (8- or 10-b), and a variety of threshold processing circuits to provide a complete high probability of intercept (HPI) receiver in a “turnkey” package (Fig. 1). These circuits then provide a digital output of RF frequency, RF amplitude, RF envelope pulse width, and time of arrival (TOA) data. The IFM receiver may also include frequency modulation on a pulse (FMOP), pulse modulation on a pulse (PMOP), pulse-on-pulse (POP), continuouswave (CW) detection, and pulse-on-CW (POCW) flag outputs. Typical frequency coverage ranges from as low as 50 MHz to 18 GHz. Also, the IFM receiver offers an instantaneous dynamic range in excess of 70 dB, with absolute accuracies better than 0.5 percent of the RF bandwidth for frequency measurements at customer-specified frequency ranges.

While being an essential component of IFM receiver systems, the DFD can be used in stand-alone configurations for integration into existing EW systems. The DFD, like the IFM, is an integrated microwave/video/digital assembly that provides digital encoding of wideband RF input signal frequency data for pulsed or CW signals. These integrated designs make it possible to meet precise system requirements for bandwidth, frequency accuracy, and resolution. Wide Band Systems, Inc. currently has configurations available that cover a frequency range from 500 MHz to 18 GHz (Fig. 2). They also provide an unambiguous measurement bandwidth in excess of 20 GHz. In addition to encoding of RF frequency, these DFDs can also provide a threshold based on the instantaneous RF signal-to-noise ratio (SNR), error detection, and various flag functions that include signal present, out of band, and FMOP information. They are designed to provide 14-b digital frequency resolution (for mean frequency resolution of 1.25 MHz), with frequency accuracy of 3 MHz RMS and peak error of 15 MHz for input signals from -55 to +15 dBm. The typical RF input VSWR is 2.0:1.

Many DFDs are designed as custom units, based on specific frequency ranges and accuracy requirements. They can make measurements on CW signals as well as on pulsed signals with pulses as short as 100 ns. The minimum input signal-to-noise ratio required for full accuracy is 3 dB, while the maximum RF input range without damage is +17 dBm. The DFDs incorporate an array of 2:1 correlators that provide a high phase measurement margin and allow accurate measurements in the presence of multiple simultaneous signals.

Both IFM receivers and DFDs have been used in a variety of military systems over the years, although the performance of current units far exceeds what was available 20 years ago. Newer units are benefiting from the advancements being offered by integrated circuit manufacturers. Improvements such as ultra-high-speed circuitry, reduction in propagation delays, and overall high-dynamic performance have lead to a significant improvement in real-time RFsignal processing. For example, older IFM and DFD designs were based on analog-to-digital converters (ADCs) that operated at clock speeds to 40 MHz and programmable read-onlymemory (PROM) chips with slow access times. This capability limited the operating clock rate of the receiver to 40 MHz; allowing for capturing of one signal sample every 25 ns limiting the minimum detectable RF pulse width, for 100 percent probability of intercept (POI), to 50 ns.

Upgraded IFM and DFD designs are incorporating ADCs with sampling rates of 100 MHz or better. This improved clock rate permits the receiver to operate at a clock rate of 100 MHz, allowing the receiver to capture one signal sample every 10 ns. This capability, along with the addition of field-programmable gate arrays (FPGA), has improved the detecting of RF signals with 100 percent POI to pulse widths of 30 ns.

While the FPGA accelerates the processing speed at which the receiver can process the digitized RF signals, they carry additional benefits. Another feature that an FPGA brings to the design is enhanced performance of the previous unit’s signal processing. The FPGA technology allows for high levels of performance and functionality from legacy receivers to be designed into an advanced, high-performance FPGA designed receiver. An additional benefit of the FPGA is its capability for “reprogramming.” This offers the flexibility to implement receiver changes or modifications without a lengthy development cycle. The FPGA programmability also permits processing design upgrades in the field with no hardware replacement necessary.

The FPGA brings a new connectivity to receivers that previous units were not able to utilize. Typically, receivers output data in a parallel data word format. Depending on the application, the number of output bits could exceed 80 b. Incorporating FPGA Ethernet MAC tools, provided by manufacturers such as Xilinx, IFM receivers are now being designed with Ethernet capability. IFM receivers at Wide Band Systems, Inc. currently have BASE-T devices, supporting 1-Gb/s Ethernet speeds. The IFM accepts an RF signal as an input and provides parametric signal information via a gigabit Ethernet interface. Pulse descriptor words (PDW), comprised of frequency, amplitude, pulse width, TOA, and associated flags generated by the IFM are grouped into descriptor packets that are embedded into a user datagram protocol (UDP) and transferred from the IFM to the system computer over the Ethernet link. Additionally, the Ethernet link allows the system computer to control and configure the operating modes, sensitivity, and timing functions of the IFM receiver. Having this Ethernet capability, all communications between a system computer and IFM receiver are now available without the need for multiple interface connectors.

SYSTEM UPGRADES
Retrofitting or replacing a military subsystem, such as an IFM, is usually a matter of providing a cost-effective solution while exceeding the RF performance of its predecessor. Additionally, these systems are endangered by obsolescence. While the RF/microwave technology is still capable of processing modern threat signals, manufacturers are leveraging the use of commercial-off-the-shelf (COTS) components to the fullest extent possible, to provide a cost-effective solution while maintaining a high level of performance. Manufacturers are continually looking to upgrade methods for converting the RF envelope. One such improvement that’s being used in Wide Band Systems, Inc. IFM designs is the digital amplitude quantizer (DAQ). This monotonic DAQ samples the RF amplitude every 25 ns and produces an 8-b digital RF amplitude data word. With a dynamic range over 70 dB, the DAQ has become an alternative to the successive detection log video amplifier (SDLVA) used in many IFM receivers. Its small size (2 x 3.65 x 0.4 in.) has also made the DAQ a potential product in military applications where space is an issue.

The receiver’s signal-processing capabilities are also being upgraded. Because of the improvements that the FPGA brings to processing signals, manufacturers are continually equipping their units with additional features. One improvement that’s being integrated into new IFM receivers is the capability of improving the TOA resolution from 25 to 0.3 ns. TOA measurements define the exact time when an RF signal is received. This data is of substantial utility in the identification of the type and mode of an RF emitter. Many radar warning receivers (RWRs) employ TOA data to sort and identify the emitter, analyzing the TOA of each sequentially received RF pulse to establish the pulse repetition interval (PRI) and, thereby, identify the emitter. The TOA resolution of those receivers had been limited to the receiver’s internal clock, usually 25 ns for 40-MHz units. However, by including a precision TOA processor patented by Wide Band Systems, Inc. into IFMs, receivers are providing accurate TOA data with time resolution in the subnanosecond region and absolute root-mean-square (RMS) accuracies on the order of 1 to 2 ns.

Manufacturers are also improving the way that digital data is received and transmitted by their units. In newer units, high-speed serial technology is becoming the standard communication between the RF component and the processor. In addition to Ethernet, HOTLink Transmitter/Receiver products, made by manufacturers like Cypress, are being designed into receivers thus providing a high-speed serial capability over fiber, coaxial, or twisted-pair lines. With the HOTLink, data can be transferred over links at speeds to 330 Mb/s. Also, this pointto- point serial communication link eliminates the bulky connectors and cabling that was associated with predecessor units.

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