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Instantaneous Frequency
Measurement Receiver (IFM)
 
Digital Frequency Discriminator (DFD)

Sage Laboratories produces a wide range of instantaneous frequency measurement products tailored to the requirements of state of the art EW receivers. High speed frequency measurement is achieved either by discrimination or channelization or a combination of the two techniques. Sage's proprietary Suspended Substrate Stripline (SSS) technology is used together with custom Application Specific Integrated Circuit (ASIC) processing circuits to ensure high reproducibility, small size and low cost.

The use of high speed microwave frequency measurement products in EW equipment is becoming increasingly important as the complexity of the signal environment continues to increase. Sage's Digital Frequency Discriminators (DFD) are available in a wide range of designs offering coverage of either specific frequency bands or full 2-18 GHz coverage in a single unit. Units with frequency resolution of up to 14 bits have been produced.

DFDs find applications in both low cost RWRs (to enhance the identification of threat signals against dense non threat background signals) and sophisticated ESM and ECM receivers, where high probability of intercept and high resolution are required to enable pulse by pulse signal de-interleaving.

Most IFM employ some form of RF input conditioning to both band define and amplify the signals. Additional features at this stage may include signal threshold detectors for triggering the digitizer circuits and protective limiting to avoid overload. The performance of this component chain is critical to the correct operation of the receiver. Great care must be taken to ensure that the RF gain vs frequency flatness is tightly controlled to ensure optimum sensitivity and noise performance. The RF gain and detector outputs must also be stable over temperature to ensure correct operation of the triggering functions.

Sage's DFD's employ a proprietary three phase discriminator design which combines the advantages of SSS construction and MIC techniques to form a unique solution. With only three video outputs the Sage design provides the optimum solution to obtaining unambiguous phase information across 360 degrees. In practice very simple RF structures are employed to yield the discriminator characteristics and are suitable for operation over large bandwidths. Sage has obtained 4:1 bandwidths from meander-lines and Schiffman sections, for decade bandwidths stepped impedance structures have been developed. In either realization Sage uses SSS as the circuit medium for both the transmission lines and the phase shifters. This well proven technology has distinct advantages over conventional realizations. SSS being a quasi-TEM structure has very well behaved temperature characteristics. Delay lines realized in SSS do not require heaters to maintain stability and are considerably more compact than conventional SiO₂ cables. The phase shift network is very simple to produce, requires no fine geometry and hence is very reproducible. The only additional components that are required on the discriminator circuit are the detector diodes, chip resistors and bias chokes. SSS lends itself very well to the integration of several tiers of discrimination on a single substrate.

Most DFD require multiple discriminators. The RF power has to be fed to each discriminator tier by a distribution network. Sage DFDs use an array of Wilkinson power splitters realized on softboard microstrip for ease of integration. These networks are designed to take into account the losses associated with each tier. The tiers with the longer transmission line lengths have more loss and require higher drive levels than the shorter ones. The distribution network ensures that all the detected video outputs are of a similar level.

The digitization network converts the raw video voltages from the discriminator outputs into a digital format suitable for processing before outputting as final data. The system requirements usually determine the level of digitization, Sage typically uses 12 or 24 levels. All Sage DFD's use a custom ASIC to perform the digitization latching and decoding function. The use of ASICs reduces the overall circuit parts count, power consumption and size and increases reliability.

Because of the need for advanced EW systems to operate at the limits of sensitivity there is inevitably the chance of false alarms and incorrect triggering of the DFD may occur. It is therefore very desirable for a frequency measuring receiver to give an indication of the validity of the frequency data. This is normally very difficult to achieve in low S/N or simultaneous signal conditions. Sage is able to exploit the accurate characterization of the phase margin of the printed discriminator and offer bad data flags or error correction optimized to the system False Alarm Rate and Probability of Detection.